Genome-based analysis of biogenic/phytoplankton influence on the global sulphur cycle.

Sulphur-aerosoles might function as condensation nucleators for water vapor and thereby contribute to the production of ocean clouds. The goal of this project has been to investigate how two ecologically important sulfur compounds, dimethylsufoniopropionate (DMSP) and dimethylsulfide (DMS), are produced by unicellular marine algae. We have identified enzymes involved in biosynthesis of DMSP in the diatom Thalassiosira pseudonana. Genome wide transcription analyses have been performed to investigate regulation and changes in cell metabolism under variable conditions and to identify candidate genes. Transcriptional analyses of the effects of silicate on DMSP in cells of T. pseudonana showed that intracellular DMSP levels were induced by silicate starvation. Candidates corresponding to all steps of the biosynthetic steps were identified based on induced expression in silicate depleted cultures. Another study in diatoms resulted in identification of five candidates to the biosynthetic pathway; three of those showed induced gene expression in silicate depleted conditionsl, supporting a role of these in DMSP production. DMSP is processed to DMS by the enzyme DMSP lyase in bacteria. We have identified a gene in the coccolithophore Emiliania huxleyi encoding a protein with similarity to DMSP lyase. The gene was sequenced in two E. huxleyi strains with low and high DMSP lyase activity respectively, and several differences on protein level were found. Gene expression analyses clearly showed that the DMSP lyase gene was expressed at a higher level in the strain with the high DMSP lyase activity. Further biochemical characterization of the DMSP lyase enzyme is needed to confirm that this is the major player in the degradation of DMSP.

Dimethylsulphide (DMS) is a semivolatile organic sulphur compound that accounts for 50-60% of the total natural reduced sulphur flux to the atmosphere. In the atmosphere, DMS is oxidised to acidic sulphur aerosols which can act as condensation nuclei for water vapour, leading to formation of clouds over the ocean. DMS is produced mostly through biogenic processes, mainly through enzymatic cleavage of dimethylsulfoniopropionate (DMSP), a compound that is produced in several groups of marine phytoplankton. DMSP appears to act both as an osmolyte and a cryoprotectant as well as an antioxidant. However, the molecular mechanisms behind biosynthesis, processing, sensing and uptake of DMSP in phytoplankton are unknown. Here, we propose to use functional genomi cs tools in combination with biochemical and molecular biology analyses to identify the molecular components and pathways involved in the cycling of DMSP and DMS in the oceans. We utilise the available genome sequences of three ecologically important phyt oplankton species: the diatom T. pseudonana, the haptophyte E. huxleyi and the prasinophyte M. pusilla. Whole-genome microarrays will be designed for each species. Microarray experiments will be performed based on unique properties for each species with r egard to different aspects of DMSP synthesis, processing and sensing. These experiments will be combined with chromatographic measurements of DMS and DMSP and other physiological data. The resulting datasets will be mined for candidate genes. Selected gen es will be subjected to functional analyses using molecular methods. Purified recombinant protein will be used for biochemical analyses. Results from this project will provide novel knowledge on key processes in the cycling of DMSP and DMS in three impor tant classes of phytoplankton. Implementation of this knowledge into suitable models will improve prediction of DMS production and its effect on the global sulphur cycle and cloud formation in future scenarios.