Harbouring Oil-Degrading Bacteria is a Potential Mechanism of Adaptation and Survival in Corals

Coral reef systems are very diverse and complex habitats, harbouring a huge diversity of microbes in their tissues, mucus and skeleton; known as a holobiont system. Corals are currently under threat from anthropogenic stressors such as crude oil pollution released from ships, oil tankers and damaged pipelines. Occasionally, oil can also be released naturally through cracks in the seabed. When exposed to oil pollution, the coral secretes excess of mucus to try and slough off the oil, in turn leaving their tissues bare thus more prone to degradation. Abnormalities in reproduction have also been recorded, as well as an increase in mucus cell size which causes them to rupture; resulting in a lack of their most important defence mechanism.

Despite these negative effects of oil on corals, a phenomenon has been found north of the Arabian Gulf surrounding Quaro Island. Corals in this area are said to be some of the healthiest and most developed; yet are subjected to continuous natural oil seepage. This study shows how harbouring hydrocarbon degrading bacteria helps these corals to survive.

Samples were collected from corals near two islands, Quaro Island and Umm Al-Maradim Island. Tissue and mucus samples were taken from Porites compressa and Acropora clathrata from both sites. The tissue and mucus samples were put in microcosms with 2 different quantities of crude oil and incubated. At stages of 1, 4, 8, 12 and 16 weeks, samples were removed for denaturing gradient gel electrophoresis (DGGE). Between and within site comparisons were completed with a non-parametric Mann-Whitney U test and Kruskal Wallis test.

Results showed that the two coral species from both sites had an equivalent number of oil-degrading bacteria in their tissues; but a significantly higher number in their mucus. In Qaro, the higher polluted area, corals produced a greater quantity of mucus in response compared to Umm Al-Maradim, the less polluted area. Coral mucus in Qaro also contained a higher number oil-degraders. Corals inhabiting Umm Al-maradim depend mainly on bacteria in their tissues as mucus production has a very high energy cost, therefore less efficient in the low polluted area. Comparisons within sites found that P. compressa uses sloughing off and oil-degrading bacteria in their tissues whereas A. clathrata depends on only the latter.

During incubation in microcosms it was expected that the number of oil-degraders would decrease over time, however the opposite was observed. Even though oil has toxic effects on bacteria it seemed to stimulate metabolic activities resulting in an increase in tolerant species. Effects were more pronounced when the higher concentration of oil was present. The DGGE results showed that the microbial population shifted to the most effective and tolerant groups; suggesting that corals can adapt to their surrounding environment by selecting the most beneficial bacteria.

A higher diversity of oil-degraders was found in the Qaro corals. It has been suggested that this is due to oil components being degraded by tolerant bacteria into more toxic compounds thus stimulating the growth of bacteria that can utilize them. Ummm Al-Maradim corals receive a lower concentration of oil and therefore not as toxic, so this process doesn’t occur.

In conclusion, this study shows that oil-degrading bacteria inhabit corals where there is a continuous release of crude oil and give them an advantage to survive and adapt. The benefits of hydrocarbon metabolism genes are currently being studied, including the role of viruses transferring the DNA between bacteria.

Al-Dahash, L.M. and Mahmoud, H.M. (2013) Harboring oil-degrading bacteria: A potential mechanism of adaptation and survival in corals inhabiting oil-contaminated reefs. Marine Pollution Bulletin 72: 364-374