Effects of iron limitation and UV radiation on Phaeocystis antarctica growth and dimethylsulfoniopropionate, dimethylsulfoxide and acrylate concentrations

Abstract

Environmental context Low iron concentrations and solar ultraviolet radiation can affect the growth of marine algae. We observed reduced growth and substantial increases in dissolved dimethylsulfoxide and cellular acrylate concentrations in low-iron cultures of a prevalent Southern Ocean algal species, Phaeocystis antarctica, with comparatively small increases observed for cellular dimethylsulfoniopropionate concentrations. Exposure of P. antarctica to high levels of ultraviolet and visible light had very little effect on concentrations of these compounds in culture, even under iron-limitation. Our results highlight the importance of iron to P. antarctica. Abstract Iron is a key nutrient regulating primary production in the Southern Ocean. We investigated the effect of iron limitation with and without exposure to ultraviolet radiation (UVR, 290–400nm) on concentrations of dimethylsulfoniopropionate (DMSP), dimethylsulfoxide (DMSO) and acrylate in axenic batch cultures of Phaeocystis antarctica, a dominant algal species in Antarctic waters. Cellular concentrations of DMSP and acrylate, and cell-number normalised dissolved DMSO concentrations were 1.4-, 11.5- and 6.9-fold higher in iron-limited cultures compared to iron-replete cultures, which we propose resulted from (1) increased reactions of DMSP and dimethylsulfide (DMS) with reactive oxygen species to produce DMSO and (2) increased DMSP cleavage under iron limitation to produce acrylate. Short-term exposure (4h) of iron-limited and iron-replete cultures to a range of photosynthetically active radiation (PAR) and UVR+PAR irradiances did not appreciably affect P. antarctica biomass or total DMSP, DMSO or acrylate concentrations, except at high UVR intensities, suggesting that iron limitation was the primary driver regulating growth and changes in concentrations of these compounds in P. antarctica. High millimolar cellular DMSP and acrylate concentrations under both iron-replete and iron-limited conditions indicated that these two compounds served as de facto antioxidants allowing P. antarctica to thrive under high UVR exposure and low iron concentrations. High dissolved acrylate concentrations indicate significant carbon removal possibly as part of an overflow mechanism during unbalanced growth.