Phosphorus Acquisition Strategies Among Phytoplankton and Free-Living Bacterial Communities in the Baltic Proper

Nutrient limitation in the Baltic Proper exhibits temporal variations, with nitrogen limiting diatom and dinoflagellate-dominated spring blooms, while phosphorus constraints characterise the cyanobacterial summer blooms. Phosphorus is a key element for cellular functions and poses significant challenges for planktonic microbial communities under limited availability. Numerous studies have explored 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 planktonic microbial communities acquire phosphorus over a year-long monitoring at an offshore station. Targeting genes related to phosphorus degradation, transport and membrane remodelling, we unveil diverse strategies employed by planktonic microbial communities to acquire phosphorus. Our findings highlight transporter and membrane remodelling-related genes are expressed at high levels across the year, suggesting their important role in coping with phosphorus acquisition. Our dataset reveals distinct strategies between phytoplankton and free-living bacteria 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 physiological adjustments of marine microbial communities to fluctuating phosphorus availability in the Baltic Sea.