Photoinhibition in marine picocyanobacteria

Marine Synechococcus and Prochlorococcus cyanobacteria have different antenna compositions although they are genetically near to each other, and different strains thrive in very different illumination conditions. We measured growth and photoinhibition of PSII in two low‐light and one high‐light Prochlorococcus strains and in one Synechococcus strain. All strains were found to be able to shortly utilize moderate or even high light, but the low‐light strains bleached rapidly in moderate light. Measurements of photoinhibition in the presence of the antibiotic lincomycin showed that a low‐light Prochlorococcus strain was more sensitive than a high‐light strain and both were more sensitive than the marine Synechococcus. The action spectrum of photoinhibition showed an increase from blue to ultraviolet wavelengths in all strains, suggesting contribution of manganese absorption to photoinhibition, but blue light caused less photoinhibition in marine cyanobacteria than expected on the basis of earlier results from plants and cyanobacteria. The visible‐light part of the action spectrum resembled the absorption spectrum of the organism, suggesting that photosynthetic antenna pigments, especially divinyl chlorophylls, have a more important role as photoreceptors of visible‐light photoinhibition in marine cyanobacteria than in other photoautotrophs.