Diatoms are eukaryotic single-celled algae whose outermost layer of cells is a siliceous (mainly silica) cell wall, so diatom cells often form beautiful, regular geometric shapes. According to the structure and shape of the diatom shell, it can be divided into a central order of radiation symmetry and a plume of symmetry on both sides.
Figure 1 Diatoms (image from the Internet)
Diatoms can survive in seawater, freshwater, brackish water and moist soils, especially in the ocean, and are abundant, up to 100,000 species, and are the main components of marine planktonic algae. At the same time, due to its huge biomass in the ocean, it can provide about 20% of the primary productivity of the earth every year, playing an important role in global climate change and elemental circulation (carbon, nitrogen, oxygen, silicon, etc.).
In the ocean, the penetration of visible light will become weaker and weaker as the depth of the sea increases, and the penetration of blue-green light and green light is the strongest. In order to capture and utilize more sunlight energy, planktonic algae have their own abilities, such as cyanobacteria and red algae evolved algal bile to capture green light, while diatoms, dinoflagellates and brown algae evolved unique light-catching antenna systems.
The light-catching antenna of the diatom is the unique fucoxanthin-chlorophyll a/c protein (FCP), whose special pigment composition allows the diatom to capture more blue-green light for photosynthesis when it is underwater. Diatoms are generally reddish-brown, and their FCP light-catching antennas have a large number of special carotenoids such as fucoxanthin, diatoxanthrane, and diatomanthin compared to photosynthetic organisms such as green algae and green plants (Figure 2). In the composition of chlorophyll, diatoms contain chlorophyll a and chlorophyll C, rather than chlorophyll a and chlorophyll b, which are common in green plants.
Figure 2 Green algae and diatoms
Recently, researchers from the Institute of Botany of the Chinese Academy of Sciences have cultured marine diatom cells, isolated their chloroplast cystoid membranes, and purified the photosystem I (PSI) of diatoms and the super pigment protein complex (PSI-FCPIs) of peripheral FCP light-catching antennas, and then analyzed the three-dimensional structure of PSI-FCPIs with 2.38 angstrom resolution by single-particle cryo-EM technology. On October 8, 2020, the results of the study entitled Structural basis for energy transfer in a huge diatom PSI-FCPIsupercomplex were published in the journal Nature Communications.
The researchers found that the reaction center of diatom PSI-FCPI has 12 subunits, surrounded by 24 FCPIs photo-catching antennas. In the core subunits, subunits such as PsaA, PsaB and other eukaryotic photosynthetic organisms have higher conservatism, but the PsaG, PsaH, PsaK, PsaO, and PsaN subunits are lost, and the PsaR and PsaS subunits are newly discovered, which may be involved in stabilizing peripheral FCPI antennas and energy transfer from the FCPI subunit to the PSI core, respectively (Figure 3).
Fig. 3 Three-dimensional structure of PSI-FCPI supercomplex. a, A top view of PSI-FCPI on the stromal side of the thylakoid membrane, with 24 FCPI antenna subunits bound around the periphery; b, a side view of PSI-FCPI, a new PsaS subunit was found on the stromal side.
The PSI of eukaryotic photosynthetic organisms all bind to a certain number of peripheral photocaping antenna proteins, 4 photocaping antenna proteins (Lhca) have been found on the periphery of the PSI reaction center of higher plants to assist in light capture, and up to 10 photocaping antenna proteins (Lhca) binding have been found in green algae. The 24 FCPIs (Lhcr proteins) of the diatom form a three-layer around the outside of the PSI reaction center, the inner layer of 11 FCPI subunits form a closed structure (Figure 4), the 10 subunits of the second layer form a half-circle structure, and the outermost layer is 3 subunits up to 16 nm away from the core. This is currently the single optical system supercomplex with the largest number of integrated light-catching antennas found.
Fig. 4 Photoretch ratio of diatoms to green algae and the light system I of higher plants
The PSI-FCPI structure combines 326 chlorophyll a, 34 chlorophyll c, 102 fucoxanthin, 35 diatomycevins, 18 β-carotene and a large number of electron transporters (Figures 5 and 6), lipids, and water molecules, which are significantly different from the pigment composition of the photosystem I complex of cyanobacteria, red algae, green algae, and higher plants, greatly increasing the light-catching cross-section of diatom PSI-FCPI, which can help diatoms absorb more blue-green light for light reaction. A vast network of delicately designed pigments and intricately arranged energy transfer pathways efficiently transfer captured layers of solar energy to the center of the reaction.
Figure 5 PSI-FCPI is a large pigment network on the stromal side (a) and cystic side (b), respectively, and possible energy delivery pathways
Fig. 6 Chlorophyll a (green), chlorophyll c (blue), fucoxanthin (brown) and diacoxanthin (yellow) bound in Lhcr (a figure) and Lhcf (b figure) type light-catching antenna
In 2019, the research team took the lead in deciphering the 1.8 Å resolution crystal structure of the FCP photo-catching antenna dimer of the phylloidium diaztom-triangular brown finger algae (Science2019, 363:eaav0365), and collaborated with the team of academician Sui Senfang of Tsinghua University to further analyze the cryo-electron microscopy structure of the optical system II-light-catching antenna II complex of Slender Horned Algae (Science2019, 365:eaax0446). At present, the three-dimensional structure of the main pigment protein complex on the photosynthetic membrane of diatoms has been resolved by the research team and its collaborators.
Compared with the previously analyzed optical system I and photocaping antenna complexes of higher plants and green algae, it is found that the light system I of diatoms, combined with such a large number of antenna subunits and pigments (Figures 5 and 6), not only greatly expands the light-catching cross-section of the diatom light system I, but also helps it to fully capture and utilize light energy in the low-light environment under deep water. Some very interesting scientific questions have also been raised: with such a large difference in the light-catching cross-section, are the efficiencies with which diatoms, green algae, and higher plant light systems I capture photons, transmit, and convert light energy are similar? How did the wonderful light-catching antenna structures of evolution come into being? These answers have yet to be further explored by researchers.
Source: Institute of Botany, Chinese Academy of Sciences