Global genetic capacity for mixotrophy in marine picocyanobacteria.
Alexis P. Yelton, Silvia G. Acinas, Shinichi Sunagawa, Peer Bork, Carlos Pedrós-Alió, Sallie W. Chisholm

The assimilation of organic nutrients by autotrophs, a form of mixotrophy, has been demonstrated in the globally abundant marine picocyanobacterial genera Prochlorococcus and Synechococcus. However, the range of compounds used and the distribution of organic compound uptake genes within picocyanobacteria is unknown. Here we analyze genomic and metagenomic data from around the world to determine the extent and distribution of mixotrophy in these phototrophs. Analysis of forty-nine Prochlorococcus and eighteen Synechococcus isolate genomes reveals that all have the transporters necessary to take up amino acids, peptides, and sugars. However, the number and type of transporters and associated catabolic genes differ between different phylogenetic groups with low-light IV Prochlorococcus, and 5.1B, 5.2 and 5.3 Synechococcus strains having the largest number. Metagenomic data from sixty-eight stations from the Tara Oceans expedition also indicate that the genetic potential for mixotrophy in picocyanobacteria is globally distributed and differs between clades. Phylogenetic analyses implicate gradual organic nutrient transporter gene loss in the observed clade differences. The phylogenetic differences in genetic capacity for mixotrophy combined with the ubiquity of picocyanobacterial organic compound uptake genes suggests that mixotrophy plays a more central role in their ecology than was previously thought.

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