Relationships among <i>Prochlorococcus</i> ecotypes across oceanic temperature ranges

Prochlorococcus, the world’s smallest known photosynthetic organism, is an open ocean cyanobacterium, thought to be globally significant in nutrient cycling. Genetically and physiologically distinct “ecotypes” of Prochlorococcus populate the world’s subtropical and tropical oceans. A few of these key ecotypes comprise the majority of these populations, with the dominant ecotypes frequently varying as a function of depth and latitude. The mechanisms underlying the specific distributions of the ecotypes remain poorly understood, but temperature was believed to play a key role in latitudinal partitioning. Quantitative PCR (Q-PCR) was used to assess ecotypic abundances across transects spanning the Pacific Ocean from 27°N to 37°S latitude. We concentrated on an extremely warm region of the Western Pacific Warm Pool (WPWP), the warmest body of open ocean water (+30°C) in the Pacific basin, and on temperatures at transition zones in high north and south latitudes. Our studies indicated that, consistent with prior studies of Atlantic Ocean populations, areas of elevated temperatures are dominated by the eMIT9312 ecotype, and in cooler zones the eMED4 ecotypes prevail as the dominant Prochlorococcus representative. Contributions of other ecotypes varied as a function of latitude and/or depth, consistent with their physiological properties. We also show that the two dominant ecotypes, eMIT9312 and eMED4 co-exist in the surface mixed layer of the oligotrophic waters of the Atlantic and Pacific Oceans in a ratio that changes as a function of temperature. Finally, we developed a novel competition technique under laboratory settings to formally assess fitness of these ecotypes. Our preliminary results confirm that the relative fitness of the eMED4 and eMIT9312 ecotypes vary as a function of temperature.