Pronounced seasonal dynamics in transcription of vitamin B1 acquisition strategies diverge among Baltic Sea bacterioplankton

Vitamin B1 (thiamin) is essential to life; yet little is known of the regulation of its availability in marine environments or how it varies seasonally. Since microbes are the key synthesizers of the vitamin in marine environments, we here used metatranscriptomics to examine the seasonal dynamics of B1 acquisition strategies (including both uptake and synthesis pathways) in Baltic Sea bacterioplankton. Elevated B1-related gene expression was observed in summer, coinciding with increased temperatures and bacterial activity and decreased nutrient availability. Different bacterial taxa exhibited distinct B1 acquisition strategies. We identified filamentous Cyanobacteria of the order Nostocales as critical to sustaining B1 production during summer, potentially compensating for limited synthesis in heterotrophic bacteria, especially for 4-amino-5-hydroxymethylpyrimidine (HMP) synthesis. Also, Pelagibacterales accounted for major portions of the community transcription, primarily taking up and salvaging the B1 precursor HMP during summer. This study highlights the partitioning of B1 synthesis, salvage, and uptake among microbial taxa, underscoring that transcriptional activity was more dynamic over time than changes in the genomic potential. We emphasize the influence of environmental conditions on microbial community dynamics and B1 cycling in general, and the potential implications of global change-induced increases in filamentous Cyanobacteria blooms on vitamin food web transfer in particular.