Light and brominating activity in two species of marine diatom

Marine organisms mediate the formation of volatile inorganic (e.g. HOBr) and organic halogens (e.g. CHBr3) and contribute to the sea-to-air emission of bromine and iodine. This air-sea halogen exchange has implications for atmospheric chemistry. It is important to establish the physiological function of halogen metabolism in key groups of marine organisms to permit predictive model development. In this study a series of laboratory experiments was performed to investigate the link between the availability of photosynthetically active radiation (PAR) and brominating activity, as measured by the bromination of phenol red, in two cold-water marine diatoms (Thalassiosira antarctica, CCAP 1085/25; Porosira glacialis, CCMP 668). Brominating activity in T. antarctica was found to change in response to short term changes in photon flux density and to have a strong positive linear relationship with gross photosynthetic rate up to 260 µmol O2 (mg chla)-1 hr-1. Experiments performed across multiple diel cycles showed that light-phase brominating activities in T. antarctica were a factor of 2.8 (±1.0) higher than those measured in the dark. Whilst P. glacialis showed no response to short term changes in PFD, measurements across a number of diel cycles revealed that light-phase brominating activities in this diatom were significantly higher than those in the dark by a factor of 1.3 (±0.3). The addition of 0.1 µM H2O2 to the medium of T. antarctica cultures led to a significant increase in brominating activity by a factor of 2.4 (±0.3) relative to no-addition controls but no such response was seen in P. glacialis. These results suggest that there is a link between PAR light availability and brominating activity in marine diatoms but that the nature of this relationship differs between species. By establishing a potential link with common ecosystem model state variables (light and photosynthesis) this work provides the first step towards developing a predictive capability for brominating activity in the marine environment. More work is needed to assess the potential for developing generalised parameterisations between PAR light availability and brominating activity in diatom species representative of a wider range of ocean regions.