Dealing with phosphorus deficiency: contrasting strategies in marine phytoplankton and bacteria.

Phosphorus (P) and nitrogen (N) are essential nutrients for microbial growth, playing crucial roles in regulating the biological productivity of marine ecosystems. Over the last decades, the relatively higher increase in anthropogenic N compared to P inputs is causing a continuous increase in the N:P supply ratio to the global biosphere. The high N:P ratio of riverine discharge may seasonally cause P limitation in estuaries and river-dominated continental shelf waters. We conducted a mesocosm experiment simulating a P-deplete and a P-replete riverine discharge to coastal waters in NW Spain to assess the functional response of marine microplankton using a metatranscriptomic approach. By examining the expression of 40 well-documented genes related to P-metabolism in prokaryotic and eukaryotic gene expression, we uncovered pronounced changes in microbial P-metabolism induced by riverine N:P ratio in this productive system. Remarkably, heterotrophic bacteria and eukaryotic phytoplankton exhibited contrasting phosphate metabolism strategies in response to P deficiency, with the former mostly expressing genes coding for high-affinity transporters and the latter mostly transcribing genes related with low-affinity transporters. Our results also highlight distinct regulatory and adaptive mechanisms across different members of the prokaryotic and eukaryotic communities when exposed to varying P concentrations. Our findings shed light on the broader ecological and functional roles of these genes in nutrient cycling within aquatic ecosystems, with potential application for the design of diagnostic tools for P status in coastal productive systems.