Crustose coralline algae biofilms and recruitment of coral larvae

The larvae of numerous marine invertebrates, including many important reef-building coral species, are known to settle and metamorphose in response to cues from bacterial biofilms. Bacterial biofilms from the surface of crustose coralline algae (CCA) such as Neogoniolithon fosliei are known to play a role in inducing metamorphosis. Webster et al. (2011) carried out a study investigating the effect that a rise in sea surface temperature (SST; projected to be 1.8-4^oC by 2100) would have on CCA N. fosliei and its ability to induce metamorphosis. They collected discs of CCA from Davies Reef, Queensland, and after an initial 2 day acclimatisation period these were exposed to 27, 29, 31 and 32^oC for 7 days and then returned to 27^oC for a further 7 days to see if they could recover. Sediments collected from the same areas were kept under the same exposure conditions. The mean SST from the area of collection is around 27^oC. They also performed coral metamorphosis assays by adding 8-day-old Acropora millepora larvae to a CCA disc from 7 days at each exposure; 24 hours was chosen as the end point for scoring metamorphosis.

It was found that after 7 days the 32^oC treatment had bleached completely with the 31^oC also significantly bleached. This was supported by a reduction in maximum quantum yield (a measurement of photosynthetic ability) of 80 and 53% respectively in comparison to the 27^oC treatment. The 31^oC treatment did improve within the recovery period but the 32^oC treatment could be seen to be growing green endolithic algae so any recovery in quantum yield may be down to this. The 32^oC treatment also induced settlement and metamorphosis in just 58% of corals where all others induced in ~93%. DGGE and phylogenetic analyses showed that none of the sediment samples had considerably different biofilms at any of the temperature treatments, but the 7-day 32^oC CCA treatment had a very different community composition from any of the others. There was a shift from domination of Alphaproteobacteria to Bacteroidetes. Interestingly, the Alphaproteobacteria that was lost at 32^oC had 96% sequence similarity to a symbiont of Montastrea faveolata (a Great Barrier Reef coral species) suggesting that this may explain the loss of colouration of the N. fosliei also. The Bacteroidetes strain that was found to make up 39% of the 7-day 32^oC made just 1.6% of the 2-day 32^oC community and had 99% sequence similarity to a Flavobacterium with anti-oxidative properties which is a major utilizer of high molecular weight DOM (so it may just be feeding on the N. Fosliei). Deltaproteobacteria was found only in the 7-day 32^oC treatment and was found to have 98% sequence similarity to a Desulfovibrio that has previously been associated with black band disease and black patch disease.

The significance of this is clearly that a rise in SST may lead to lower recruitment levels of coral and other invertebrate larvae. As there are a number of possibilities as to what is actually producing the cue for metamorphosis, the next step is to determine exactly which part of the CCA biofilm or the CCA is producing this cue. It was a positive that any species that are induced by sediment biofilms may remain relatively stable, though. It was good that in this paper the authors had used several different methods so that the results they found were verifiable in several different ways.  

Webster, N., Soo, R., Cobb, R. & Negri, A. (2011) Elevated seawater temperature causes a microbial shift on crustose coralline algae with implications for the recruitment of coral larvae. ISME Journal. 5, 759-770
http://www.ncbi.nlm.nih.gov/pubmed/20944682