Doris Slezak

Effects of solar radiation on the fate of dissolved DMSP and conversion to DMS in seawater

Abstract.The effect of ultraviolet radiation (UVR) and photosynthetically-active radiation (PAR) on the conversion of dissolved dimethylsulfoniopropionate (DMSPd) to dimethylsulfide (DMS) was studied in coastal, shelf and open ocean waters. Unfiltered and 0.8 μm filtered seawater samples were incubated in the dark or exposed to solar radiation for ~6 h followed by post-exposure, dark incubations with tracer additions of 35S-DMSPd. End-products resulting from 35S-DMSPd metabolism were quantified, including 35S-DMS, total volatile 35S and particle-assimilated 35S. Exposure of productive coastal and shelf waters of the Gulf of Mexico to UVR+PAR inhibited the initial rates of 35S-DMSPd consumption and the rates of 35S assimilation into cellular macromolecules by 12 to 87% and 13 to 81% respectively, compared to dark controls. After 24 h of post-exposure, dark incubation, however, the assimilation of 35S in the UVR+PAR treatments was the same as observed in dark controls. In contrast, the 35S-DMS yield from DMSPd consumption was always higher in UVR+PAR treatments than in dark controls after 24 h post-exposure, dark incubation. Exposure of mesotrophic Mediterranean Sea or oligotrophic Sargasso Sea water samples to UVR+PAR resulted in variable effects on DMS yields, with two out of four experiments showing lower, and two out of four showing higher DMS yields from 35S-DMSP compared with dark controls. In the Gulf of Mexico and Sargasso Sea, the higher 35S-DMS yields caused by UVR+PAR exposure were offset by strong inhibitory effects of UVR+PAR on 35S-DMSPd consumption rates, leading to lower 35S-DMS production overall. When DMS production from DMSPd was compared to DMS production from total DMSP, we found that only 20 to 75% of the produced DMS came from DMSPd, in one case with the lowest contributions from DMSPd in UVR+PAR treatments. Our results suggest that UVR exposure is likely an important factor promoting higher DMS yields from DMSPd in productive coastal waters, and that a substantial fraction of DMS production comes from non-DMSPd-derived sources.

Distribution and cycling of dimethylsulfide, dimethylsulfoniopropionate, and dimethylsulfoxide during spring and early summer in the Southern Ocean south of New Zealand

Abstract.Concentrations of total dimethylsulfoniopropionate (DMSPt) and its degradation products, dissolved dimethylsulfide (DMS) and dimethylsulfoxide (DMSOd), were measured in surface waters along three transects between 49 to 76°S latitude (November 2003 & 2005 and December 2004) in the New Zealand sector of the Southern Ocean. Most water samples were collected from the ship’s underway pump system, and concentrations of DMSPt, DMS and DMSOd obtained with this method showed excellent agreement with Niskin bottle-collected samples. Dissolved DMSP (DMSPd), on the other hand, was significantly higher in pump samples. Biological consumption rates for DMSPd and DMS were also measured in Niskin-collected surface waters at selected stations. Concentrations of DMSPt (12 to 52 nM) and DMS (0.6 to 3.2 nM) were moderate in open waters north of the seasonal sea ice (north of 63°S), and very low (< 12 nM DMSPt and < 1 nM DMS) where sea ice coverage was >80% (65 – 73°S; November transects). High concentrations of DMSPt (up to 95 nM) and DMS (up to 30 nM), and high DMS:DMSPt ratios, were observed on the northern boundaries of the seasonal sea ice (63 to 68°S) and in the northern Ross Sea (74 – 76°S). Surface water DMSOd concentrations were variable (1 – 55 nM), but generally higher in ice melt zones and the northern Ross Sea, especially in summer (December 2004). Rates of biological DMS and DMSPd consumption were elevated in ice melt zones, but were generally quite low (<1 nMDMSd−1 and<2.5nMDMSPd d−1), except in the Ross Sea polynya where consumption rates reached 7 nM DMS d−1 and 13 nM DMSPd d−1. This data set provides much needed information regarding the distribution and cycling of DMSP, DMS and DMSO in a poorly studied area of the Southern Ocean. Our findings show that areas of ice melt and the northern Ross Sea are zones of elevated DMS concentrations and sea-air fluxes and also rapid cycling of DMSP and DMS.

Chemical “light meters” for photochemical and photobiological studies

Abstract.Nitrate and nitrite solar actinometers or chemical ‘light meters’ were used to quantify light doses in photochemical and photobiological experiments involving dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycling. Light doses were calculated based on the photochemical production of salicylic acid (SA) from benzoic acid in these actinometers, with SA quantified by either spectrofluorometry or high performance liquid chromatography. Nitrate and nitrite actinometers were modified for deployment at low temperatures in Antarctic waters by addition of sodium chloride as a freezing point depressant. The addition of salt did not affect the solar response of the actinometers; however, the solar response did change slightly with latitude. In the Antarctic, peak response wavelengths (and bandwidths) for the Mylar D-wrapped actinometers in quartz tubing were 326 nm (319 – 333 nm) and 353 nm (325 – 380 nm) for nitrate and nitrite, respectively, and these were 2 – 5 nm blue shifted compared to the peak response wavelengths and bandwidths observed in the Sargasso Sea. Excellent agreement was observed when comparing the integrated irradiance determined with the actinometers to that determined with a spectroradiometer. Likewise, diffuse attenuation coefficients for downwelling irradiance (Kd(λ)) calculated from water column actinometer measurements agreed well with Kd(λ) values calculated from irradiance measurements determined with a Biospherical PUV-511 profiling radiometer. Actinometers were used to measure light doses in experiments involving DMS and DMSP transformations during several field campaigns in the Ross Sea, Antarctica and the Sargasso Sea. Based on actinometer measurements, it was determined that DMS photolysis was dependent on UV irradiation between approximately 325 – 380 nm, while biological consumption rates of DMS and DMSP were inhibited by radiation at wavelengths less than approximately 333 nm. When DMS photolysis rate constants were expressed in terms of light dose rather than time, it was possible to 1) directly determine photolysis rate constants in the water column and 2) directly compare photolysis rate constants across diverse oceanographic regions.