Methods

A broad range of methods are applied within the project, ranging from hydroaocustic remote sensing techniques, in situ techniques deployed at the seafloor to state-of-the-art microbiological lab methods. Some selected methods are presented in the following.

Gas bubble emissions from the seafloor cause strong backscattering of sound waves in the water column and can, therefore, be traced by hydroacoustic techniques as described by Nikolovska et al 2008 G-cubed (doi:10.1029/2008GC002118). Gas bubble emissions appear as so-called flares in echosounder records. The figure shows the real-time display of the EM 710 multibeam echosounder. In addition, the horizontally looking sonar of ROV Quest was used for the quantification of gas bubble emissions.

The Gas Bubble Sampler collects gas and transports it under in situ pressure to the sea surface. It is operated by ROV Quest. On board ship, the gas is released under controlled conditions and sub-samples for the chemical and isotopic analyses of gas taken. The image shows how gas hydrate shells for around bubbles collected in the funnel of the Gas Bubble Sampler (Sahling et al 2009 JMPG doi:10.1016/j.marpetgeo.2009.01.010).

ROV-operated in situ payloads (like Microprofiler and Benthic Chamber) are used to investigate the micro-heterogeneity of the chemosynthetic habitats and the underlying biogeochemical processes as well as fluid and gas release rates. Main aims are to determine the spatial scales on which sediment biogeochemistry varies and to investigate if these variations are also reflected in the distribution of bacterial and fauna diversity. Combining the habitat-specific rate measurements with video observation enables us to estimate the spatial distribution of the different habitats providing first data on methane and oxygen budgets of a variety of cold seeps worldwide.

The quantification of microbial carbon fixation and the identification of metabolically active microbial groups in marine sediments has been one of the great challenges in subsurface microbiology. With the dual (D2O and 13CDIC) stable isotope approach we developed a new method which allows screening for total lipid-production and inorganic carbon fixation rates without addition of energy substrates. The D2O approach allows to track lipid formation also in metabolically less active environments. Furthermore the use of lipids allows to discriminate microbial activity of different phylogenetic groups, and it allows to differentiate between autotrophic and heterotrophic carbon fixation. The figure shows an example of the D2O 13CDIC dual stable isotope approach: Total bacterial lipid production and bacterial inorganic carbon fixation within a 8cm core of Swedish fjord sediments. The ratio of two rates indicates the strength of auto- and heterotrophic carbon fixation in the sediment.