The role of diatoms in the production of dimethylsulfide and dimethylsulfoniopropionate: from synthesis to global significance

Diatoms contribute as much as 40% of oceanic primary production and play key roles in the global biogeochemical cycling of carbon, nitrogen, phosphorus, silicate and other elements. The potential importance of diatoms in the sulfur cycle has become increasingly evident. Dimethylsulfoniopropionate (DMSP) is the cellular precursor of the volatile trace gas dimethylsulfide (DMS). DMS oxidises to sulfate aerosols in the atmosphere and these can act as cloud condensation nuclei (CCN) forming clouds. Aerosols and clouds increase global albedo resulting in climate cooling. DMSP is produced by various micro- and macroalgal taxa, yet diatoms have a significantly lower DMSP:C ratio than other microalgal groups. This had led to much of the research in this field focusing on coccolithophores and prymnesiophytes. However, given up-regulations of diatom intracellular DMSP concentrations with changing environmental conditions, and the high global biomass, their contribution to global DMSP and DMS pools may have been underestimated. In this thesis changes to intracellular DMSP concentration of five diatom species were characterised, revealing increases in intracellular DMSP concentration ranging from between 36- and 186-fold under nitrogen-limitation. Stable isotope techniques were used to quantify the first DMSP synthesis rates of 62.56 (± 6.04) mmol L−1 CV d−1 and 74.24 (± 8.17) mmol L−1 CV d−1 for the model diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum respectively, under nitrogen-limited conditions. In the same species, the first measurements of DMSP breakdown rates (71.20 ± 6.49 and 82.27 ± 6.05 mmol L−1 CV d−1), exudation rates (4.30 ± 0.75 and 1.05 ± 0.43 mmol L−1 CV d−1) and DMSP intracellular half-life (26.63 ± 7.99 and 71.45 ± 3.32 hours) in microalgae were made, and underlying biological function of DMSP was assessed by comparing synthesis rates for nitrogen-limited cultures with those experiencing chemically induced oxidative stress by methyl viologen (2.23 ± 0.37 and 0.47 ± 0.07 mmol L−1 CV d−1). To better understand the influence of diatoms on global DMSP and DMS pools, these novel rates were incorporated into the global biogeochemical model PlankTOM10, whilst also making the first comparisons between the PlankTOM10 and the Lana et al., (2011) climatology (LN11). Computations revealed poor agreement between PlankTOM10 and LN11. Changes to diatom DMSP:C ratios with nitrogen limitation were implemented and caused a maximum 4% increase in global annual average surface layer DMS concentration. PlankTOM10 might underestimate diatom contributions to DMSP and DMS pools overall due to its inherent exclusion of coastal regions. It is common for models of this nature to do this to exclude unrealistically high DMS values for coastal regions from oceanic predictions. Diatoms can dominate in these very dynamic regions where nutrients are known to fluctuate on seasonal and much shorter timescales. Overall, this study suggests that in order to improve the modelled contributions of diatoms in the future, and point the way to further worthwhile DMSP and DMS culture and field studies, renewed attention should be paid to coastal and upwelling zones