The marine silica cycle, tightly intertwined with the carbon and nitrogen cycles, is driven by the activity of biosilicifying microbes in surface waters. These biosilicifiers transform dissolved silica (silicic acid; dSi) into amorphous biogenic silica (bSi) structures. Traditionally, silica cycling and biosilicification research has focused on diatoms, an important biosilicifying phytoplankton group.

However, diverse microbes can biosilicify or utilize dSi in some capacity, though the cellular mechanisms underlying these processes and their roles in silica cycling remain poorly understood. In this thesis, the microbial dynamics of the Baltic Sea silica cycle were explored through isolate and natural community experiments, as well as time series samplings at the Linnaeus Microbial Observatory (LMO) sampling station.

BSi accumulation ability was explored in different picoplankton (<3 μm) groups and the contribution of natural picoplankton communities to bSi stock at LMO was monitored. Brackish and marine picoeukaryotes were found to accumulate bSi, identifying a previously overlooked group as relevant to silica cycling, whereas freshwater strains of picocyanobacterium Synechococcus did not accumulate bSi in comparison to brackish and marine strains, revealing a link between bSi accumulation ability and phylogeny. In natural communities, the picoplankton contribution to bSi stock varied year-round and was proportionally significant in summer. Furthermore, experiments showed that environmental factors like phosphorus can influence bSi accumulation in picoplankton.

Meanwhile, in larger plankton (>3 μm), bSi stock varied strongly over time at LMO, and correlated with changes in diatom carbon biomass, underscoring the dominant role of diatoms in silica cycling dynamics. Seasonal expression dynamics of silicon transporters (SITs), a protein used for active dSi uptake and widely distributed in most eukaryotic groups, including 12 taxonomic classes at LMO, were also investigated. Diatom SIT expression followed clear seasonal patterns while, unexpectedly, silicoflagellate SIT expression exceeded that of diatoms over the sampling period, highlighting the oftentimes hidden dynamics behind silica cycling.

This thesis expands our knowledge of the biological component of the Baltic Sea silica cycle, uncovering its complex microbial community dynamics over the seasons, and emphasizes the importance of investigating diverse organism groups to better understanding silica cycling.