Marine isoprene-degrading bacteria

A review of: Alvarez, L.A., Exton, D.A., Timmis, K.N., Suggett, D.J. & McGenity, T.J., (2009), Characterisation of marine isoprene-degrading communities, Environmental Biology, 11(12): 3280-3291

Isoprene is the second most abundant natural hydrocarbon in the atmosphere. It is volatile and can result in the formation of tropospheric ozone, the third most powerful greenhouse gas. Marine algae are known as a significant source of isoprene, whilst some freshwater bacteria are able to degrade the compound. Despite its high energy content, only hints of isoprene degradation in marine and coastal environments have been observed. Significant proportions of gases like methane are oxidised before they can enter the atmosphere and models of marine isoprene flux assume a bacterial sink, however clear evidence is still lacking. This study aims to determine the extent of isoprene degradation in marine environments, whilst identifying the organisms responsible and their relationship with the microalgae.

All samples were taken from various points in an estuary. Isoprene-degrading bacteria obtained from the samples were cultured on agar in isoprene-saturated atmosphere and analysed by DGGE and pyrosequencing of the 16s rRNA genes. Some were also incubated with phytoplankton species to analyse their relationship.

They found that Isoprene concentrations decreased from the head of the estuary to the mouth. They noted that the higher nutrient levels at the head of the estuary elevated microalgal production in the water column which may have led to the higher levels. Isoprene added to the samples of the marine and estuarine water and sediment slurry was significantly degraded, with degradation at least an order of magnitude faster in sediments then water samples and the onset and rate of biodegradation being more rapid at the head of the estuary then at the mouth. Also interestingly, biodegradation was more than 10-fold faster in all samples when isoprene was added at 0.001% than when added at 0.1% v/v, suggesting that degradation is favoured at lower concentrations. It could also be that the high concentrations may have killed the degraders, as isoprene and its degradation products can be toxic.

The degrading bacteria were mainly Actinobacteria, such as Rhodococcus, which were found in all samples and seem to be the main organism responsible for isoprene degradation in coastal samples. Bacteroidetes and Psuedomonas were also present, which were not previously known as isoprene-degraders. Studies generally suggest that Bacteriodetes benefit from the extracellular polymeric substances produced by algae, however this study explains that isoprene is also an important algal product that supports bacterial growth.

They also found that isolated isoprene-degrading bacteria are nutritionally versatile and most degrade n-alkanes for carbon and energy. They explain that isoprene-degrading capacity is widespread in diverse phyla, questioning whether such microbes are specialist isoprene degraders or generalists. In contrast to specialist methane-oxidizing microbes and many coastal microorganisms, these degraders opportunistically utilize a wide range of compounds from the dissolved organic carbon pool. Suggesting that, in the absence of spilled petroleum hydrocarbons, algal production of isoprene could maintain viable populations of hydrocarbon-degrading microbes.

This study confirms that Isoprene-producing algae support the growth of a mixture of isolates which utilise the carbon source and act as marine isoprene-sinks, overall providing a clearer picture of the relationship between consuming bacteria and producing algae and how this affects the isoprene flux and the proportion emitted into the atmosphere.