Marvellous Mucus

Many coral reefs are found in nutrient poor zones, but are none-the-less high primary production ecosystems. Mucus secreted by some corals (mucopolysaccharide) carry energy and nutrients to a range of planktonic and benthic consumers, and supports a large microbial growth within the reef system.The conductors of this study decided to investigate which bacteria in seawater are favoured by the release of freshly detached mucus material. Short-term changes in organic carbon and nitrogen, and microbial community composition were simultaneously analysed in mucus enriched seawater.

Coral mucus and seawater were collected from the lagoon of Dahab, in the southern part of the Gulf of Aqaba, northern Red Sea, in May 2004. Polyps of one of the most common Red Sea scleractinian corals (Fungia sp.) with a diameter of 5-10cm were collected from depths of 3-8m, and then exposed to air for 3 minutes in order to trigger mucus production. Mucus produced in the first minute was discarded to reduce contamination of bacteria from the coral surface and mucus produced in the further 2 minutes was aseptically collected and bottled. The coral mucus was homogenised and mixed with seawater (1:10) obtained from the same site within 60 minutes and bottled; bottles of seawater which hadn’t been enriched with mucus were kept for control. All bottles were kept incubated for 50 hours at in situ temperature (24^oC) and light conditions (500-800 µmol quanta m^-2day^-1) at a depth of 1m.

Each bottle was portioned and filtered onto precombusted GF/F filters. The concentrations of particulate organic C (POC) and particulate N (PN) and stable isotope ratios of C to N were determined using dried filters with THERMO NA 2500 elemental analyser coupled to THERMO/Finnigan MAT Delta plus isotope ratio mass spectrometer. The POC and PN concentrations were two- to threefold higher in the mucus amended incubations when compared with the controls.

15ml samples were taken after 0, 2, 4, 6, 10, 26 and 50 hours for bacterial cell counting and fluorescent in situ hybridisation (FISH). Portions were filtered onto polycarbonate membrane filters, after 24h of fixation in buffered paraformaldyhide solution. Total cell numbers were determined by staining with 4’, 6-diamino-2-phenylindole and automated epifluorescence microscopy. The initial cell numbers in the mucus-seawater mixture were marginally higher than the control cell numbers. Additions of mucus lead to an almost fourfold increase in bacterial abundance after 10 hours; whereas the control approximately doubled over the whole 50 hours.

Addition of mucus to the seawater clearly favoured the growth of the Gammaproteobacteria. The relative abundance of these microorganisms initially developed similarly in the mucus and control incubations. Between 4 and 10h, the proportions of Gammaproteobacteria in the mucus incubations rose steeply, to around 60%after 10h and around 70% toward the end of the experiment. The Gammaproteobacteria never exceeded 40% of the total cell counts in the controls.

A review of:

Allers, E., Niesner, C., Wild, C., and Pernthaler, J. (2008). Microbes Enriched in Seawater after Addition of Coral Mucus. Applied and Environmental Biology. Vol. 74 (10). p. 3274-3278