A molecular study of the bacterioneuston and its role in the air-sea exchange of trace gases
The two-way transfer of gases between oceans and air exerts an important control on the atmospheric inventories of climatically active gases such as CO2, CH4 and N20. Numerous physical variables are important to gas transfer at the sea surface and have complex interactions. Because of this, our current knowledge of the gas exchange process remains somewhat limited. Such a situation is urgently in need of attention if the effects of global change into the 21st century are to be adequately addressed. The major rate-limiting step in sea-air gas exchange is slow molecular transport across the so-called "surface microlayer", a region only tens of microns thick at the sea-air interface formed by chemical and microbiological components. This environment is microbiologically distinct from the underlying water, containing enhanced populations of marine bacteria that have only been identified in recent years following advances in molecular identification. In such a microbiologically rich environment, active bio-cycling of gases such as CO2 and CH4 might reasonably be expected to impact their sea-air transfer rates, but this has never previously been demonstrated. We examined directly the microbiological nature of the sea-surface microlayer for the first time and evaluated its potential for modifying the sea-air flux of CH4, using our expertise in molecular microbiology and air-sea gas exchange. Microlayer (neuston) and subsurface (pelagic) seawater samples (North Sea) were collected and subsequent DNA extracts used to construct clone libraries and for subsequent identification of microbiologically active sites specific to CH4-oxidising microbes. The results showed significant differences in the microbial communities of the neuston and pelagic samples, with Vihrio and Pseudoalteromonas spp. dominating the neuston layer. We subsequently examined the potential role methanotrophic bacteria in the sea-air exchange of CH4 in controlled experiments in a laboratory gas exchange tank in which the CH4 compositions of the air and water phases and the methanotroph content of the water could be selected and modified. Our results showed a small but significant enhancement of sea-air CH4 exchange in the presence of methanotrophic bacteria. This suggests that a previously ignored, small bacterial consumption term should be taken account of in sea- air CH4 exchange and that similar "sinks" may apply for other trace gases at the sea surface. If so, current gas exchange models may include errors that could potentially compromise global trace gas budgeting.