Metabolite diversity among Prochlorococcus strains belonging to divergent ecotypes

The euphotic zone of the surface ocean contains distinct physical-chemical regimes that vary inversely in light and nutrient concentrations as a function of depth. The most numerous phytoplankter of the mid- and low-latitude ocean is the picocyanobacterium Prochlorococcus , which consists of ecologically distinct subpopulations (i.e., "ecotypes"). Ecotypes have different temperature, light and nutrient optima and display distinct relative abundances along gradients of these niche dimensions. As a primary producer, Prochlorococcus fixes and releases organic carbon to neighboring microbes as part of the microbial loop. However, little is known about the specific molecules Prochlorococcus accumulates and releases or how these processes vary among its ecotypes. Here we characterize metabolite diversity of Prochlorococcus by profiling three ecologically-distinct cultured strains: MIT9301, representing a high-light adapted ecotype dominating shallow tropical and sub-tropical waters, MIT0801, representing a low-light adapted ecotype found throughout the euphotic zone and MIT9313, representing a low-light adapted ecotype relatively most abundant at the base of the euphotic zone. In both intracellular and extracellular metabolite profiles, we observe striking differences across strains in the accumulation and release of molecules. Some differences reflect variable genome content across the strains, while others likely reflect variable regulation of genetically-conserved pathways. In the extracellular profiles, we identify molecules that may serve as currencies in Prochlorococcus ' interactions with neighboring microbes and therefore merit further investigation.