Seasonal DMSP production dynamics in temperate waters driven by significant contributions from both eukaryotic and prokaryotic autotrophs

The influence of dimethyl sulfide (DMS) on climate is potentially large but highly uncertain. Some of this uncertainty results from the over-simplification of biological drivers of marine DMS production within predictive models. The available models rely on chlorophyll (chl) as the sole biological parameter and often fail to replicate observations, particularly during highly productive events with elevated seawater DMS concentrations. The major precursor for DMS is dimethylsulfoniopropionate (DMSP), an abundant compatible solute produced by many marine eukaryotes and prokaryotes. Despite its importance, knowledge of how, why and by what DMSP is produced is limited. For example, haptophytes and dinoflagellates typically produce 50-100 times more DMSP per unit chl than diatoms and prochlorophytes - yet the relevant enzymes have until recently been poorly characterised, limiting our understanding. Furthermore, although the DMSP synthesis genes and their transcripts are widespread in surface ocean bacterial communities, the contribution by this group of organisms to total DMSP production is so far unquantified.

Here, we explore the diversity and expression of functional DMSP synthesis and lyase genes alongside DMS/P biogeochemical states and rates over a spring-summer time series (March – July 2021) at an established time series station in temperate shelf sea waters, to characterise the biological drivers of DMSP and DMS production.

DMSP concentrations ranged from <5 nmol L^-1 to 160 nmol L^-1, with peaks in mid April and late June. The April peak coincided with significant increases in the transcription of eukaryotic DMSP biosynthesis genes. As the season progressed, the eukaryotic transcripts fell dramatically, whilst an increase in transcription of prokaryotic (cyanobacterial) DMSP biosynthesis genes was observed. DMS concentrations in the spring/summer productive period were characterised by three sharp peaks in early May (27 nmol L^‑1), early June (13 nmol L^-1) and late June (20 nmol L^-1)_,interspersed with lower concentrations of ~2 - 6nmol L^-1. Each peak was associated with distinct prokaryotic community composition. Some relationships were observed between the DMS peaks and the transcription of eukaryotic DMS production genes and prokaryotic DMS degradation genes, demonstrating the fine balance of processes which determine net DMS production in the surface ocean.  Overall, we observed that both eukaryotic and prokaryotic autotrophs significantly contributed to seasonal variation in DMSP production in these temperate waters.