Production and cross-feeding of nitrite within Prochlorococcus populations.

Prochlorococcus is an abundant photosynthetic bacterium in the open ocean, where nitrogen (N) often limits phytoplankton growth. In the low-light-adapted LLI clade of Prochlorococcus , nearly all cells can assimilate nitrite (NO 2 - ), with a subset capable of assimilating nitrate (NO 3 - ). LLI cells are maximally abundant near the primary NO 2 - maximum layer, an oceanographic feature that may, in part, be due to incomplete assimilatory NO 3 - reduction and subsequent NO 2 - release by phytoplankton. We hypothesized that some Prochlorococcus exhibit incomplete assimilatory NO 3 - reduction and examined NO 2 - accumulation in cultures of three Prochlorococcus strains (MIT0915, MIT0917, and SB) and two Synechococcus strains (WH8102 and WH7803). Only MIT0917 and SB accumulated external NO 2 - during growth on NO 3 - . Approximately 20-30% of the NO 3 - transported into the cell by MIT0917 was released as NO 2 - , with the rest assimilated into biomass. We further observed that co-cultures using NO 3 - as the sole N source could be established for MIT0917 and Prochlorococcus strain MIT1214 that can assimilate NO 2 - but not NO 3 - . In these co-cultures, the NO 2 - released by MIT0917 is efficiently consumed by its partner strain, MIT1214. Our findings highlight the potential for emergent metabolic partnerships that are mediated by the production and consumption of N cycle intermediates within Prochlorococcus populations. IMPORTANCE Earth's biogeochemical cycles are substantially driven by microorganisms and their interactions. Given that N often limits marine photosynthesis, we investigated the potential for N cross-feeding within populations of Prochlorococcus , the numerically dominant photosynthetic cell in the subtropical open ocean. In laboratory cultures, some Prochlorococcus cells release extracellular NO 2 - during growth on NO 3 - . In the wild, Prochlorococcus populations are composed of multiple functional types, including those that cannot use NO 3 - but can still assimilate NO 2 - . We show that metabolic dependencies arise when Prochlorococcus strains with complementary NO 2 - production and consumption phenotypes are grown together on NO 3 - . These findings demonstrate the potential for emergent metabolic partnerships, possibly modulating ocean nutrient gradients, that are mediated by cross-feeding of N cycle intermediates.