The Deepwater Horizon oil spill in 2010
showed that there is still an urgent demand for the development and
optimisation of offshore bioremediation techniques. The biggest challenge that
scientists face is to develop a bioremediation technique with a universal
applicability in different geographical locations. Gertler et al. (2012) aimed to determine the
response of indigenous microbial consortia, from different geographical
locations, to a simulated oil spill. They hypothesised that because of global
exchanges of sea water and the relative stable properties of the seawater
system, comparable oil-degrading microbial communities would be present at all
locations despite distinct site characteristics. In fact, some species of
obligate hydrocarbon-degrading bacteria (OHCB) have been linked to hydrocarbon
producing microalgae, providing a natural niche for OHCB, which may explain the
global distribution of these microorganisms.
A standardised mesocosm experimental design
was used with two different treatments, filtered and unfiltered sea water, in
order to investigate the effect of reduced grazing. They used sea water samples
from the Irish (Menai Bridge), North (Helgoland) ad Mediterranean (Messina)
Seas, all differing in meteorological and hydrological parameters as well as in
nutrient availability (N/P ratio). Fifteen millilitres of crude oil and some slow-release
fertiliser were added to each system. Changes in microbial community
composition were assessed by ARISA (Automated Ribosomal Intergenic Spacer
Analysis) and DGGE fingerprinting and 16 S rRNA gene library analysis over a
period of 50 days (30 for Helgoland mesocosms).
Gertler et
al. (2012) found that specific key stone species forming a central and
integral part of the oil-degrading consortia were found at all sites and were
related with Alcanivorax (=alcane
eater) borkumensis. This finding is
in accordance with previous studies that showed the overwhelming prevalence of
microorganisms from the genus Alcanivorax
in oil-degrading microbial consortia in temperate zones. The effective primary
colonisation of oil/water interfaces by Alcanivorax in temperate zones is thought to be due to
multiple alkane hydroxylases, strong mineral nutrient scavenging, biosurfactant
production and biofilm formation capabilities. Apart from this common set of
bacteria, 58 of 64 OTUs detected in the clone libraries were site specific, highlighting
the complexity of microbial ecology in marine oil degradation. These
site-specific microbial consortia were detected at early and late stages of the
experiment, yet fingerprint patterns of all three locations showed intense
convergence during oil degradation, consisting mainly of Alcanivorax.
The authors highlight the complexity of
indigenous marine oil-degrading microbial communities, questioning the
necessity of bioaugmentation practises. Instead they propose that
biostimulation of these readily available, locally adapted OHCB (by stimulating
the growth of Alcanivorax without
hindering the development of location-specific consortia) would be more
beneficial compared with less cost-efficient bioaugmentation approaches.
I think this study illustrates well how
wide-spread OHCB are in the marine environment and that stimulating, rather
than augmenting, these populations may help in the bioremediation of
oil-polluted waters. Although similar studies have been carried out at each
geographical location, lack of standardisation in methodology hindered direct
comparison, Gertler et al. managed to
provide a better insight in the community structures of different geographical
locations, vital to develop universally applicable bioremediation techniques,
designed to suit microbial consortia independently of both location and oil type.
Unfortunately the filtration of sea water had no effect on the
occurrence of marine protozoa (only larger protozoa were excluded), so future
studies should modify their methodology to improve removal of grazers from the
water samples.
Gertler, C., Näther, D. J., Cappello, S., Gerdts, G., Quilliam, R. S., Yakimov, M. M. & Golyshin, P. N. 2012. Composition and dynamics of biostimulated indigenous oil-degrading microbial consortia from the Irish, North and Mediterranean Seas: a mesocosm study. FEMS microbiology ecology, 81, 520–36. Available at: http://www.ncbi.nlm.nih.gov/pubmed/22462472
Hi Anna,
ReplyDeleteAnother great post! It seems we have gathered quite some interest in oil degradation on the blog. Starting back in October with Roybn’s review of the Deepwater Horizon transcriptomic paper by Mason et al. (2012), since then I think we’ve had four other posts and lots of comments on the topic. In February Sophie blogged about a review article by Yakimov et al. (2007) who were interested in obligate oil-degrading bacteria like your current blog, they had reviewed the latest results relating to the biogeography, ecophysiology, genomics and potential for biotechnological applications of obligate hydrocarbonoclastic bacteria (OHCB). I was quite interested to read that by the end of 2006, more than 250 Alcanivorax-affiliated bacteria had been isolated in all types of marine environments. A week or so later Kathryn reviewed another Deepwater Horizon paper where the authors surveyed the succession of deep oil flocs and found a dominance of Colwellia. Then of course it’s your current blog, and now in March we’ve had another one. Most recently Georgia reviewed a study looking at the effects of oil on shoreline communities, Kostka et al. (2012) found that the strains mainly found in sand were: Gammaproteobacteria, including bacteria from genera Alcanivorax, Marinobacter, Pseudomonas, and Acinetobacter, and that (not surprisingly) the presence of oil changed community structure.
I was particularly interested in your blog as it is the only study we’ve discussed which is actually experimental rather than a survey. As you might know, I’m a huge fan of mesocosms and I think they’re a great tool which microbiologists should make more of (especially for climate change studies but that’s another story). Do the authors propose any particularly exciting ideas for how to efficiently simulate the naturally occurring OHCBs? Also, I wondered what affect you think grazers might have?
Thanks,
Vicky
Hi Vicky
ReplyDeletethank you for your comment
It's a shame the experiment on grazing did not quite work out as planned, but the authors still included it as they did get some results after all. Basically, estimations of microbial diversity using Shannon and Simpson indices suggested that in absence of predator microbial diversity declines. In fact, predators may be "killing the winners" and prevent domination of a single species. There is no indication in the paper why OHCBs would be more susceptible to predation than other organisms, so, in my opinion, in an natural environment, we would probably observe the characteristic cyclic curve of a predator-prey relationship. However, grazing on OHCBs is poorly understood and there is still a lot to learn, for instance species of oil-tolerant heterotrophic nanoflagellates were identified to be an important grazer on OHCBs in a previous study by Dalby et al. 2008 (http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2007.01428.x/full). So it is really difficult to predict how predation would affect bioaugmentation attempts and whether predation could negate any benefits of this method.
No exciting ideas were proposed to stimulate growth of local OHCBs. I think the main concept is to provide key limiting nutrients such as hydrocarbons, nitrogen and phosphorus. In this experiment they used a slow-release fertilizer, the provision of which selected for Alcanivorax borkumensis.
I hope this helps
Anna