Corals and DMS: implications for the future.

Coral-associated Symbiodinium are known to produce dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP); that potentially play a role in the algal antioxidant system and also climate regulation. Corals resistance to bleaching and oxidative stress partly depends upon the capacity of their symbionts to respond to environmental stressors, such as rising temperatures, and DMS and DMSP are shown in this study to contribute to this response.

Production of DMS and DMSP was measured under thermal stress in two clades of Symbiodinium (C1 and D1) that were known to exhibit different thermal tolerances, collected from two Acropora species from the Great Barrier Reef (GBR). Cultures of the Dinoflagellates were kept in axenic conditions (without other living organisms present), which was essential as this allowed only the synthesis of DMS and DMSP by Symbiodinium to be measured. Cultures were kept under control conditions (26°C) and also increased temperatures, which were increased 1°C per day from 26°C to 31°C which fitted with the worldwide bleaching thresholds and severe climate change scenarios.

In the thermally tolerant clade, D1, DMSP consumption was not greatly affected by an increase in temperature. However in clade C1, which is known to be more sensitive to variations in temperature, both DMS and DMSP were consumed at a higher rate under thermal stress. Given that DMS and DMSP are ROS scavengers under oxidative stress, these results suggest the use of sulphur compounds as antioxidants in the response to thermal stress in clade C1.

DMS is a volatile sulphur substance that could take part in a climate feedback that decreases solar radiation and sea surface temperatures (SSTs) by being involved in the formation of cloud condensation nuclei (CCN), leading to increased low-cloud cover over the Great Barrier Reef producing a negative feedback mechanism. However a decrease in DMS under thermal stress in the thermal sensitive clade C1 of Symbiodinium could lead to reduced DMS emissions under predicted rises in SSTs (IPCC, 2007) in the GBR, which is currently clade C dominant. This could eventually result in reduced CCN and have a positive feedback on the regional climate in the GBR. As this study shows that different Symbiodinium clades may have different tolerances to climate change (please not, more research needs to be done to confirm this), it could be that corals have the ability to adapt to increasing SSTs by associating with more thermally tolerant symbionts such as clade D. However D1 has been identified as one of the lowest producers of DMSP among Symbiodinium spp., so reduced DMS emissions might also be expected if the GBR shifts toward a clade D dominance. There is also the recent discovery that coral polyps also have the capacity to produce DMSP and to increase its production under thermal stress so maybe corals have the ability to adapt to climate change more than we think.

I feel that this study gives a platform for new research, to investigate how concentrations of biogenic sulphur compounds may change in the future due to rising temperatures and potentially other climate change factors. Other research may also prevail how climate change may decrease importance and relevance of the CLAW Hypothesis, as previously mentioned. Due to the worldwide importance of corals, it would also be interesting to see if important coral species can adapt to rising sea temperatures by changing their Symbiodinium clades to more thermally tolerant ones to avoid bleaching. 

Reference: Deschaseaux, E. S. M., Beltran, V. H., Jones, G. B., Deseo, M. A., Swan, H. B., Harrison, P. L., & Eyre, B. D. (2014). Comparative response of DMS and DMSP concentrations in Symbiodinium clades C1 and D1 under thermal stress. Journal of Experimental Marine Biology and Ecology, 459, 181-189.

Can be found via: http://www.sciencedirect.com/science/article/pii/S0022098114001373