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 (K d (l)) calculated from water column actinometer measurements agreed well with K d (l) 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.

Spectral irradiance dependence of sunlight effects on plankton dimethylsulfide production

“…Field studies using radioisotope additions have shown that solar radiation inhibits DMS consumption, roughly, as much as bacterial leucine incorporation (Toole et al 2006;Kieber et al 2007). Thus, in sunlit incubations, BC rates were calculated as the product of BC dark and the fractional inhibition of bulk bacterial leucine incorporation rates (I LIR ), as BC 5 BC dark 3 I LIR .…”

“…To address this issue we conducted sensitivity tests where two extreme scenarios were considered: no photoinhibition of BC in sunlight, and total photoinhibition of BC in the harshest exposure (T bottles). According to published data, both extreme assumptions are unlikely to happen (Toole et al 2006;Kieber et al 2007). These exercises showed that calculating BC in alternative ways would have little effect on GP rates, and that the change would be generally smaller than the measurement uncertainty.…”

“…At the ecosystem level, sunlight modulates microbial DMSPd utilization , and the relative importance of heterotrophic and autotrophic DMSPd consumers (RuizGonzá lez et al 2012b). On the other hand, DMS sinks are also related to sunlight: UVR inhibits bacterial DMS consumption (Toole et al 2006;Kieber et al 2007) and drives DMS photolysis (Toole et al 2003).…”

Inhibition of Phytoplankton and Bacterial Productivity by Solar Radiation in the Ross Sea Polynya

“…Indeed, UV and PAR exposure in the Ross Sea polynya were not high compared to other observations in Antarctic waters (Kieber et al, 2007;Pakulski et al, 2007). Thus, the assemblages must be particularly sensitive to solar exposure in order for effects to be so pronounced despite moderate exposure levels.…”

Phytoplankton photosynthetic response to solar ultraviolet irradiance in the Ross Sea Polynya: Development and evaluation of a time‐dependent model with limited repair

“…Screen coverage was adjusted over the primary tank for each experiment to approximate average exposure around 1-m depth (based on observed E z ) with no screens during overcast conditions to as many as two layers of nylon window screen during sunny conditions. Effective transmission through the screens was measured using UV actinometers (Kieber et al 2007) and a 4p quantum scalar laboratory sensor (QSL 2100) for PAR and incorporated into transmission factors, a l and a PAR , for each wavelength of UVR and PAR, respectively. The effect of the screens was generally greater in the UV range compared to PAR; for example, one layer of screen resulted in transmission of 64% of broadband UV and 78% of PAR.…”