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 (NO₂^-), with a subset capable of assimilating nitrate (NO₃^-). LLI cells are maximally abundant near the primary NO₂^- maximum layer, an oceanographic feature that may, in part, be due to incomplete assimilatory NO₃^- reduction and subsequent NO₂^- release by phytoplankton. We hypothesized that some Prochlorococcus exhibit incomplete assimilatory NO₃^- reduction and examined NO₂^- accumulation in cultures of three Prochlorococcus strains (MIT0915, MIT0917, and SB) and two Synechococcus strains (WH8102 and WH7803). Only MIT0917 and SB accumulated external NO₂^- during growth on NO₃^-. Approximately 20-30% of the NO₃^- transported into the cell by MIT0917 was released as NO₂^-, with the rest assimilated into biomass. We further observed that co-cultures using NO₃^- as the sole N source could be established for MIT0917 and Prochlorococcus strain MIT1214 that can assimilate NO₂^- but not NO₃^-. In these co-cultures, the NO₂^- 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 NO₂^- during growth on NO₃^-. In the wild, Prochlorococcus populations are composed of multiple functional types, including those that cannot use NO₃^- but can still assimilate NO₂^-. We show that metabolic dependencies arise when Prochlorococcus strains with complementary NO₂^- production and consumption phenotypes are grown together on NO₃^-. These findings demonstrate the potential for emergent metabolic partnerships, possibly modulating ocean nutrient gradients, that are mediated by cross-feeding of N cycle intermediates.