The role coral reefs may play in the sea surface microlayer

The sea surface microlayer (SML) is important in the exchange of gases between the atmosphere and the oceans, the enrichment of organic compounds in the SML has been shown to suppress gas exchange. The SML is home to a distinct microbial community most studies looking into microorganisms in the SML have been focused in temperate regions with much less conducted in tropical regions before the study by Nakajima et al (2013), there were no studies conducted focusing on the SML in coral reef areas.

In coral reef ecosystems scleractinian corals release a large amount of organic matter which is known as coral mucus. Corals have been found to release up to half of the organic carbon provided by zooxanthellae into the surrounding seawater. The gel like mucus contains gas bubbles which result in positive buoyancy as the mucus travels up the water column it traps various organic particles which allows it to become enriched in microorganisms. The authors looked at the difference in enrichment of microorganisms in the SML compared to SSW over coral reefs, if this microbial abundance and production varied with coral cover and how microbial biomass in the SML of coral reefs compared with other marine ecosystems.

The study investigated a fringing coral reef at Bidong Island, Malaysia samples were collected over the course of 3 days using a metal mesh sampler which collected the upper 250 μm of the water layer. The sites were categorised into higher live coral coverage (HCC) and lower coral coverage (LCC) samples were collected from both the SML (surface microlayer) and SSW (subsurface water).

Findings

With the exception of bacterial production and growth rate enrichment in the SML was significant compared to the SSW for every biological parameter. With increasing coral coverage enrichment factor (EFs) also increased. This higher bacterial abundance in the SML could be due to production of bacteria in the SML or supply of bacteria from the SSW through adhesion to rising bubbles. The result of the paper suggest that it is likely the latter which explains this trend, although bacteria production was slightly higher in the SML the enrichment factor was lower than those for bacterial abundance suggests the SSW is likely the major source of bacteria. A reason why the bacterial growth rate may have been lower in the SML is that bacterial growth may be inhibited in the SML due to higher exposure to UV radiation this would be especially important in the lower latitudes where this coral reef was found. In the HCC sites due to higher coral coverage resulting in a higher release of coral mucus this explains why there is a higher enrichment factor for the HCC sites however the study did not directly measure particulate organic matter which would of allowed finding to be more quantitative.

Heterotrophic nanoflagellates (HNF) were the most enriched microorganisms in the SML showing a significantly higher abundance in the SML. HNF abundances were found to increase exponentially as numbers of bacteria and cyanobacteria increased which suggest that HNF grazed these organisms and therefore increased. The ratio of bacteria/HNF was found to be lower in the SML compared to the SSW indicating that grazing rates are higher in the SML HNF/bacteria and HNF/cyanobacteria rates were also lower at the HCC site suggesting that grazing of flagellates on bacteria and cyanobacteria is enhanced in the SML with high coral coverage.

The microbial composition of the SML is important because higher microbial heterotrophic metabolism in the SML is known to cause a  CO2 saturation which drives CO2 emission at the SML this is important to the findings of this study because the higher microbial abundance in the SML of coral reefs may result in a higher metabolic activity which may potential influence gas exchange between the ocean and the atmosphere furthermore higher coral coverage which results in a higher abundance of microbes may further increase this process. Due to differences in methods used it was hard for the authors to compare the EFs from their study to others one big method difference was that most recent studies use flow cytometry in contrast to the use of microscopy for cell counting in older studies. However comparing results showed that the EFs for the abundance of bacteria cyanobacteria and HNF were considerably higher in the coral reef sites compared to other sea environments.

I found this paper interesting as it looked at the impact coral mucus may have on the MSL and its part in effecting the gas exchange between the ocean and atmosphere, It showed that coral mucus does appear to play a role in influencing the community in the SML through it buoyant behaviour and it source of microbes to the SML. I think further study is needed as this paper only looked at 2 coral sites and an offshore coral site. A disadvantage I found with this paper was I do not think it’s very well structured I found that it tended to repeat itself a number of times and was not very concise.  

Nakajima, R., Tsuchiya, K., Nakatomi, N., Yoshida, T., Tada, Y., Konno, F., Toda, T., Kuwahara, V., Hamasaki, K., Othman, B., Segaran, T. and Effendy, A. (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, pp.11-22.