Is Mucus Maximising Microlayer Microbes?

The Sea-surface Microlayer (SML) is a gelatinous biofilm-like habitat with a distinct microbial community teaming with bacteria, flagellates, algae and fungi. These communities are significantly 'enriched' compared to the water below due to rising TEPs (Transparent Exopolymer Particles) that supply nutrition and attached microbes to the microlayer habitat. Although TEP is largely formed from phytoplankton DOM, coral mucus may play a similar role. Corals produce vast amounts of mucus, some of which rises to the surface as transparent filaments and strings, due to trapped air bubbles. This could potentially supply both microorganisms and nutrients to the microlayer in waters above reefs. Therefore, it may be that the communities on the SML above reefs could be more enriched with bacteria than other ecosystems and enrichment may increase as coral coverage does.

To test this, Nakajima et al. sampled a fringing coral reef off Bidong Island, Malaysia for three consecutive days in June at three sites. Two shallow sites, one with high coral cover and the other with low coral cover (dominated by Acropora nobilis and A. formosa) and also an offshore site, where I assume coral was absent. The SML was sampled using a metal mesh sampler, which collected the upper 250µm of water and subsurface water samples were taken a depth of 10cm. Water samples were immediately brought to the lab on ice. DOC and Chlorophyll a concentrations were measured. Microbes were enumerated using epi-fluorescence after filtration onto membrane filter, 0.2µm for heterotrophic bacteria and 0.8µm for cyanobacteria, heterotrophic and autotrophic nanoflagellates. Bacterial (heterotrophic) production was the measurement of bromodeoxyuridine incorporation in the dark and bacterial growth rate was calculated. Enrichment factors were calculated for all parameters and multivariate statistics carried out. Unsurprisingly, the SML was significantly enriched over every parameter bar bacterial production and growth rate. More interestingly, enrichment factors for all the microbial groups maybe higher than in other marine habitats. These higher enrichment factors could have implications for the exchange of gases, if matched to an increased metabolism. However, it is difficult to confirm this due to varying methodologies used in different studies. Remarkably, the high coral cover site had the highest enrichment of heterotrophic bacteria. This may support the hypothesis that coral mucus transports trapped pelagic and coral-associated bacteria into the SML. Furthermore, transport into the SML may be more important than in-situ production given a low bacterial growth rate. Additionally, at the high coral sites there was a very low ratio of heterotrophic flagellates to bacteria/cyanobacteria possibly suggesting that there could be a higher level of grazing above the high coral site.

I found the study very interesting, but it can be summarised as being very 'old school', given that there was no focus on taxonomy. A full phylogenetic analysis using 16S/18S rRNA sequences would have allowed direct comparison between the SML communities above the coral with other marine systems and the coral microbes themselves. It would also have been much more informative to sample a number of sites with differing coral coverage to give a clearer picture of whether the enrichment really increased with increasing coverage. Although I am cautious about some of the conclusions made, I think that the study does raise interesting questions and provides a good starting point for future studies where more powerful techniques could be used.  

Ref: Nakajima, R., Tsuchiya, K., Nakatomi, N., Yoshida, T., Tada, Y. Konno, F., Toda, T., Kuwahara, V.S., Hamasaki, K., Othman, B.H.R., Segaran, T.C., Effendy, A.W.M. (2013). Enrichment of microbial abundance in the sea-surface microlayer over a coral reef: implications for biogeochemical cycles in reef ecosystems. Marine Ecology Progress Series, 490, 11-22.