The photosynthetic strategies of the wild Prochlorococcus marinus

Abstract

Prochlorococcus marinus, the most abundant photosynthetic organism on the planet, can be found stratified throughout the marine water column from the surface waters to the bottom of the euphotic zone. Strains of Prochlorococcus at different depths exhibit diverse responses to light, however, they are notoriously difficult to culture and consequently few of these strains have been studied in vitro. As the past research that has been conducted on Prochlorococcus demonstrates novel and diverse photosynthetic strategies, we have analyzed mass spectrally detected Prochlorococcus peptides, obtained from North Pacific Ocean water samples, to investigate the photosynthetic strategies of wild Prochlorococcus in their natural habitat. We focussed on the major photosynthetic structures: the light harvesting Prochlorophyte Chlorophyll Binding proteins, photosystem II, cytochrome b6f, and photosystem I as well as the rate-limiting enzyme in the Calvin cycle, Rubisco, the NADPH Dehydrogenase complex, plastiquinol terminal oxidase, the ATP synthase complex and the FtsH proteases purportedly involved in photosystem repair. We found that Prochlorococcus in the North Pacific Ocean do not photosynthesize below 200 m and may become heterotrophic under low oxygen and light conditions, suggesting potential for mixotrophy under other conditions. It was further found that some Prochlorococcus in the North Pacific Ocean are deficient in Cytochrome b6f and may therefore be using novel alternative electron transport strategies. We found a strong correlation between Rubisco expression and oxygen, suggesting increased capacity to fix carbon and perform linear electron transport in high oxygen conditions. We further found that lowlight preferring strains have little capacity to repair their photosystems due to deficiency in FtsH proteases. The overall results of the present research show that Prochlorococcus uses ecotype, clade and strain-specific photosynthetic strategies that are associated with not only depth (and consequently light) but oxygen concentrations as well. Therefore, it is proposed that a dichotomous classification considering both depth/light and oxygen preferences of each strain would better reflect the diversity of photosynthetic capacities of individual strains. The results of this research give insight into future directions for the continued study of Prochlorococcus.