Phosphorus acquisition among planktonic microbial communities across a salinity gradient.

The Baltic Sea, with its pronounced gradient in temperature, salinity, and nutrient availability, provides a natural system to investigate microbial responses to eutrophication and climate change effects. While nutrient limitation in this system has been extensively studied, direct comparisons of phosphorus acquisition strategies across co-occurring prokaryotic and eukaryotic communities remain limited. This study provides a comprehensive analysis of how microbial plankton communities respond at the molecular level to shifts in phosphorus availability, by resolving both prokaryotic and eukaryotic strategies across spatial and seasonal gradients. By combining size-fractionation, community composition profiling, stoichiometry, and metatranscriptomics along a transect from the Bothnian Bay to the Bothnian Sea across spring and summer conditions, we identify both shared and domain-specific strategies of P acquisition. Results reveal that phosphorus acquisition is shaped by strong environmental gradients, with prokaryotes primarily relying on transporter-mediated uptake, while eukaryotes preferentially employ membrane remodelling and degradation pathways. These contrasting strategies varied across basins and seasons, reflecting differences in nutrient availability, temperature, and community structure. This integrative approach provides new insights into the functional ecology of microbial communities under shifting nutrient regimes and improves our ability to anticipate ecosystem response to eutrophication and climate change in the Baltic Sea.