Are corals cooling the seas?

While searching for papers relating to symbiosis following Colins lecture yesterday I came across a really interesting paper linking symbiosis, the marine sulphur cycle and DMSP production that has strong evidence to suggest that it is not only algae that are responsible for DMSP production.

Dimethylsulphoniopropionate (DMSP) in an integral part of the sulphur cycle within the marine environment, which once metabolized to dimethylsulphide (DMS) is though to induce cloud formation and in turn may help reduce temperatures and also phytoplankton growth as part of a feedback mechanism (although this is still under contention).

Raina et al. have studied the Acropora corals of the indo-Pacific region as they acquire their symbiotic algae (Symbiodinium) from the environment following their larval stage.  Juveniles of two coral species, Acropora millepore and Acropora tenuis were cultured in an algae free environment and the absence of Symbiodinium was confirmed using five DNA markers.  DMSP levels were monitored and despite the total lack of symbionts, the larval corals still produces high levels of DMSP.  The levels detected were in fact greater than those reported for benthic algae found in the same region.  Once the larvae had settled the DMSP levels within the coral continued to increase (by 44% in A. millepore and 54% in A. tennis).

When subjected to thermal stress (32°C) over 6 days both species again showed a marked increase in DMSP levels.  They also exhibited a decrease in levels of the antioxidants acrylate and DMS; presumably this is attributed to their use in ROS detoxification.

To support the findings in the juveniles, adult corals were subjected to thermal stress until 84% of their Symbiodinium had been expelled and the remaining algal cells were all displaying signs of advanced necrosis.  Under these conditions the adult corals contained 68% more DMSP and 36% less acrylate than the control, confirming that the DMSP production in these corals is not just at one particular stage in its life.

Raina et al. then looked at the genetic level for evidence of homologous gene sequences that are found in other species.  These genes correspond to each of the enzymes in the four steps associated with the biosynthesis pathway for DMSP recently discovered within a diatom species.  They found orthologues for two of these genes, the first of which codes for an NADPH reductase enzyme that has many other orthologues in nature.  The second gene found however is thought to be specific to the DMSP pathway, and codes for methyltransferase which is responsible for regulating intracellular DMSP levels and until now has only been found in photosynthetic organisms and two eukaryotes.   The expression of this gene was initially high but decreased following settlement at a time when the coral would normally acquire its Symbiodinium. 

This research shows that the coral itself actually may contribute around half of the DMSP produced and I find it interesting that at a time of stress when the corals may have lost all symbionts they could still be capable of producing DMSP which has been linked to reducing water temperature as part of a feedback mechanism.  This could possibly give the corals time to recover and take on new symbionts that may be more tolerant to an overall increase in temperature. 

Raina, J. B., Tapiolas, D. M., Forêt, S., Lutz, A., Abrego, D., Ceh, J., ... & Motti, C. A. (2013). DMSP biosynthesis by an animal and its role in coral thermal stress response. Nature.