Effects of UV radiation and nitrate limitation on the production of biogenic sulfur compounds by marine phytoplankton

We tested the effects of UV radiation (UVR) and nitrate limitation on the production of dimethylsulfide (DMS), particulate dimethylsulfoniopropionate (DMSPp), and particulate dimethylsulfoxide (DMSOp) in natural seawater from the Gulf of Mexico and in phytoplankton cultures. DMS/Chl a ratios in PAR-only and PAR + UV-exposed seawater were 0.44–2.0 and 0.46–1.9 nmol DMS mg 1 Chl a, respectively, whereas the ratios in cultures of Amphidinium carteraewere 1.0–2.2 nmolmg 1 in PAR-exposed samples and 0.91–2.1 nmolmg 1 in PAR + UV-exposed samples. These results suggested that UVR did not substantially affect DMS/Chl a ratios in seawater and A. carterae culture samples. Similarly, UVR had no significant effect on DMSOp/Chl a in seawater samples (0.83–1.6 nmol DMSO mg 1 Chl a for PAR + UV vs. 0.70–1.5 nmol mg 1 for PAR-exposed seawater samples, respectively) or in A. carterae cultures (0.20–1.3 and 0.19–0.88 nmol DMSO mg 1 Chl a in PAR + UVand PAR-exposed cultures, respectively). In an experiment with the diatom, Thalassiosira oceanica, the culture was grown in high nitrate (30 mM) or low nitrate (6 mM) media and exposed to PAR-only or PAR + UV. The low nitrate, PAR-only samples showed an increase of intracellular dimethylsulfoniopropionate (DMSP) concentration from 2.1 to 15 mmol L 1 in 60 h, but the increase occurred only after cultures reached the stationary phase. Cultures of T. oceanica grown under UVR had lower growth rates than those under PAR-only (m0 = 0.17 and 0.32 d , respectively) and perhaps did not experience severe nitrate limitation even in the low nitrate treatment. These results suggest that the elevated UVR in low nitrate environments could result in reduction of DMSP in some species, whereas DMSP concentrations would not be affected in eutrophic areas. 2008 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: [email protected] (H. Harada).