Summary

Recent advances in astronomical observations have found a significant number of extrasolar planets that can sustain surface water, and the search for extraterrestrial life on such planets is gaining momentum. A team of astrobiologists from Astrobiology Center, National Institute for Basic Biology, and SOKENDAI have proposed a novel approach for detecting life on ocean planets. By conducting laboratory measurements and satellite remote sensing analyses, they have demonstrated that the reflectance spectrum of floating vegetation could serve as a promising biosignature. Seasonal variations in floating vegetation may provide a particularly effective means for remote detection.The results of this research will be published in the journal Astrobiology on February 2, 2025.

Background 

Astronomical surveys have discovered nearly 6,000 exoplanets, including many habitable planets, which may harbor liquid water on their surfaces. The search for life on such planets is one of the most significant scientific endeavors of this century, with direct imaging observation projects currently under development.

On Earth-like planets, the characteristic reflectance spectrum of terrestrial vegetation, known as “vegetation red edge”, is considered as a key biosignature. However, ocean planets, with most of their surfaces covered by water, are unlikely to support terrestrial vegetation. To broaden the scope of life detection on ocean planets, this study examined the characteristics of reflectance spectra from floating plants and tested their detectability.

Results

The study investigated the reflectance spectra of floating plants across different scales, from individual leaves in laboratory settings to large-scale observation via satellite remote sensing of lake vegetation.

Although floating leaves exhibit considerable morphological variation among species, their general trend reveals a pronounced red edge, often comparable to or even exceeding that of terrestrial plants. This enhancement is attributed to air gaps in sponge tissue that provide buoyancy and specialized epidermal structures that offer water repellency. While floating leaves show slightly reduced reflectance when wet, they still display a more distinct red edge than submerged water plants (Figure 1).

However, on a larger scale, the red edge signature of floating vegetation weakens due to lower vegetation density and reduced leaf overlap on the water surface. Landscape-scale analyses using satellite remote sensing (Sentinel-2; ESA) with the Normalized Difference Vegetation Index (NDVI) flourishes in summer and disappears in winter, causing the NDVI to be relatively low when averaged over the year. Nevertheless, the fluctuation between minimum and maximum NDVI values is more pronounced for floating vegetation compared to forests. To further investigate this pattern, a large-scale survey of 148 lakes and marshes across Japan was conducted. The study revealed a characteristic seasonal NDVI variation, shifting from negative values in winter to positive values in summer (Figure 2). Importantly, while water suppresses the reflectance of floating vegetation, its own reflectance is even lower and remains stable. It enhances the detectability of seasonal NDVI fluctuations, which remain robust against atmospheric and cloud interference, suggesting that this method could be promising for detecting life on habitable exoplanets in the future.

The post Can we find floating vegetation on ocean planets? first appeared on Astrobiology Center, NINS.

A former Center for Astrobiology researcher’s research topic on photosynthesis has progressed into a new technology for smart agriculture!

This is the result of research that the lead author of the paper, Dr. Kozuma, was still conducting when he was at ABC!

 

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小型センサで植物を見守るスマート農業の新技術を開発

〜クラウド連携でいつでも、どこでも健康状態のモニタリングが可能に〜

Key Points：

• We have developed a compact sensor that is attached to the underside of plant leaves to detect changes in leaf color, chlorophyll (Note 1) content, and stress response.
• The developed sensor is water-resistant and battery-powered for long-term continuous measurement, and the measurement data can be viewed anytime and anywhere via the cloud.
• The relatively low cost of the sensor makes it possible to monitor plant health over a wide area or at high densities by installing many sensors.

Abstract：

Due to the effects of climate change and population growth, agriculture faces an urgent need for efficient resource management and increased productivity. Against this backdrop, smart agriculture is attracting attention, especially for technologies that can remotely monitor plant conditions.

Associate Professor Koichiro Miyamoto of the Graduate School of Engineering at Tohoku University and Assistant Professor Kaori Agatsuma of the Graduate School of Life Sciences (currently Graduate School of Agricultural Science, Kyoto University) have devised and developed a new compact sensor that can remotely monitor plant health using a smartphone or other terminal. The sensor can be attached to the underside of a plant leaf to accurately measure the physiological response of the leaf without blocking sunlight. The measured data is shared via online storage and can be monitored remotely and in real time. It can also detect leaf color, chlorophyll content, and environmental stress, paving the way for long-term continuous and multi-point simultaneous measurement systems. This compact sensor can be fabricated for only a few thousand yen and is expected to be used in the agricultural and other fields.

These results were published in Sensing and Bio-Sensing Research on September 24, 2024.

For details, please refer to the ohoku University press release.

 

Publication：

Journal：Sensing and Bio-Sensing Research 46(2024),100688.

Title：Analysis of plant physiological responses based on leaf color changes through the development and application of a wireless plant sensor

Authors：Kaori Kohzuma, Ko-ichiro Miyamoto

Responsible Author1: Kaori Kohzuma, Assistant Professor, Graduate School of Life Sciences, Tohoku University

(Current affiliation: Graduate School of Agricultural Science, Kyoto University; Former affiliation: Graduate School of Science, The University of Tokyo; Research Center for Astrobiology, National Institutes of Natural Sciences)

Author2： Associate Professor Koichiro Miyamoto, Graduate School of Engineering, Tohoku University

DOI：10.1016/j.sbsr.2024.100688

URL：https://doi.org/10.1016/j.sbsr.2024.100688

 

The post New Technologies for Smart Agriculture from Astrobiology first appeared on Astrobiology Center, NINS.

Plants possess a safety valve called Non-Photochemical Quenching (NPQ) to actively dissipate excess light energy as heat during the energy conversion process of photosynthesis. In recent years, there has been progress in understanding the molecular mechanism of NPQ, highlighting its important role in environmental adaptation. However, the impact of heat dissipation through NPQ has yet to be well-studied or quantified.

A research team led by Associate Professor Kenji Takizawa from the Astrobiology Center of the National Institutes of Natural Sciences, Assistant Professor Eunchul Kim from National Institute for Basic Biology, Professor Jun Kikawada, and Aoi Murakami from the Graduate University for Advanced Studies calculated the amount of heat emitted by NPQ in standard plants under sunlight. They estimated the effect of the resulting temperature increase inside leaves due to NPQ and the effect of the ground temperature increase when averaged on a global scale.

While the heat generated by NPQ is relatively small compared to the overall energy balance at both the cellular and global levels, the study demonstrated that it is non-negligible and could contribute to temperature increases under conditions where heat transfer is restricted.

This result was published in Frontiers in Plant Science (March 20, 2024).

Research Findings:

• Impact of NPQ on leaf internal temperature:
  Using a combination of the amount of solar irradiance at noon in middle-latitude areas with absorption rates by light-harvesting complex and energy distribution rate at the photosynthetic reaction centers , they estimated that the thermal energy released as NPQ is approximately 64 Wm-2. By modeling the leaf structure consisting of epidermal tissue, palisade tissue, and spongy tissue, and calculating the internal temperature gradient when this heat is emitted from the central palisade tissue, they found that the normal temperature increase is slight (less than 0.1 degrees). However, under special conditions where heat conduction is limited to the air layer within the spongy tissue, the internal leaf temperature could potentially rise by up to about 1 degree Celsius.
  
• Impact of NPQ on Earth’s Surface Temperature:
  Considering variations in solar irradiance due to latitude, season, and time of day, as well as vegetation coverage, they estimated the overall average calorific value due to NPQ to be 2.2 Wm-2. This corresponds to approximately 0.55% of the total infrared radiation emitted from the Earth’s surface. While this percentage is small, it suggests that NPQ could have a comparable impact on Earth’s environment to recent greenhouse gas effects.

Prospects for the future

Perspectives in Plant Physiology :

Although the heat generated by NPQ is not effective in warming the entire leaf, it may locally and temporarily increase the temperature within the cell and chloroplast. The development of tiny temperature sensors such as metal nanoparticles is expected to clarify the temperature gradient around the chloroplast thylakoid membrane, thereby elucidating the energy distribution in the photosynthetic reaction process and its control by temperature.

Perspectives in Astrobiology :

Most plants on earth have evolved to maximize light collection and release excess energy as heat. Vegetation cover of the earth’s surface reduces reflectance and thus offsets the cold temperatures caused by the absorption of carbon dioxide. If plants that reflect most of the solar radiation and emit less heat evolve on an exoplanet different from Earth, the expansion of terrestrial vegetation could lead to more rapid cooling than on Earth.

Publication Journals

Magazine name：Frontiers in Plant Science

Publication date：March 20, 2024

Paper Title： How much heat does non-photochemical quenching produce?

Authors：Murakami A, Kim E, Minagawa J, and Takizawa K

DOI：10.3389/fpls.2024.1367795

The post Does Heat Release from Photosynthetic Safety Valves Impact Plants and the Earth’s Environment? first appeared on Astrobiology Center, NINS.

• Plants and algae typically utilize only visible light for photosynthesis in the sunlight spectrum. Certain algae species thriving in Antarctica can harness infrared light for photosynthesis, but the mechanism was previously unknown.
• Using a device called a cryogenic electron microscope, they revealed the protein structure these algae utilize to conduct photosynthesis with infrared light.
• Many planets discovered outside the solar system orbit stars which have so lower-temperature than the Sun that they emit primarily infrared light. This suggests the potential for life that utilizes infrared light for photosynthesis. This achievement may provide insights into exploring such possibilities of life beyond Earth.

Research Overview:

A research team led by Dr. Makiko Kosugi from the Astrobiology Center (currently a Project Assistant Professor at the National Institute of Basic Biology and a Collaborative Researcher at Chuo University), along with researchers from Institute of Materials Structure Science at the High Energy Accelerator Research Organization (KEK) including Associate Professor Masato Kawasaki, Project Assistant Professors Naruhiko Adachi and Toshio Moriya, and Professor Toshiya Chida, from Tohoku University Associate Professor Minoru Shibata, from Akita Prefectural University Associate Professor Kojiro Hara, from Tokyo University of Agriculture Professor Shinichi Takaichi, from the National Institute of Basic Biology RMC Professor Yasuhiro Kamei, from University of Hyogo Associate Professor Yasuhiro Kashino , from the National Institute of Polar Research Professor Sakae Kudo, and from Chuo University Professor Hiroyuki Oda, identified the light-harvesting antenna protein (Pc-frLHC) responsible for absorbing far-red light (700-800 nm) in the green algae Prasiola crispa, which is known to conduct oxygenic photosynthesis utilizing far-red light, part of infrared light. The team also revealed the three-dimensional structure of this molecule by Single Particle Analysis (Note 1) using a cryogenic electron microscope at KEK, revealing that Pc-frLHC forms a large complex composed of 11 identical proteins in a ring structure (see Figure 1). Each protein binds 11 chlorophyll molecules, with five of these chlorophylls suggested to be special for absorbing far-red light. Spectroscopic analysis indicated that part of the far-red light energy absorbed by these special chlorophylls is converted within Pc-frLHC to energy equivalent to visible light, which is then utilized for photosynthesis. This result was published in the British scientific journal “Nature Communications” on February 15, 2023 (Kosugi et al., 2023, “Uphill energy transfer mechanism for photosynthesis in an Antarctic alga”).

Research Background：

Photosynthesis by plants and algae uses the energy of visible light (350-700 nm) in sunlight to break down water into oxygen, hydrogen, and electrons, and the reducing power obtained is used for carbon dioxide assimilation. It is usually not used for water splitting because its energy is lower than that of visible light. Although some cyanobacteria (see Note 2) have been known to utilize infrared light for photosynthesis and have been analyzed, eukaryotic photosynthetic organisms such as plants and algae (see Note 2) have not been analyzed.

Kosugi et al. at the Center for Astrobiology (now the National Institute for Basic Biology) and colleagues have recently shown that the green alga Nankyoku Kawanori, a eukaryotic photosynthetic organism that grows on land in Antarctica, uses far-red light (700~800 nm), which is part of infrared light, for photosynthesis with an energy conversion efficiency as high as that of visible light (Kosugi (Kosugi et al. 2020). The terrestrial environment in Antarctica is extremely dry with low temperatures and frequent freezing. In addition, the extremely strong ultraviolet rays during the summer make it impossible for many organisms to survive. Nankyoku Kawanori is extremely resistant to desiccation and freezing, and can quickly regain its metabolic activity after being dried out or frozen for a long period of time by applying water. These characteristics make it one of the few photosynthetic organisms that can grow in the Antarctic terrestrial environment. Nankyo kawanori forms colonies (aggregates) with many layers of cells (Fig. 2).

Near the surface of the colony, sunlight can reach enough to use visible light for photosynthesis, but this has the disadvantage of damaging the cells due to the ultraviolet rays contained in the sunlight. On the other hand, the lower layers of the colony are less likely to be damaged by ultraviolet light, but visible light is absorbed by the algae on the surface layer and used for photosynthesis, so very little visible light reaches the colony. It is thought that Nankei-Kawanori acquired a system that utilizes infrared light for photosynthesis during its evolution and increased the amount of photosynthesis in the lower layers of its colony, allowing it to reproduce even in the extremely harsh environment of Antarctica (Fig. 3).

The existence of organisms that can use infrared light for photosynthesis has attracted much attention in the field of astrobiology. This is because many of the exoplanets discovered so far are located around low-temperature stars that are fainter than the sun and emit more infrared light than visible light (Note 3). Oxygen emitted into the atmosphere by photosynthetic organisms is considered to be one of the traces of life that can be observed from Earth when studying the presence of life on exoplanets. Elucidating the mechanism and evolutionary process of photosynthesis using infrared light on Earth is important for discussing the possibility of detecting oxygen on exoplanets around low-temperature stars. It has been suggested that infrared-enabled photosynthesis in Nankyoku Kawanori involves an uphill energy transfer (Note 4), in which low energy excites molecules at high energy levels, and the mechanism that achieves high light-use efficiency may involve a previously unknown quantum biological reaction The mechanism of high light-utilization efficiency may involve a previously unknown quantum biological reaction. Therefore, we purified and identified an infrared harvesting antenna protein from Nankyo kawanori and elucidated its molecular structure to elucidate the mechanism of infrared-utilizing photosynthesis.

Research Findings:

The Nankyoku Kawanori used in the experiment were collected during the activities of the 49th and 54th Expeditions to the Antarctic region. The cells of the Nankyoku Kawanori were crushed and separated according to protein size and charge, and a protein with a pronounced absorption band in far-red light was purified and named Pc-frLHC (Prasiola crispa far-red light harvesting Chl-binding protein The protein was named Pc-frLHC (Prasiola crispa far-red light harvesting Chl-binding protein complex). Analysis of the amino acid sequence of the protein revealed that Pc-frLHC is a four-transmembrane (Note 6) LHCI (Light harvesting chlorophyll a/b binding complex of photosystem I), which binds to photosystem I (Note 5) in some green algae. This four-transmembrane LHCI has been reported to absorb the longest wavelength of visible light in the green alga Chlamydomonas, but can hardly absorb far-red light (Mozzo et al. 2010). Furthermore, Pc-frLHC functions as an antenna for photosystem II (Note 5), which performs water splitting, rather than photosystem I, suggesting that the absorption band of the long-wavelength absorbing LHC originally possessed by green algae moved to an even longer wavelength and evolved as an antenna for photosystem II.

Single-particle analysis using cryo-electron microscopy has succeeded in obtaining a three-dimensional structural molecular model of Pc-frLHC with high resolution. The general photosystem II antenna protein of green algae is a structure of three proteins bound together, but the Pc-frLHC analyzed in this study is a novel complex structure with 11 proteins bound together in a ring (Fig. 4). 11 chlorophylls are bound to one protein, and all of them in the ring chlorophylls are located at a distance that allows energy transfer, forming an energetically connected network. Chlorophyll normally absorbs visible light, but it is known that when multiple chlorophyll molecules interact with each other in close proximity, a portion of the absorption band shifts to the longer wavelength side. In Chlamydomonas quadruple transmembrane LHCI, which can absorb relatively long-wavelength light, two chlorophylls were reported to be in close proximity (Mozzo et al. 2010), but in Pc-frLHC, these two chlorophylls were found to be in close proximity to another chlorophyll, indicating that five chlorophylls strongly interact. This chlorophyll structure is the basis of the Pc-frLHC. This chlorophyll structure is thought to be responsible for the far-red light absorption of Pc-frLHCs.

To understand how far-red light energy absorbed by Pc-frLHC is transferred, we excited the long-wavelength absorbing chlorophyll of Pc-frLHC with ultrashort laser pulses of far-red light and examined how chlorophyll fluorescence (see Note 7) changes over time. The fluorescence from the long-wavelength-absorbing chlorophyll is detected at 713 nm, while the fluorescence from normal chlorophyll is detected at 680 nm; by examining how the fluorescence at 680 nm increases with time, we can see that the energy between the long-wavelength-absorbing and normal chlorophylls goes within 25 picoseconds (= 0. It has been found that the energy goes back and forth within 25 picoseconds (= 0.000000025 seconds). This result indicates that an uphill excitation energy transfer from long-wavelength-absorbed chlorophyll to normal chlorophyll is indeed occurring within Pc-frLHC. In this process, part of the energy of far-red light is converted to visible light energy, and the subsequent photosynthetic reaction is thought to proceed in the same way as when visible light is absorbed.

Perspectives：

• Elucidating the Details of Uphill Excitation Energy Transfer
  In order to elucidate the full extent of the high excitation efficiency of photosystem II by far-red light, it is necessary to analyze the details of the energy transfer from Pc-frLHC to photosystem II. To this end, we will purify a supercomplex of Pc-frLHC and photosystem II from Nankei-kawanori cells to elucidate the excitation energy transfer process
  
• Evolutionary aspects of far-red light-utilizing oxygen-evolving photosynthetic organisms
  Although the protein has not been identified or structurally analyzed, several eukaryotic algae with a prominent far-red light absorption band have been reported in addition to Nankyoku Kawanori, and it is possible that a far-red light-absorbing light-harvesting protein similar to the Pc-frLHC found in Nankyoku Kawanori in this study exists in other eukaryotic algae as well Pc-frLHC found in Nankyokawanori. We will obtain amino acid sequences of far-red light-utilizing light-harvesting proteins in various algae, clarify their evolutionary lineages, and analyze the homology and diversity of far-red light-utilizing mechanisms.
  
• Astrobiological Aspects
  The search for life targeting exoplanets is expected to make great progress in the future with the development of the next-generation Very Large Telescope. Oxygen is a promising bio-signature, but is there any possibility of detecting “photosynthetically derived” oxygen on exoplanets around low-temperature stars? By clarifying the details of infrared oxygen-evolving photosynthesis on Earth, we will explore the possibility of the evolution of photosynthetic life on exoplanets around low-temperature stars.

Annotation:

1) Single particle analysis by cryo-electron microscopy:
Protein structure analysis technology has developed rapidly in recent years. Conventional X-ray protein structure analysis requires protein crystals, but cryo-electron microscopy eliminates the need for crystals, facilitating analysis of samples that are difficult to crystallize, or samples that can only be obtained in small quantities, such as the Antarctic-derived organisms in this study.

2) Cyanobacteria and eukaryotic photosynthetic organisms:
Cyanobacteria are the most primitive oxygen-evolving photosynthetic organisms and are considered to be the ancestors of chloroplasts. Cyanobacteria evolved through intracellular symbiosis into eukaryotic photosynthetic organisms such as algae and plants. The mechanism of photosynthesis by far-red light is different between cyanobacteria and eukaryotic photosynthetic organisms, and it is important to clarify both mechanisms.

3) Low-temperature stars:
A star that is lighter and cooler than the Sun (G-type), also known as an M-type dwarf star. They are also called M-type dwarfs. They are considered important targets in the search for life because of their overwhelmingly large percentage of the stars in the Universe. Because the ratio of infrared light is higher than that of visible light, the environment of the surrounding exoplanets is also dominated by infrared light.

4) Uphill excitation energy transfer:
Excitation energy transfer between chlorophyll molecules is usually passed from a higher energy level molecule to a lower energy level molecule, and this reverse reaction is called uphill excitation energy transfer. Uphill excitation energy transfer is considered to occur when the energy difference between molecules is compensated by thermal energy.

5) Photosystem I and Photosystem II:
Proteins involved in the electron transfer system in the thylakoid membrane of chloroplasts. It has a special chlorophyll reaction center that causes charge separation by light energy. Photosystem II breaks down water, and photosystem I increases the energy level of electrons received from photosystem II to a level that allows the reduction of electron transfers needed for carbon dioxide fixation. Excitation of photosystem II requires higher light energy at shorter wavelengths than that of photosystem I.

6) Four transmembrane LHCs:
Most light-harvesting antenna proteins in algae are folded and embedded in a lipid bilayer called the thylakoid membrane in chloroplasts. The number of times they penetrate the membrane depends on the number of folds.

7) Chlorophyll fluorescence:
Light emitted when chlorophyll is electronically excited by light to a lower energy state.

論文情報：

Journal：Nature Communications

“Uphill energy transfer mechanism for photosynthesis in an Antarctic alga”

Author： Makiko Kosugi, Masato Kawasaki, Yutaka Shibata, Kojiro Hara, Shinichi Takaichi, Toshio Moriya, Naruhiko Adachi, Yasuhiro Kamei, Yasuhiro Sagano, Sakae Kudo, Hiroyuki Koike, Toshiya Senda

DOI： 10.1038/s41467-023-36245-1

URL：https://www.nature.com/articles/s41467-023-36245-1

Co-Publishing Institution：

• National Institute for Basic Biology　Press Release
• Chuo University　Press Release
• National Institute of Polar Research　Press Release
• High Energy Accelerator Research Organization　Press Release
• Tohoku University　Press Release(Tohoku University Website, Laboratory Website)
• University of Hyogo　Press Release

The post Revealing the Mechanism of Antarctic Algae’s Photosynthesis Using Infrared Light. A New Key to Extraterrestrial Life? first appeared on Astrobiology Center, NINS.

The search for life on exoplanets is one of the most important themes in the field of astrobiology. As evidence for the existence of such life, it is expected to detect biosignatures (Note 1) that show characteristic patterns of photosynthesis-derived light. One of these is the red edge (Note 2), a spectroscopic feature of the light spectrum reflected by vegetation. For example, the light environment of planets around stars lighter than the Sun (M dwarfs), which are currently the target of observation, is very different from that of the Earth in our solar system, and it is under discussion how the red edge appears.

In photosynthesis, light energy absorbed from sunlight is either used for photochemical reactions or released as fluorescence (Note 3) or heat (Figure 2). Remote sensing of the Earth has recently observed the fluorescence as well as the red edge. The red edge allows us to measure the amount of vegetation covering the planetary surface, whereas the fluorescence is used to estimate more detailed photosynthetic activity, such as stress conditions. We, therefore, tested the promise of photosynthetically derived fluorescence as an advanced biosignature in addition to the red edge.

In this study, we simulated how fluorescence appears in planetary spectra for a Sun-like star and an Earth-like planet orbiting two M-type dwarfs (GJ667C and TRAPPIST-1), respectively, assuming different planetary atmospheres and surface conditions. We used two light absorption and fluorescence spectra of photosynthetic organisms: typical vegetation with chlorophyll a and b (Chl) and purple bacteria with bacteriochlorophyll b (BChl). We determined the fluorescence intensity by appropriately scaling it according to the number of photons acquired under radiation fields in the habitat. Using these light absorption spectra, we also calculated the leaf reflection spectra by means of radiation transfer calculations (Note 4). In this way, we developed a model that consistently handles light absorption, fluorescence, and reflection, and investigated how they appear in planetary spectra.

Numerical simulations showed that, in the case of BChl, in the absence of clouds or strong absorbers around 1,000 nm, the fluorescence, together with the detection of red edges, can be a good biosignature to identify traces of photosynthesis (Figure 3). However, a noise model assuming NASA’s planned future 6 m aperture space telescope (previously considered as LUVOIR, now called the Habitable Worlds Observatory) around solar-type stars, we also found that it takes a very long observation time to identify the fluorescence. Even so, ultra-cool stars such as TRAPPIST-1 have strong absorption of vanadium oxide (VO), iron hydride (FeH), and potassium in the stellar atmosphere, and interestingly these lead to significantly larger apparent reflectance at wavelengths where the flux from the star is small due to this stellar absorption, and fluorescence emission from the planet. This may be a good feature for observing fluorescence at high dispersion by future large ground-based telescopes such as TMT and needs to be verified in the future. Furthermore, it is important to consider the conditions for large fluorescence emission from the physiological perspective of photosynthesis and to capture the nonlinear response of biological fluorescence relative to the incident light since fluorescence is also generated nonbiologically.
At ABC, young researchers actively collaborate across the boundaries of research fields between astronomy and biology, and observation, experiment, and theory. This study results from such activities and has been compiled as an academic paper. This is truly an achievement that links biology and astronomy, and theory and observation.

Footnotes :
(Note 1) Spectral features of atmospheric molecules such as oxygen, ozone, and methane, and the surface feature, e. g., due to vegetation.

(Note 2) A feature in which the reflectance spectrum of leaves increases sharply around 700 nm.

(Note 3) The light emitted when electronically excited states by light quenched to a low-energy state.

(Note 4) A calculation method that deals with light propagation.

Publication Information :
Journal: The Astrophysical Journal
Title: Photosynthetic Fluorescence from Earth-like Planets around Sun-like and Cool Stars

Authors: Yu Komatsu 1,2, Yasunori Hori 1,2, Masayuki Kuzuhara 1,2, Makiko Kosugi 1,2,3, Kenji Takizawa 1,3, Norio Narita 4,1, Masashi Omiya 1,2, Eunchul Kim 3, Nobuhiko Kusakabe 1,2, Victoria Meadows 5, Motohide Tamura 1,2,4
1) Astrobiology Center, 2) National Astronomical Observatory of Japan, 3) National Institute for Basic Biology, 4) University of Tokyo, 5) University of Washington

DOI: 10.3847/1538-4357/aca3a5

arXiv: https://arxiv.org/abs/2301.03824

The post Can We Detect Photosynthetic Fluorescence in Space? first appeared on Astrobiology Center, NINS.

Solar energy is an essential energy source for all life. Plants harvest solar energy by utilizing photosynthesis reactions, in which they optimize the efficiency of sunlight utilization. One mechanism of optimization is “state transition”, a system that balances the driving of two photosystems (Photosystem I and Photosystem II). In this study, they extracted the Photosystem I State Transition Supercomplex (PSI-LHCI-LHCII supercomplex), where Photosystem I, antenna system LHCI providing light energy to this system, and LHCII conveyed from Photosystem II are bound, from green algae cells, and determined its three-dimensional structure using a cryo-electron microscope. This method revealed details of the state transitions involving the light-harvesting antenna LHCII between two large photosynthetic complexes. This research was achieved by an international team including Assistant Professor Ryutaro Tokutsu and Professor Jun Minagawa at the National Institutes of Natural Sciences, National Institute for Basic Biology (Current affiliation: Graduate School of Science, Kyoto University), Project Professor Kazuyoshi Murata at National Institute for Physiological Sciences, Project Associate Professor Kenji Takizawa at the Astrobiology Center, Assistant Professor Tomohito Yamasaki at Faculty of Science and Engineering, Kochi University, and Dr. Mei Li at Chinese Academy of Sciences.

The achievements were published in Nature Plants on July 8, 2021.

Release Details: RIKEN Press Release

Journal Information

Title: Structural basis of LhcbM5-mediated state transitions in green algae
Authors: Xiaowei Pan, Ryutaro Tokutsu, Anjie Li, Kenji Takizawa, Chihong Song, Kazuyoshi Murata, Tomohito Yamasaki, Zhenfeng Liu, Jun Minagawa, Mei Li

Nature Plants
https://www.nature.com/articles/s41477-021-00960-8

DOI:10.1038/s41477-021-00960-8

The post Determining the Structural Basis of Photosynthetic State Transition first appeared on Astrobiology Center, NINS.

Abstract：

The Antarctic land surface is an extreme environment exposed to low temperatures, freezing, aridity, and strong ultraviolet rays during the summer. Understanding the adaptive strategies of organisms that can thrive in such environments is important for understanding the possibility of life phenomena in various environments, not only on Earth but also in the universe.

Dr. Kosugi and his research team at the Center for Astrobiology have shown for the first time that Antarctic minnows, collected in Antarctica, perform a series of photosynthetic reactions not only in visible light, which is used by most photosynthetic organisms, but also in near-infrared light, depending on the light environment. Since the energy of near-infrared light is lower than that of visible light, it was predicted that photosynthetic efficiency would be greatly reduced. However, measurements of oxygen-evolving activity and redox reactions in the photosystem revealed that the efficiency of utilization of photons absorbed by the algae for photosynthesis is the same as that of visible light.

Since the algae grow in multiple layers, visible light is absorbed mainly in the upper layers, and the light reaching the lower layers is dominated by near-infrared rays rather than visible light. The system that enables the use of near-infrared light for photosynthesis is thought to help increase the photosynthetic efficiency of the entire Nankei-Kawanori community.

The existence of photosynthetic organisms that utilize near-infrared light in the extreme environment on Earth provides various hints for the evolution of life on planets around stars (red dwarfs), where the near-infrared light fraction is very high. We believe that further clarification of the evolution and mechanism of near-infrared-utilizing photosynthesis will lead us to the possibility of the existence of oxygen-evolving photosynthetic organisms on such planets.

The results were published in the biological journal Biochimica et Biophysica Acta – Bioenergetics.

Key points of the publication：

• Near-infrared-induced oxygen-evolving photosynthesis was discovered in an algae dominant in the polar regions.
• Highly efficient photosynthetic reactions by uphill-type (Note 1) excitation energy transfer were suggested.

(Remark 1) A phenomenon in which excitation energy is transferred from a molecule with a lower energy state at the time of excitation to a molecule with a higher energy state at the time of excitation. See the right figure in Figure 3.

Research Background：

Photosynthesis, which is carried out by algae and plants on the earth today, is a reaction that uses light energy to break down water and produce organic matter from carbon dioxide using the reduction force obtained. Oxygen is released during the water decomposition process. This oxygen-evolving photosynthesis was initiated by prokaryotic cyanobacteria (cyanobacteria) about 2.7 billion years ago, transforming the anaerobic global environment into an aerobic one, where oxygen was virtually non-existent. The increase in the concentration of oxygen in the atmosphere led to the flourishing of aerobic respiring organisms, which is believed to have had a significant impact on the evolution of life on Earth.

It has been believed that oxygen-evolving photosynthetic reactions require the energy of visible light. This is because at lower light energies, it becomes difficult to obtain the reducing power needed to break down water and fix carbon dioxide. Since the 1990s, however, a number of organisms have been discovered that perform oxygen-evolving photosynthesis using only near-infrared light. It was reported that some cyanobacteria synthesize photosynthetic pigments (chlorophyll d, f) that absorb near-infrared light and use them as reaction centers for charge separation reactions (direct infrared use). On the other hand, efficient energy transfer from near-infrared absorbing chlorophyll to visible light absorbing chlorophyll (indirect infrared utilization) has been suggested in some cyanobacteria and eukaryotic photosynthetic organisms, and an uphill-type (Note 1) energy transfer mechanism that makes this possible has attracted attention The following is a list of the most important examples.

The Nanjing riverine algae (Prasiola crispa) is a terrestrial green alga that is widely distributed in cold regions at high latitudes and is known to form large colonies in polar environments (Figure 1). The research team has been studying the stress tolerance ability and growth environment of Nankiola crispa in detail with the aim of elucidating the adaptive strategies of organisms growing in polar regions. In the process, the team identified a near-infrared absorption band, which is not found in common green algae, and analyzed the role of this band.

Research：

Nankyoku Kawanori collected in Antarctica (Figure 2) has a near-infrared absorption band with a peak absorption around 710 nm as a shoulder to the normal red visible light absorption band (680 nm). The size of the near-infrared absorption band varies greatly among individuals collected in the field and disappears after long-term incubation under fluorescent light, which contains almost no near-infrared light, suggesting that its expression is adjusted according to the light environment. Measurements of the light wavelength dependence of photosynthetic activity revealed that light energy absorbed in the near-infrared absorption band is used for photosynthesis with the same level of efficiency as visible red light. In algae, water decomposition reactions require light energy equivalent to visible light, suggesting that an Uphill-type (Note 1) excitation energy transfer is taking place. It is possible that the Uphill excitation reaction (Note 1), which greatly exceeds the range compensated by thermal fluctuation, is taking place, and further elucidation of the molecular mechanism is expected in the future. Figure 3 is a schematic diagram summarizing the results of this study.

Publication：

Authors：
Makiko Kosugi, Shin-Ichiro Ozawa, Yuichiro Takahashi, Yasuhiro Kamei, Shigeru Itoh, Sakae Kudoh, Yasuhiro Kashino, Hiroyuki Koike,

Title：
Red-shifted chlorophyll a bands allow uphill energy transfer to photosystem II reaction centers in an aerial green alga, Prasiola crispa, harvested in Antarctica,

Journal：
Biochimica et Biophysica Acta – Bioenergetics, 2020年 Vol. 1861 (2), 148139,
DOI:10.1016/j.bbabio.2019.148139

The post Discovery of Green Algae that Perform Oxygen-Evolving Photosynthesis Using Near-Infrared Light in the Antarctic Terrestrial Environment first appeared on Astrobiology Center, NINS.

Abstract:

Recently, red dwarfs (M-type stars), which are low-temperature stars with less than half the mass of the Sun, have been attracting attention as targets for the search for habitable planets that can harbor life like the Earth. Many of the stars close to the Sun are red dwarfs, and it is hoped to observe signs of life (biomarkers) on such planets in the near future.

One promising biomarker for exoplanets is the reflectance spectrum called the red edge, which is produced by vegetation on land. However, since the position of the red edge (wavelength of about 0.7 μm) is determined by the wavelength of light used by vegetation for photosynthesis, it has been thought to depend on the wavelength of light emitted by the star. For example, since near-infrared light, which has a longer wavelength than visible light, is dominant on planets around red dwarfs, it was expected that the red edge would also move to the near-infrared wavelength side.

A joint research team consisting of Kenji Takizawa, Project Associate Professor and Nobuhiko Kusakabe, Project Specialist at the National Astrobiology Center of the National Institutes of Natural Sciences, Jun Minagawa, Professor at the National Institute for Basic Biology, and Kenpo Narita and Motohide Tamura, Assistant Professor at the University of Tokyo, has been studying the wavelengths at which red edges appear in the assumed light environment of a life-supporting planet around a red dwarf star. The team theoretically investigated the wavelengths at which the red edge appears in the light environment of a life-supporting planet around a red dwarf from the perspective of photosynthesis. As a result, the team proposed for the first time that photosynthetic organisms that first land on the surface of a red dwarf star, even if they originate and evolve in water, use visible light for photosynthesis just like on Earth because infrared radiation is absorbed by water, and that, contrary to previous expectations, red edges are likely to appear in the same locations as vegetation on Earth. This study may provide important guidance for key biomarkers and wavelengths for future observations of life on exoplanets.

The results of this study will be published in the British online scientific journal Scientific Reports on August 8, 2017 at 10:00 a.m. UK time (6:00 p.m. EDT).

Background：

Observations by NASA’s Kepler spacecraft have revealed that extrasolar planets are ubiquitous in our galaxy, and with the successive discoveries of terrestrial planets in the habitable zone for life, the discovery of a second Earth that harbors life around a star near the Sun is expected to be a feasible goal. If observations of exoplanets confirm the presence of oxygen in planetary atmospheres, it would be a sign (biomarker) of the existence of life, but this alone is not conclusive evidence, since non-living oxygen evolution is also a possibility. In addition, many of the stars close to the Sun are red dwarfs, which are the most important targets for future observations, so there is an urgent need to advance research on biomarkers on terrestrial planets around red dwarfs.

Plants on Earth absorb visible light from blue to red, which they use for photosynthesis, and reflect near-infrared light, which they do not use, and thus show a characteristic reflectance spectrum called “red edge. If this red edge can be observed on exoplanets, it will be a biomarker that can more reliably support the existence of life, along with the presence of oxygen. However, the wavelength of the red edge may not be the same on an exoplanet with a different light environment than on Earth. Red dwarfs, which are important targets for future observations, emit more near-infrared light than visible light, and it has been thought that the wavelength used for photosynthesis shifts from visible light to near-infrared light, which in turn shifts the position of the red edge to the longer wavelength side. In this study, based on the latest photosynthesis research, we examined whether the shift of the red edge in accordance with the light environment is reasonable or not.

Research Findings：

We estimated the light environment on land and in water for an Earth-like planet in the life-supporting region of the red dwarf star Leo AD in the vicinity of the solar system and predicted the optimal photosynthetic utilization wavelength for that environment. If the terrestrial planet evolved to increase photosynthetic productivity due to the abundance of near-infrared light, it would use light up to 900 nm or 1,100 nm for photosynthesis, and the red edge would appear on the longer wavelength side of that wavelength. On the other hand, because near-infrared light is attenuated by water molecules in water, the existence of photosynthetic organisms that depend solely on visible light is predicted even around red dwarfs, as it is on Earth. As a possible evolutionary pathway of the photosynthetic mechanism from visible light-utilizing organisms in water to near-infrared-utilizing organisms on land, we can assume a transient photosynthetic mechanism in which one of the two reaction centers utilizes visible light and the other utilizes near-infrared light. However, if the two reaction centers with different absorption wavelengths cannot be excited in a balanced manner, the acquired energy will generate dangerous reactive oxygen species, which is rather detrimental to survival. We tested whether this transient photosynthetic mechanism can adapt to the light environment in the boundary region between underwater and terrestrial environments by comparing it with examples of terrestrialization that have actually occurred on Earth. The results show that for oxygen-evolving photosynthetic organisms to rapidly evolve from water to land:

• The emission spectrum of the main star is close to the light transmission spectrum of water molecules
• Excitation wavelengths of multiple reaction centers are close
• A mechanism to maintain the excitation balance of the reaction centers is in place.

The following three conditions are necessary. While the light environment and photosynthetic mechanism on Earth satisfy these three conditions, the radiation peak of the main star of a planet around a red dwarf is shifted to the long wavelength side, making it difficult to maintain the excitation balance of the reaction centers if their excitation wavelengths differ greatly, making landing difficult.

On the other hand, if only visible light is used, the reaction center excitation wavelengths are close and thus it is possible to maintain the excitation balance even under the irradiation of the red dwarf, and the photosynthetic organisms that land first are likely to have a photosynthetic mechanism similar to that of the Earth, and the position of the red edge is likely to be similar to that of the Earth. This suggests that even on planets around red dwarfs, the first plants to land on

Prospects：

By examining not only the adaptation of organisms to the environment on a red dwarf planet, but also the processes that lead to such adaptation, it was shown for the first time that visible light utilization is maintained during the process from the birth of oxygen-evolving photosynthetic organisms to the transition to terrestrial life. It is important to further examine whether the evolution to the near-infrared utilization type, which is inhibited in water, progresses quickly on land, from the viewpoints of both photosynthetic function and evolutionary process. Future exoplanet observation instruments such as the 30-meter telescope (TMT) and space telescopes should cover a wide range of wavelengths from visible light to near-infrared, and should also capture the shift of the red-edge position on red dwarfs toward longer wavelengths in accordance with the evolution of terrestrial vegetation.

Translated with DeepL.com (free version)

Terminology：

Kepler spacecraft：NASA’s exoplanet probe launched in 2009, which discovered more than 2,000 exoplanets, showing that there are as many planets as there are stars.

Habitable zone：A region at a certain distance from a star where radiated energy keeps water in a liquid state on the planet’s surface. Also called habitable zone.

Photosynthetic reaction center：A pigment-protein complex that converts light energy into chemical energy. In oxygen-evolving photosynthesis, a pair drives a series of electron-transfer reactions.

Light-harvesting antenna：A pigment-protein complex located around the reaction center that collects light and transfers energy to the reaction center.

Research Support：

This research was supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Publication：

Title：Red-edge position of habitable exoplanets around M-dwarfs

Journal：Scientific Reports

Authors：Kenji Takizawa, Jun Minagawa, Motohide Tamura, Nobuhiko Kusakabe, Norio Narita

Related Links：

National Astronomical Observatory Press Release
Institute for Basic Biology Press Release

The post Photosynthesis in a different light environment from Earth: New predictions of wavelengths as indicators for life exploration on exoplanets first appeared on Astrobiology Center, NINS.