Effects of irradiance and iron limitation on Phaeocystis antarctica growth and DMSP, DMSO, DMS, and acrylate concentration

Phaeocystis antarctica forms reoccurring blooms in the austral spring covering hundreds of km2 and accounting for ~23% of primary production in the Southern Ocean. Coinciding with P. antarctica blooms are high concentrations of dimethylsulfoniopropionate (DMSP), dimethylsulfoxide (DMSO), dimethylsulfide (DMS), and acrylate. I investigated the effects of irradiance and iron concentration on the growth of axenic P. antarctica cultures and on cellular and dissolved concentrations of acrylate, DMSP, DMSO, and DMS. Cellular DMSP concentrations were high (199-403 mmol L-1) and did not vary more than ~20% with irradiance intensity or spectral quality; cellular DMSP did increase up to 35% under iron limitation. Cellular acrylate concentrations were also high, reaching 45 mmol L-1 with high irradiance, representing 0.5-1.8% of the total cellular carbon in P. antarctica. High dissolved acrylate concentrations, up to 38 microM, were also detected, suggesting that acrylate may be actively transported through the cell membrane and/or produced from extracellularly-located DMSP lyase. The high dissolved acrylate could constitute 0.2 to 2.1% of total DOC in the Ross Sea. Cellular and dissolved DMSO concentrations increased on average from 3- to 10-fold, but up to 20-fold in some cases, with increasing irradiance intensity, iron limitation, and growth phase. Exposure of P. antarctica to ultraviolet radiation (UVR, 290-400 nm) resulted in biological weighting functions (BWFs) that were strongly weighted in the UV-B (290-320 nm) for dissolved DMSP and acrylate, but spanned the entire UVR for particulate and dissolved DMSO. Stratospheric ozone depletion could therefore result in increases up to 15% for dissolved DMSP and less than 1% for acrylate and DMSO in the Ross Sea. Overall, increases in DMSO and acrylate under conditions that give rise to increased oxidative stress (e.g., high light intensity, UVR exposure, iron limitation) are consistent with antioxidant activity. However, since cellular concentrations of DMSP and acrylate are quite high in P. antarctica, they are de facto antioxidants and it is unlikely that P. antarctica regulates cellular DMSP or acrylate for this function. The high dissolved acrylate concentrations suggest that DMSP cleavage may also allow for removal of excess sulfur and carbon as part of an overflow mechanism.