Effects of elevated temperature and acidification on sulfate assimilation and reduction of microalgae

Increased temperature and acidification are two important environmental factors affecting algal growth in marine ecosystems with the increase of atmospheric CO 2 . The dinoflagellate Amphidinium carterae and the diatom Phaeodactylum tricornutum were chosen to study the effect of warming and acidification on their sulfate assimilation and reduction processes by continuous incubation at different temperatures (15, 20 and 25 °C) and pH NBS values (8.10, 7.80 and 7.60). Variations in associated sulfur compounds, namely sulfate, dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS) and acrylic acid (AA) were observed. The largest sulfate uptake was at 25 °C for A. carterae and at 20 °C for P. tricornutum , however, the optimal growth temperature for both microalgae was 20 °C. The release of DMSP and DMS decreased in A. carterae while they increased in P. tricornutum under the condition of increased temperature. Seawater acidification increased the uptake of sulfate and promoted the growth of the microalgae. Acidification also reduced the release of DMSP, dissolved DMSP (DMSPd), DMS and AA from A. carterae with mean values of 55%, 22%, 9% and 40%, respectively. However, acidification increased the release of DMSP and DMSPd by P. tricornutum with mean values of 44% and 186%, the release of DMS was inhibited (25%) and with no significant difference in the release of AA (2%). Amino acids were found to inhibit the uptake of sulfate by the two microalgae, and the inhibitory effect of cysteine was found to be stronger than that of methionine. The inhibitory effect of amino acids was temperature sensitive and relatively weak at 20 °C. Besides, acidification could enhance the inhibitory effect and was evident in A. carterae . The sulfur metabolism intermediates (cysteine and methionine) have a feedback regulation effect on the sulfate absorption process of algae.