Production and cross-feeding of nitrite within populations.

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