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Concept: Cyanobacteria





Bacterial phototaxis was first recognized over a century ago, but the method by which such small cells can sense the direction of illumination has remained puzzling. The unicellular cyanobacterium Synechocystis sp. PCC 6803 moves with Type IV pili and measures light intensity and color with a range of photoreceptors. Here, we show that individual Synechocystis cells do not respond to a spatiotemporal gradient in light intensity, but rather they directly and accurately sense the position of a light source. We show that directional light sensing is possible because Synechocystis cells act as spherical microlenses, allowing the cell to see a light source and move towards it. A high-resolution image of the light source is focused on the edge of the cell opposite to the source, triggering movement away from the focused spot. Spherical cyanobacteria are probably the world’s smallest and oldest example of a camera eye.

Concepts: DNA, Cyanobacteria, Archaea, Bacteria, Organism, Light, Synechocystis, Pilus


Based on previous comparative genomic analyses, a set of nearly 600 polypeptides, of which ~300 have unknown physiological function, was identified that is present in green algae and flowering and nonflowering plants, but not present (or highly diverged) in non-photosynthetic organisms. The gene encoding one of these GreenCut proteins, CPLD38, is in the same operon as ndhL in most cyanobacteria; NdhL is part of a complex essential for cyanobacterial respiration. A cpld38 mutant of Chlamydomonas reinhardtii did not grow on minimal medium, was high light sensitive under photoheterotrophic conditions, had lower accumulation of photosynthetic complexes, reduced photosynthetic electron flow to P700+, and reduced photochemical efficiency of photosystem II; these phenotypes were rescued by a wild-type copy of CPLD38. Biophysical and biochemical analyses demonstrated that cytochrome b6f function was severely compromised, and levels of transcripts and polypeptide subunits associated with the cytochrome b6f complex were also significantly lower in the mutant; the subunits of the cytochrome b6f complex turned over much more rapidly in mutant than in wild-type cells. Interestingly, PTOX2 and NDA2, two major proteins involved in chlororespiration, were more than 5-fold higher in mutant relative to wild-type cells, suggesting a shift from photosynthesis toward chlororespiratory metabolism in mutant cells, which is supported by experiments that quantify the reduction state of the plastoquinone pool. These findings support the hypothesis that CPLD38 impacts the stability of the cytochrome b6f complex and may play a key role in balancing redox inputs to the quinone pool from photosynthesis and chlororespiration.

Concepts: Cyanobacteria, Photosynthesis, Protein, Gene, Carbon dioxide, Photosystem, Plastocyanin, Plastoquinone


Iron is an essential component in many protein complexes involved in photosynthesis, but environmental iron availability is often low as oxidized forms of iron are insoluble in water. To adjust to low environmental iron levels, cyanobacteria undergo numerous changes to balance their iron budget and mitigate the physiological effects of iron depletion. We investigated changes in key protein abundances and photophysiological parameters in the model cyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803 over a 120 hour time course of iron deprivation. The iron stress induced protein (IsiA) accumulated to high levels within 48 h of the onset of iron deprivation, reaching a molar ratio of ∼42 IsiA : Photosystem I in Synechococcus PCC 7942 and ∼12 IsiA : Photosystem I in Synechocystis PCC 6803. Concomitantly the iron-rich complexes Cytochrome b6f and Photosystem I declined in abundance, leading to a decrease in the Photosystem I : Photosystem II ratio. Chlorophyll fluorescence analyses showed a drop in electron transport per Photosystem II in Synechococcus, but not in Synechocystis after iron depletion. We found no evidence that the accumulated IsiA contributes to light capture by Photosystem II complexes.

Concepts: Cyanobacteria, Photosynthesis, Oxygen, Carbon dioxide, Iron, Photosystem, Light reactions, Plastocyanin


Cyanobacteria-plant symbioses play an important role in many ecosystems due to the fixation of atmospheric nitrogen (N) by the cyanobacterial symbiont. The ubiquitous feather moss Pleurozium schreberi (Brid.) Mitt. is colonized by cyanobacteria in boreal systems with low N deposition. Here, cyanobacteria fix substantial amounts of N2 and represent a potential N source. The feather moss appears to be resistant to decomposition, which could be partly a result of toxins produced by cyanobacteria. To assess how cyanobacteria modulated the toxicity of moss, we measured inhibition of bacterial growth. Moss with varying numbers of cyanobacteria was added to soil bacteria to test the inhibition of their growth using the thymidine incorporation technique. Moss could universally inhibit bacterial growth, but moss toxicity did not increase with N2 fixation rates (numbers of cyanobacteria). Instead, we see evidence for a negative relationship between moss toxicity to bacteria and N2 fixation, which could be related to the ecological mechanisms that govern the cyanobacteria - moss relationship. We conclude that cyanobacteria associated with moss do not contribute to the resistance to decomposition of moss, and from our results emerges the question as to what type of relationship the moss and cyanobacteria share.

Concepts: Cyanobacteria, Oxygen, Bacteria, Evolution, Plant, Symbiosis, Nitrogen, Mosses


Despite our increasingly sophisticated understanding of mechanisms ensuring efficient photosynthesis under laboratory-controlled light conditions, less is known about the regulation of photosynthesis under fluctuating light. This is important because-in nature-photosynthetic organisms experience rapid and extreme changes in sunlight, potentially causing deleterious effects on photosynthetic efficiency and productivity. Here we report that the chloroplast thylakoid lumenal protein MAINTENANCE OF PHOTOSYSTEM II UNDER HIGH LIGHT 2 (MPH2; encoded by At4g02530) is required for growth acclimation of Arabidopsis thaliana plants under controlled photoinhibitory light and fluctuating light environments. Evidence is presented that mph2 mutant light stress susceptibility results from a defect in photosystem II (PSII) repair, and our results are consistent with the hypothesis that MPH2 is involved in disassembling monomeric complexes during regeneration of dimeric functional PSII supercomplexes. Moreover, mph2-and previously characterized PSII repair-defective mutants-exhibited reduced growth under fluctuating light conditions, while PSII photoprotection-impaired mutants did not. These findings suggest that repair is not only required for PSII maintenance under static high-irradiance light conditions but is also a regulatory mechanism facilitating photosynthetic adaptation under fluctuating light environments. This work has implications for improvement of agricultural plant productivity through engineering PSII repair.

Concepts: Cyanobacteria, Photosynthesis, Chloroplast, Plant, Photosystem, Chlorophyll, Thylakoid, Light-dependent reactions



Concepts: Cyanobacteria, Protein, Archaea, Bacteria, Biology, Organism, Eutrophic, Cyanotoxin


Braarudosphaera bigelowii (Prymnesiophyceae) is a coastal coccolithophore with a long fossil record, extending back to the late Cretaceous (ca. 100 Ma). A recent study revealed close phylogenetic relationships between B. bigelowii, Chrysochromulina parkeae (Prymnesiophyceae), and a prymnesiophyte that forms a symbiotic association with the nitrogen-fixing cyanobacterium UCYN-A. In order to further examine these relationships, we conducted transmission electron microscopic and molecular phylogenetic studies of B. bigelowii. TEM studies showed that, in addition to organelles, such as the nucleus, chloroplasts and mitochondria, B. bigelowii contains one or two spheroid bodies with internal lamellae. In the 18S rDNA tree of the Prymnesiophyceae, C. parkeae fell within the B. bigelowii clade, and was close to B. bigelowii Genotype III (99.89% similarity). Plastid 16S rDNA sequences obtained from B. bigelowii were close to the unidentified sequences from the oligotrophic SE Pacific Ocean (e.g. HM133411) (99.86% similarity). Bacterial16S rDNA sequences obtained from B. bigelowii were identical to the UCYN-A sequence AY621693 from Arabian Sea, and fell in the UCYN-A clade. From these results, we suggest that; 1) C. parkeae is the alternate life cycle stage of B. bigelowii sensu stricto or that of a sibling species of B. bigelowii, and 2) the spheroid body of B. bigelowii originated from endosymbiosis of the nitrogen-fixing cyanobacterium UCYN-A.

Concepts: DNA, Cyanobacteria, Photosynthesis, Evolution, Organelle, Phylogenetics, Endosymbiont, Endosymbiotic theory





Complete structural elucidation of natural products is often challenging due to structural complexity and limited availability. This is true for present-day secondary metabolites, but even more for exceptionally preserved secondary metabolites of ancient organisms that potentially provide insights into the evolutionary history of natural products. Here we report the full structure and absolute configuration of the borolithochromes, enigmatic boron-containing pigments from a Jurassic putative red alga, from samples of less than 50 µg using microcryoprobe NMR, CD spectroscopy, and DFT calculations, and reveal their polyke-tide origin. The pigments are identified as spiroborates with two pentacyclic sec-butyl-trihydroxy-methyl-benzo[gh]tetraphen-one ligands and less substituted deriva-tives. The configuration of the sec-butyl group is found to be (S). Because the exceptional benzo[gh]tetraphene scaffold is otherwise only observed in the recently discovered polyke-tide clostrubin from a present-day Clostridium bacterium, the Jurassic borolithochromes now can be unambiguously linked to the modern polyketide, providing evidence that the fossil pigments are almost originally preserved secondary metabolites and suggesting that the pigments in fact may have been produced by an ancient bacterium. The borolithochromes differ fundamentally from previously described boronated polyketides and represent the first boronated aromatic polyketides found so far. Our results demonstrate the potential of microcryoprobe NMR in the analysis of previously little-explored secondary metabolites from ancient organisms and reveal the evolutionary significance of clostrubin-type polyketides.

Concepts: DNA, Cyanobacteria, Photosynthesis, Bacteria, Evolution, Metabolism, Organism, Fungus