Photosynthetic microorganisms play a crucial role in the marine environment. In vast areas of the oceans, marine primary
productivity is performed by cells smaller than 2-3 µm (picoplankton). A survey of picophytoplankton diversity was carried out by designing general degenerate DNA primers based on well conserved amino acid regions of the D1 protein. D1 encoding gene,
psbA, is found in both cyanobacteria and the chloroplast genomes of algae and higher plants and is widely used as a phylogenetic marker. During this study the existence of a new uncultured
cyanobacterial-like group was observed. Further work combining bacterial artificial chromosome (BAC) libraries and denaturing gradient electrophoresis (DGGE) data analysis has linked
psbA genes attribute this group to marine bacteriophages. A comparison of cyanobacterial and bacteriophages
psbA genes suggests, a strong support for a monophyletic group composed of viral environment PCR amplified, DGGE
psbA sequences and BAC clones, to the exclusion of any cultured cyanobacterial sequences. Based on statistical analysis we hypothesize that all
photosynthetic-gene sequences which cluster in the phage group, belong to mobile genetic element (MGE) assemblages from phages, prophages that affect the evolution of marine cyanobacterial photosynthesis through possible mechanism of lateral gene transfer.A quantitative analysis of DNA assemblages (scaffolds) from Sargasso Sea and GOS shotgun sequencing projects was done based on an amino-acids signature motif for identification of phage-
psbA. This motif was further used to estimate the prevalence of viral genes in the environment and their expression in the marine samples. Surprisingly, the results indicate that viral photosynthetic gene pool composes more than half of the total cyanobacterial
psbA pool, and that viral
psbA genes expressed in the marine environment.The authors therefore concluded that viral D1 genes are actively expressed in the marine environment and represent functional diversity of viral variants. The phage-encoded proteins may play a direct role on bacterial photosynthesis determining the level of photosynthetic productivity in the oceans (oxygen evolution and carbon fixation) and redefining the possible role of phages in the overall biosphere.
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