Production and cross-feeding of nitrite in Prochlorococcus populations

Prochlorococcus is an abundant marine cyanobacterium in the oligotrophic subtropical gyres where nitrogen (N) is often the proximal limiting nutrient. Prochlorococcus genomes encode a variety of N assimilation traits that are differentially distributed among cells in the population. Nitrate (NO₃^-) assimilation is generally restricted to cells belonging to a few high-light adapted clades and the LLI clade of Prochlorococcus. Of the various low-light adapted clades, cells belonging to the LLI clade are the most abundant and typically found at the top of the nitracline near the primary nitrite (NO₂^-) maximum layer. Nearly all LLI cells can assimilate nitrite, but only around 40% can assimilate both nitrate and nitrite. The former must be reduced to the latter, and then to ammonium (NH₄⁺) before assimilation into the amino acid pool. Thus, in these cells, nitrite acts as an intermediate in the pathway. Given that the LLI Prochlorococcus cells are often associated with the primary nitrite maximum, we hypothesized that those that are growing on nitrate may accumulate and release nitrite due to a bottleneck during nitrite reduction to ammonium. Compared to the initial reduction of nitrate to nitrite, the further reduction of nitrite to ammonium requires greater investment of iron and reducing power. Using nitrate as the sole N source, we examined nitrite accumulation in batch culture for 3 Prochlorococcus strains: MIT0915 (LLI clade), MIT0917 (LLI clade), and SB (HLII clade). While MIT0915 did not accumulate nitrite in the bulk culture medium, MIT0917 and SB excreted nitrite at rates of 2.0 and 0.4 fmol nitrite cell^-1 d^-1, respectively, at a light intensity of 50 μmol photons m^-2 s^-1. Approximately 20% of the nitrate transported into the cell by MIT0917 appears to be released as nitrite, with the balance assimilated into biomass. We further observed that stable co-cultures using nitrate as the sole N source could be established for MIT0917 and LLI strains that can assimilate nitrite but not nitrate. In these co-cultures, the nitrite released by MIT0917 is drawn down to undetectable concentrations by its partner strain. Our findings highlight the potential for emergent metabolic partnerships within Prochlorococcus populations that are mediated by the production and consumption of the N cycle intermediate, nitrite.