Oil pollution as a result of anthropogenic activities is
a major cause of damage to corals and is becoming a worldwide problem. The
presence of crude oil causes sections of the exoskeleton to weaken and break
off of the coral surface, the remaining tissue of which is then degraded. Oil
pollution has also been observed to have detrimental effects on the
zooxanthellae that are endosymbiotic to corals, thus resulting in coral
bleaching. However some corals in the Arabian Gulf, which are constantly
exposed to crude oil released from cracks in the seabed, surprisingly appear to
be in a perfectly healthy state despite these conditions. Al-Dahsah and Mahmoud
(2012) investigated the hypothesis of oil degrading bacteria being present on
coral as an adaptive mechanism to enable the survival of oil in the surrounding
environment.
The sample sites included two locations in the North
Arabian Gulf. Qaro Island is an offshore reef system polluted by oil seeping
from cracks in the seabed and Um Al-Maradim Island in South Kuwait is an unpolluted
site that was used as the control. Coral
and mucus samples were collected from five colonies of the corals Acropora clathrata and Porites compressa at each site. Oil
degrading bacteria was isolated both from fresh samples and from Porites compressa that had been enriched
using a microcosm. These bacteria were tested for growth in a range of
aliphatic and aromatic hydrocarbons and their ability to attenuate crude oil.
DGGE was also used to determine the variation and richness of the bacterial
populations.
They found the mucus and tissue from both sites to
contain bacteria capable of degrading a wide range of aliphatic and aromatic
hydrocarbons and crude oil. Although the control site appears to be clean by
eye, it is in fact exposed to crude oil as a result of being transported by
water currents, wind and waves from polluted regions and therefore appear to
have obtained oil degrading bacteria to enable them to remain in a healthy
state. The predominant bacteria were found to be associated with Gammaproteobacteria, Actinobacteria and
Firmicutes. Increases in the concentration of crude oil caused a shift in
the populations to include more oil degrading bacteria, suggesting that
stressful conditions such as oil exposure may cause corals to change their residential
bacterial communities or even incorporate exogenous bacterial communities
obtained from the surrounding environment. This evidence suggests corals do
have adaptations of oil degrading bacteria that enable them to survive oil
pollution. Additional research investigating the viruses transferring the genes
that enable the degradation of hydrocarbons would be of interest to gain
further knowledge of these adaptations of corals.
Al-Dahash, L.M. and Mahmoud, H.M. (2012). Harboring
oil-degrading bacteria: a potential mechanism of adaptation and survival in
corals inhabiting oil-contaminated reefs. Marine
Pollution Bulletin. Available online at: http://www.sciencedirect.com/science/article/pii/S0025326X12004298
Hi Aimee,
ReplyDeleteVery interesting post! Since mucus and tissue were both measured was there a significant difference between the bacteria present on the surface and within the coral tissue? And did a particular coral species show a greater efficiency for utilizing the oil degrading bacteria?
Thanks,
Kathryn
Hey Kathryn,
ReplyDeleteOverall the mucus samples were shown to have a greater diversity of oil degrading bacteria than the tissue. This was thought to be due to the tissue being a more selective environment, meaning a smaller amount and diversity of bacteria are able to colonise it. In comparison the mucus consists of carbohydrate, protein and lipids, making it an ideal environment with plenty of nutrients for the bacterial communities! However, differences were observed between the sites. The polluted site had a greater amount of oil degrading bacteria in the mucus, whereas the unpolluted site had a greater amount in the tissue. This was probably due to the differences in the quantities of oil present. As mucus is constantly produced it inhibits the settling of oil and thus provides a better defence mechanism, although this is energetically costly. Therefore it is more efficiently for the corals at the unpolluted site to instead harbour greater numbers of oil degrading bacteria in the tissue.
As for the differences in the coral species, a significantly greater number of oil degrading bacteria were observed in the tissue of A. clathrata, whereas there were greater numbers in the mucus of P. compressa. These differences were thought to be due to differences in the properties and mechanisms of adaptations of the coral species, making it difficult to analyse which was the most efficient. P. compressa is in a hemispherical structure and produces mucus in addition to harbouring oil degrading bacteria, whereas A. clathrata is a branched coral and only demonstrated the latter adaptation.
I hope this information helps,
Aimee