A review of Obligate oil-degrading marine bacteria Current Opinion in Biotechnology, Volume 18, Issue 3, June 2007, Pages 257-266 Michail M Yakimov, Kenneth N Timmis, Peter N Golyshin
In this paper, authors review latest results pertaining to the biogeography, ecophysiology, genomics and potential for biotechnological applications of OHCB.
Hydrocarbon degrading bacteria are found everywhere, both in marine and terrestrial environments. We should not be surprised by this : hydrocarbons and their derivates existing in different forms ( solid, liquid and gaseous fossil carbon deposits, lipids and fatty acids ... ) are ubiquitous in the biosphere and have high carbon content available for biomass.
In contrast to terrestrial hydrocarbon degraders which tend to be metabolically versatile, marine degraders are mostly highly specialized obligate hydrocarbon utilizers, so they are referred as OHCB. Their growth occurs only on substrates containing long-chain alkyl moieties, so they have a highly specialized substrate specifity.
Few OHCB have been isolated from different sites.
Alcanivorax borkumensis has been detected in all types of marine environments: water at all levels and depth, hydrotermal vents and mud vulcanoes, whale carcass, marine invertebrates and algae. This ubiquity is due to its ability to grow on many saturated petroleum fraction constituents and on biogenic hydrocarbon.
Marinobacter species are important marine hydrocarbon degraders that are metabolically more versatile than Alcanivorax, because of the 1500 additional genes that Alcanivorax lacks.
T. oleivorans and Cycloclasticus spp. are also widely distributed, but they have mostly been found in the Northern hemisphere.
Oleiphilus messinensis, initially isolated from harbor sediments ( Messina, Italy ) seems to thrive elsewhere as a sponge symbiont.
In contrast to the cosmopolitan OHCBs discussed above, Oleispira antarctica inhabits only cold waters at high latitudes.
What makes OHCBs so different and let them to take more advantage than other bacteria living in the same environment?
OHCBs exhibit features typical of oligotrophic bacteria.
Remarkably, most of these organisms have an outer cellular membrane enriched for a wide range of transport system for the capture of nutrients ( nitrate especially ) and oligo-elements ( Mg, Mo, Zn, Co ) from the generally nutrient-poor marine environment.
No genes for either passive or active carbohydrate transporters, wich are usually present in other bacteria, were identified in the genome. This observation is consistent with the inability to use monomeric sugars as growth substrates.
When OHCBs show their presence in the environment?
Several microcosms studies have shown that an influx of oil in a marine site causes population densities of OHCB increase up to 90% of the total microbial community, causing a shift in community balance.
A temporal succession of different genera occuring in the bloom has been described : aliphatic hydrocarbon-degraders, in particula Alcanivorax, are the first to bloom and are succeded by microbes such as Cyclocasticus specialized for the remaining compounds that are more difficult to degrade.
Authors point out that, although revealing, such microcosm studies are reductionist and artificial, and they lack of the complexity, diversity and dynamics of natural inputs ( mineral nitrogen and phosphorous ), exports ( cellular metabolites ) and predator grazing.
Further, some studies used tecniques unable to establish the causality between an organism and its physiology/environmental role. For istance, numerous isolates of T. oleivorans obtained from the Polar coastal area of Russia, from the Mediterranean sea and from the North sea, were identical in terms of their small subunit rRNA gene sequences, but exhibit distinct substrate preferences and temperature requirements. This aspect must be considered when developing potential mitigation strategies to combat oil pollution in marine water.
The key role of OHCB in bioremediation processes is well-known, but we must also think about other potential applications of these bacteria.
Recent studies resulted in the identification of novel enzymes : 5 groups of carboxylesterases were biochemically characterised and exhibited good potential for biosynthetic applications.
Moreover, future studies should investigate the role of predators and grazers ( viruses, procariotes, eucariotes ) on the composition, population dynamics and ecophysiological functioning of marine oil-degrading communities.
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