Differential activity of coexisting Prochlorococcus ecotypes

The population stability and abundance of the unicellular cyanobacterium Prochlorococcus is attributed to its extensive genetic diversity. At least twelve distinct ecotypes have been discovered so far and follow distinct distribution patterns over gradients in the oligotrophic ocean. However, multiple ecotypes always coexist. While genomic analysis suggests that ecotypes contribute differently to biogeochemical cycles, no studies have tested the differential activity of coexisting ecotypes in natural assemblages of Prochlorococcus. We performed DNA stable isotope probing (DNA-SIP) experiments with ¹³C-labeled bicarbonate to identify ecotypes actively assimilating carbon. We identified cells in G1 and G2 phases of the cell cycle to identify ecotypes engaged in cell division. Closely related ecotypes and sub-ecotypes exhibited differential activity in carbon assimilation and cell division. In the case of cell division, actively dividing ecotypes were the most abundant ecotypes. In the case of carbon assimilation, both rare and abundant ecotypes were among the most active. We discuss how intrinsic genomic characteristics, acclimation state, microbial interactions, and physical dynamics of the ocean could contribute to these patterns of activity.