Abstract
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 260μmol O2 (mg chla)−1h−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 0.1μM 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.
Highlights
This study has found evidence for a link between the availability of photosynthetically active radiation (PAR) and brominating activity, as measured by phenol red bromination, in two cold water marine diatoms (T. antarctica and P. glacialis)
Given the mode of action of the haloperoxidase enzymes we propose that brominating activity is linked to the breakdown of H2O2 produced during photosynthesis, and has a diel cycle consistent with the mitigation of light-induced stress
Establishing a potential link between brominating activity in marine diatoms and common ecosystem model state variables such as light availability and photosynthetic rate is a first step towards developing the capability to accurately predict spatial and temporal variability in the sea-air emission of organobromines produced by haloperoxidase activity (e.g. CHBr3)
Summary
A wide range of marine organisms are known to be involved in the production of halogenated organic (e.g. CHBr3, CH2I2) and inorganic species (I2, HOI) including bacteria (e.g. Fuse et al, 2003), cyanobacteria (e.g. Smythe-Wright et al, 2006; Hughes et al, 2011), microalgae (e.g. Moore et al, 1996; Hill and Manley, 2009; Hughes et al, 2013), seaweeds (e.g. Goodwin et al, 1997; Manley and Barbero, 2001) and invertebrates (Fielman et al, 1999). Hughes et al (2006) show that halocarbon production is not induced when diatoms are exposed to levels of light which cause high levels of oxidative stress leading to strong and irreparable photo-inhibition (i.e. Fv/Fm b 0.2) These studies provide important process information but they do not necessarily allow us to establish the reasons why diatoms put metabolic investment into driving reactive halogen formation. This is not least because halocarbon formation in seawater is believed to be controlled by both the production of reactive halogens and the availability of organic precursors (Lin and Manley, 2012) which may have external sources. Resolving the physiological and ecological functions of halogenating activity requires studies which probe the biochemical processes that drive this halogen metabolism
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