Influence of DMSP on bacterial coral assemblages

Dimethylsulfoniopropionate (DMSP) is a common compound produced by phytoplankton, which been has shown to have antioxidant defence functions as well as having a major role in climate regulation. During a pathway, known as the cleavage pathway, DMSP lyases aid in the degradation of DMSP in to dimethylsulfide (DMS), which is a main source of atmospheric sulfur. Another pathway, in which DMSP can be degraded is the demethylation pathway, which involves the demethylation of DMSP into methylmecraptopropionate (MMPA). Coral are known for their symbiotic relationship with symbiodinium These organisms produce high levels of DMSP, which coral bacterial assemblages are able to utilise as their carbon source.

In the paper by Frade, et al. (2016) three reef building coral species (Meandrina meandrites, Porites astreoides and Siderastrea siderea) were sampled at Curacao, Caribbean. Corals were sampled twice a day (dawn and noon) at two different sampling depths (5m and 25). Control samples were taken and immediately flash frozen, whereas treatment samples were also collected and were subject to air exposure for a 3-minute period before being flash frozen. DMSP concentration of each sample was determined by gas phase chromatography and symbiont density was calculated using flow cytometry. A clone library for the three DMSP demethylase gene (dmdA) subclades were created and quantitative Polymerase Chain Reaction (qPCR) was carried out to determine dmdA gene abundance within the bacterial assemblages.

The study showed that coral tissue DMSP concentration increased significantly under air exposure stress and also with increasing symbiont abundance. DMSP concentration varied between host species; S. siderea’s DMSP concentration per symbiont cell was higher than those found in M. meandrites, therefore suggesting that there are differences in the DMSP production of different species. Exposure to air as a stressor led to the DMSP concentration doubling, which shows the importance of coral symbionts in their antioxidant response to a stressor. D/3 subclade of dmdA was the only subclade which did not respond to DMSP concentrations, unlike clades A/2 and C/2.

The findings in this study show the importance of coral associated bacteria due to the extent of the presence of DMSP demethylase genes found within the bacterial population. This shows their ability to utilise DMSP in the demethylation pathway, rather than the cleavage pathway, which would result in increase of DMS production and cloud formation. This is also the first study to link the abundance of the dmdA gene within the bacterial assemblages to changes in the community dynamics it is an important milestone in understanding the importance of coral and their bacterial assemblages, within the microbial ocean processes.

Frade P. R., Schwaninger V., Glasl B., Sintes E., Hill R. W., Simó R., Herndl G. J. (2015) Dimethylsulfoniopropionate in corals and its interrelations with bacterial assemblages in coral surface mucus. Environmental Chemistry 13, 252-265. http://dx.doi.org/10.1071/EN15023