Production and cross-feeding of nitrite within Prochlorococcus populations
Authors: Berube Paul M. , O’Keefe Tyler , Rasmussen Anna , Chisholm Sallie W. 
Published: January 2022

Prochlorococcus is an abundant photosynthetic bacterium in the oligotrophic open ocean where nitrogen (N) often limits the growth of phytoplankton. Prochlorococcus has evolved into multiple phylogenetic clades of high-light (HL) adapted and low-light (LL) adapted cells. Within these clades, cells encode a variety of N assimilation traits that are differentially distributed among members of the population. Among these traits, nitrate (NO3−) assimilation is generally restricted to a few clades of high-light adapted cells (the HLI, HLII, and HLVI clades) and a single clade of low-light adapted cells (the LLI clade). Most, if not all, cells belonging to the LLI clade have the ability to assimilate nitrite (NO2−), with a subset of this clade capable of assimilating both NO3− and NO2−. Cells belonging to the LLI clade are maximally abundant at the top of the nitracline and near the primary NO2− maximum layer. In some ecosystems, this peak in NO2− concentration may be a consequence of incomplete assimilatory NO3− reduction by phytoplankton. This phenomenon is characterized by a bottleneck in the downstream half of the NO3− assimilation pathway and the concomitant accumulation and release of NO2− by phytoplankton cells. Given the association between LLI Prochlorococcus and the primary NO2− maximum layer, we hypothesized that some Prochlorococcus exhibit incomplete assimilatory NO3− reduction. To assess this, we monitored NO2− accumulation in batch culture for 3 Prochlorococcus strains (MIT0915, MIT0917, and SB) and 2 Synechococcus strains (WH8102 and WH7803) when grown on NO3− as the sole N source. 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 balance assimilated into biomass. We further observed that co-cultures using NO3- as the sole N source could be established for MIT0917 and a Prochlorococcus strain that can assimilate NO2− but not NO3−. In these co-cultures, the NO2− released by MIT0917 was efficiently consumed by its partner strain during balanced exponential growth. 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, NO2−.

SIGNIFICANCE

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. During growth on NO3−, some Prochlorococcus cells release up to 30% of their N uptake as extracellular NO2−. 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.