Microbes Enriched in Seawater after Addition of Coral Mucus

 This study investigated which bacteria in seawater are favoured by the release of freshly detached mucus material into seawater. The mucus from one of the most common Red Sea scleractinian corals (Fungia sp.) was used and amended with enrichment cultures of costal seawater in order to analyse short-term changes in organic C and N and microbial community composition.

Mucus was filtered and the concentration of particulate C and particulate N and stable isotope ratios of C to N were determined. Bacterial cell counts took place and total cell numbers were determined using automated epifluorescence microscopy. Cell numbers in the mucus-seawater mixture collected were slightly higher than the control cell numbers, when the addition of mucus occurred; there was an almost fourfold increase in bacterial abundance. This exponential growth ended within the first 24h, this was probably due to substrate of nutrient limitation. The rapid enrichment of bacterial in the mucus-amended incubations confirms previous observations of enhanced bacterial growth. Enriched bacterial cultures were identified by 16s RNA gene sequence analysis and PCR. Ninety-five distinct genotypes were obtained from 3 samples; more than 90% of the sequences were affiliated with Gammaproteobacteria. Phylogenetic analysis was carried out. All the gamaproteobacterial sequences in the mucus derived clone libraries were affiliated with Alteromonadaceae, Pseudoaltermonas spp., and Vibrio spp, 21 distinct genotypes from these groups were fully sequenced.

Changes in the gammaproteobacterial populations were analysed by FISH, it was found that the addition of mucus to seawater clearly favoured the growth of Gammaproteobacteria. Pseudoalteromonadacae accounted for less than 10% of total counts in mucus-amended incubations and were rare in untreated controls. Alteromonadaceae were the most successful of the studied bacterial populations and accounted for most Gammaproteobacteria. The Vibrionaceae was another successful bacterial group in mucus-amended enrichments. There was a delay in the formation of the largest population which indicates that this group are not able to directly consume organic material in the mucus but profited from ecoenzymatic activity of Alteromonadaceae. Both Alteronmonas ssp. and Vibrio ssp. can inhibit the growth of strains belonging to their own genus and strains belonging to other genus.

In order to further identify the dominant bacterial genotype in the enrichments, a specific probe for FISH was designed. It was found that bacterial genotypes closely related to A. macleodii were among the dominant genotypes enriched by the addition of mucus. The observed changes in cell numbers in the first hours of incubation suggested that the doubling times of these bacteria were extremely short. The close cultivated relative of A. Macleodii exhibited hydrolytic exoenzymatic activities, therefore it should be well equipped to degrade major components of coral mucus and Alteromonas spp. utilizes the resulting monomers. The versatile metabolism of these microorganisms may help exploit rapid changes in the supply of complex substrate source like coral mucus. Bacteria similar to A. Macleodii were present as free living single cells but also formed aggregates, likely on coral mucus particles.

This study didn’t aim to describe the community composition of coral mucus, but tried to asses which bacterial taxa were enriched first on mucus material freshly released into ambient water, regardless of where they originated (mucus or seawater). Genotypes related to A. Macleodii have been detected in coral tissue and mucus, suggesting that they are also common in the bacterium-rich layer of the coral-water interphase. Gammaproteobacterial lineages exhibit high sensitivity to grazing by heterotrophic nanoflagellates. Organic carbon released as mucus might be channelled to higher trophic levels via tight predator-prey interaction.  Overall I think this paper is significant and their aims were achieved.

Allers et al., 2008. 

http://aem.asm.org/content/74/10/3274.full.pdf