Historically, primary producers in the Gulf of Bothnia in the Baltic Sea were phosphorus (P) limited, but recent decades have seen elevated P levels, potentially shifting the ecosystem towards nitrogen limitation and increasing nitrogen (N2)-fixing cyanobacteria. Here we explored the diversity of P-acquisition strategies among prokaryotic and eukaryotic communities in relation to environmental conditions to better understand the effects of this shift in inorganic nutrient concentrations. Combining field observations in a north-south transect from the Bothnian Bay (BoB) to the Bothnian Sea (BoS), in spring and summer, size-fractionation, and metatranscriptomics, we found a dynamic relationship between P acquisition strategies and environmental drivers, including temperature and dissolved inorganic phosphorus (DIP). The study revealed basin-specific differences in P availability and acquisition, with the BoB showing lower P levels than the BoS. The microbial communities were adapted to low-P conditions, with prokaryotes regulating transporter expression and eukaryotes adapting through membrane remodelling. Taxon-specific variations in P acquisition were also observed, with Cyanobacteria exhibiting high transporter gene expression in response to low DIP while membrane remodelling genes increased when DIP was higher. These findings provide essential insights into microbial adaptation to P limitation and its connection to eutrophication, informing predictions of future ecosystem changes and sustainable approaches to mitigate nutrient enrichment in the Baltic Sea region. 

Nutrient limitation in the Baltic Proper exhibits temporal variations, with nitrogen limiting diatom and dinoflagellate-dominated spring blooms, while phosphorus constraints characterise the diazotrophic cyanobacterial summer blooms. Phosphorus is a key element for cellular functions including DNA synthesis and membrane formation and poses significant challenges for planktonic microbial communities under limited availability. Numerous studies have explored various strategies phytoplankton and bacteria employ to cope with phosphorus scarcity. However, the temporal dynamics of phosphorus acquisition within natural communities remain poorly understood. Using metatranscriptomics, this study addresses this gap by examining how phytoplankton and free-living bacteria acquire phosphorus over a year-long monitoring at an offshore station in the Baltic Proper. Targeting genes related to phosphorus degradation, transport, and membrane remodelling, we unveil diverse strategies employed by different planktonic microbial communities to acquire phosphorus. Our findings highlight that transporter-related genes are expressed at high levels across the year, suggesting their important role in coping with phosphorus acquisition. Our data also suggests that membrane phospholipids constitute a crucial phosphorus reserve for both free-living bacteria and eukaryotic phytoplankton. Our dataset reveals distinct strategies between these groups under nutrient-limited conditions. While eukaryotic phytoplankton appear to rely more on recycling internal stores of phosphorus via membrane remodelling processes, free-living bacteria appear more prone to optimize extracellular scavenging mechanisms. These insights reveal the complex adaptive responses of marine microbial communities to fluctuating nutrient dynamics in the Baltic Sea.

In the marine environment, the prevailing paradigm is that larger organisms like diatoms are primary contributors to phytoplankton stoichiometry. Numerous studies investigated the stoichiometry of phytoplankton groups or total community but its dynamics among different size-groups are not resolved. In exploring the influence of phytoplankton community composition and succession on seasonal stoichiometry in the Baltic Sea, our study reveals that smaller size-groups, such as nano- and picoplankton, play a more significant role than traditionally thought. During seasonal transitions in nutrient availability—from nutrient-rich spring conditions favouring diatoms and dinoflagellates to nitrogen-limited summer conditions favourable for cyanobacteria—the Baltic Proper exhibits marked shifts in community structure and offers a unique system to investigate stoichiometric dynamics. Our yearly sampling at an offshore station using a size-fraction protocol unveils that the stoichiometry within larger size fractions (>20 µm) does not reflect the overall community's stoichiometry. Instead, nano- and picoplankton dominate nutrient cycling processes despite their smaller size. On any occasion, they represent between 55 and 90% of the biomass making them critical for nitrogen and phosphorus uptake and photosynthetic carbon fixation. These findings challenge the plankton stoichiometry paradigm and highlight the necessity to include these smaller phytoplankton groups into future climate change models to improve predictions regarding ecosystem responses to eutrophication and environmental changes.