B31B-0293
The SOLAS Bergen Mesocosm Experiment 2008: The Role of the Sea Surface Microlayer in Air-Sea Gas Exchange.
During the UKSOLAS "Joint Bergen Mesocosm Experiment" we investigated the role of microbial processes in the sea surface microlayer (SML) in controlling the production, consumption and sea-to-air fluxes of methane and nitrous oxide during the development of a phytoplankton bloom. In mesocosm waters transferred to a purpose-built sealed gas exchange tank, we measured invasive and evasive exchange fluxes of methane, nitrous oxide and inert sulphur hexafluoride at selected levels of turbulence using a fully automated, coupled gas chromatographic (GC) system. From the measured fluxes we derived estimates of the so-called "apparent" gas transfer velocity, kw, normalised to a Schmidt Number (Sc) of 600 (k600). In the case where neither methane nor nitrous oxide undergoes measurable microbial cycling in the microlayer, the ratios of k600 for either methane or nitrous oxide to k600 for sulphur hexafluoride should equal unity. Significant deviations from unity thus enable us to infer and quantify any microbial modification of gas exchange rates in the microlayer.
B31B-0294
Nitrous Oxide Production and Reduction in the Sea Surface Microlayer: Nitrification or Denitrification?
Oceans are a significant source of atmospheric N2O, but may also act as a sink for both atmospheric and water column-derived N2O, mainly through reduction to N2 by denitrifying bacteria. The sea surface microlayer will play an important role in regulating the exchange of such greenhouse gases between the hydrosphere and the atmosphere, particularly as biological activity and bacterial abundance at this interface are greater than in the bulk water. Despite this, little is known of the role of the sea surface microlayer in ocean-atmosphere trace gas exchange. Here we adopted a stable isotope (15N-enrichment) approach to source partition this N2O between nitrification (ammonia oxidation) and denitrification throughout an artificial phytoplankton bloom. Water was sampled from the Raunefjord, Norway, and 16 ́mol nitrate L-1 and 1 ́mol phosphate L-1 were added to stimulate the bloom. We then applied a further 2 mM N as either 14NH415NO3 or 15NH415NO3 (10 atom percent excess 15N), to partition the measured 15N-N2O and 15N-N2 between nitrification and denitrification over a 7 day period. Following shaking, gases were sampled from the headspace and analysed for the 15N-enrichment of N2O and N2 by mass spectrometry. Here we discuss results in relation to the potential for N2O reduction to N2 within the sea surface microlayer, and the possible role of phytoplankton C in driving this reduction.
B31B-0295
Nitrous Oxide Production by Bacterioneuston
Covering 70% of the Earth, the interface between the ocean and atmosphere, the sea surface microlayer, plays a key role in controlling the exchange of natural and anthropogenic materials. Knowledge of the specific role of the sea surface microlayer and its bacterial population, the bacterioneuston is currently limited. Denitrification is an important route of nitrogen loss from the marine environment. It is also both a sink and source of the greenhouse gas nitrous oxide. Here we investigated the role of denitrifying organisms in the sea-surface microlayer and their potential to lower net emissions of N2O by reducing to N2. Bacterioneuston was sampled from two sites in the Ythan Estuary and in Moray Firth at Cromarty during phytoplankton blooms in Autumn 2007 and Spring 2008 and obtained enrichment cultures from these. Incubation experiments were carried out over a 72 hour period, testing bacterioneuston cultures for N2O production using gas chromatography and nitrate, nitrite and ammonium concentrations. All of the enrichment cultures analysed produced significant quantities of N2O. Results of nitrate, nitrite and ammonium analysis were mixed. Some cultures showed a decrease in nitrate concentration possibly due to nitrate reduction by the bacterioneuston. However, an increase in the nitrate concentration measured with other cultures indicates that N2O production could be due to other nitrogen cycle processes other than denitrification, most likely ammonia oxidation. The results indicate that further research is necessary to establish which nitrogen cycle processes are being carried out the bacterioneuston cultures. Keywords: Denitrification; Bacterioneuston; Sea surface microlayer; Gas Chromatography (GC); Nitrous oxide
B31B-0296
Thermophilic Microorganisms in Sediments at Arctic Marine Mud Volcanoes – Do They Come From the Deep Biosphere?
Thermophilic endospore-forming sulfate-reducing bacteria (SRB) have been detected in the permanently cold Arctic fjord sediments (Svalbard; 79°N). Incubating these sediments at 50°C results in high rates of sulfate reduction that are due to induction of an abundant thermophilic population. Sources for such a relatively high density of thermophiles in cold sediments are unknown, but could include the warm, deep biosphere where growth-supporting habitats for thermophiles may exist. This warm environment could be connected to the cold surface sediments via upward fluid flow within the sea bed. Mud volcanoes regularly transport hot sediment from the deep biosphere to surface areas, and as such may represent transport mechanisms from warm to cold environments. Sediment samples from the Haakon Mosby mud volcano (HMMV) located south of Svalbard in the Barents Sea (72°N, 14°44' E; 1,250 m water depth), were examined to further investigate this possibility. During incubations at 50°C, sulfate reduction rates in Smeerenburgfjorden and HMMV sediments began to increase exponentially following a lag phase, during which sulfate reduction rates were not detected using a radiotracer assay. Assuming that thermophilic SRB in both sediments are similar, the magnitude and nature of their response can be used to estimate their relative abundance in different samples. Using this approach, more thermophilic SRB were detected in Svalbard sediment than in HMMV sediments. The presence of thermophilic SRB at the HMMV supports the idea that mudflow may be a legitimate mechanism for transporting thermophiles from the deep biosphere to cold surface sediments. Further depth and concentric resolution at HMMV may allow a more thorough understanding of how the transport mechanism might work. Considering the relative responses and the distance between the two study sites, thermophilic SRB in Svalbard sediments likely do not come from the HMMV. Local cold seeps around Svalbard share some similarities to HMMV, i.e., they are characterized by geo-excreted fluids and gases. Fluid flow may explain the observation of thermophiles in cold sediments that are not in the center of mud volcanoes.