Methane sources in arctic thermokarst lake sediments on the North Slope of Alaska.

The permafrost on the North Slope of Alaska is densely populated by shallow lakes that result from thermokarst erosion. These lakes release methane (CH4 ) derived from a combination of ancient thermogenic pools and contemporary biogenic production. Despite the potential importance of CH4 as a greenhouse gas, the contribution of biogenic CH4 production in arctic thermokarst lakes in Alaska is not currently well understood. To further advance our knowledge of CH4 dynamics in these lakes, we focused our study on (i) the potential for microbial CH4 production in lake sediments, (ii) the role of sediment geochemistry in controlling biogenic CH4 production, and (iii) the temperature dependence of this process. Sediment cores were collected from one site in Siqlukaq Lake and two sites in Sukok Lake in late October to early November. Analyses of pore water geochemistry, sedimentary organic matter and lipid biomarkers, stable carbon isotopes, results from CH4 production experiments, and copy number of a methanogenic pathway-specific gene (mcrA) indicated the existence of different sources of CH4 in each of the lakes chosen for the study. Analysis of this integrated data set revealed that there is biological CH4 production in Siqlukaq at moderate levels, while the very low levels of CH4 detected in Sukok had a mixed origin, with little to no biological CH4 production. Furthermore, methanogenic archaea exhibited temperature-dependent use of in situ substrates for methanogenesis, and the amount of CH4 produced was directly related to the amount of labile organic matter in the sediments. This study constitutes an important first step in better understanding the actual contribution of biogenic CH4 from thermokarst lakes on the coastal plain of Alaska to the current CH4 budgets.

The occurrence of lysogenic bacteria and microbial aggregates in the lakes of the McMurdo Dry Valleys, Antarctica.

The McMurdo Dry Valleys of Antarctica form the coldest and driest ecosystem on Earth. Within this region there are a number of perennially ice-covered (3-6 m thick) lakes that support active microbial assemblages and have a paucity of metazoans. These lakes receive limited allochthonous input of carbon and nutrients, and primary productivity is limited to only 6 months per year owing to an absence of sunlight during the austral winters. In an effort to establish the role that bacteria and their associated viruses play in carbon and nutrient cycling in these lakes, indigenous bacteria, free bacteriophage, and lysogen abundances were determined. Total bacterial abundances (TDC) ranged from 3.80 x 10(4) to 2.58 x 10(7) cells mL(-1) and virus-like particle (VLP) abundances ranged from 2.26 x 10(5) to 5.56 x 10(7) VLP mL(-1). VLP abundances were significantly correlated (P < 0.05) with TDC, bacterial productivity (TdR), chlorophyll a (Chl a), and soluble reactive phosphorus (SRP). Lysogenic bacteria, determined by induction with mitomycin C, made up between 2.0% and 62.5% of the total population of bacteria when using significant decreases and increases in TDC and VLP abundances, respectively, and 89.5% when using increases in VLP abundances as the sole criterion for a successful induction event. The contribution of viruses released from induced lysogens contributed <0.015% to the total viral production rate. Carbohydrate and protein based organic aggregates were abundant within the water column of the lakes and were heavily colonized by bacteria and VLPs. Alkaline phosphatase activity was detected within the matrix of the aggregates, implying phosphorus deficiency and consortial nutrient exchanges among microorganisms.

Physical, chemical and biological processes in Lake Vostok and other Antarctic subglacial lakes.

Over 70 lakes have now been identified beneath the Antarctic ice sheet. Although water from none of the lakes has been sampled directly, analysis of lake ice frozen (accreted) to the underside of the ice sheet above Lake Vostok, the largest of these lakes, has allowed inferences to be made on lake water chemistry and has revealed small quantities of microbes. These findings suggest that Lake Vostok is an extreme, yet viable, environment for life. All subglacial lakes are subject to high pressure (approximately 350 atmospheres), low temperatures (about -3 degrees C) and permanent darkness. Any microbes present must therefore use chemical sources to power biological processes. Importantly, dissolved oxygen is available at least at the lake surface, from equilibration with air hydrates released from melting basal glacier ice. Microbes found in Lake Vostok's accreted ice are relatively modern, but the probability of ancient lake-floor sediments leads to a possibility of a very old biota at the base of subglacial lakes.

Geomicrobiology of subglacial ice above Lake Vostok, Antarctica.

Data from ice 3590 meters below Vostok Station indicate that the ice was accreted from liquid water associated with Lake Vostok. Microbes were observed at concentrations ranging from 2.8 x 10(3) to 3.6 x 10(4) cells per milliliter; no biological incorporation of selected organic substrates or bicarbonate was detected. Bacterial 16S ribosomal DNA genes revealed low diversity in the gene population. The phylotypes were closely related to extant members of the alpha- and beta-Proteobacteria and the Actinomycetes. Extrapolation of the data from accretion ice to Lake Vostok implies that Lake Vostok may support a microbial population, despite more than 10(6) years of isolation from the atmosphere.

Perennial Antarctic lake ice: an oasis for life in a polar desert.

The permanent ice covers of Antarctic lakes in the McMurdo Dry Valleys develop liquid water inclusions in response to solar heating of internal aeolian-derived sediments. The ice sediment particles serve as nutrient (inorganic and organic)-enriched microzones for the establishment of a physiologically and ecologically complex microbial consortium capable of contemporaneous photosynthesis, nitrogen fixation, and decomposition. The consortium is capable of physically and chemically establishing and modifying a relatively nutrient- and organic matter-enriched microbial "oasis" embedded in the lake ice cover.

Extreme supersaturation of nitrous oxide in a poorly ventilated Antarctic lake.

Lake Bonney, a permanently ice-covered Antarctic lake, has a middepth maximum N2O concentration of 41.6 micromoles N (>580,000% saturation with respect to the global average mixing ratio of N2O) in its east lobe, representing the highest level yet reported for a natural aquatic system. Atmospheric N2O over the lake was 45% above the global average, indicating that this lake is an atmospheric source of N2O. Apparent N2O production (ANP) was correlated with apparent oxygen utilization (AOU), and denitrification was not detectable, implying that nitrification is the primary source for this gas. The slope of a regression of ANP on AOU revealed that potential N2O production per unit of potential O2 consumed in the east lobe of Lake Bonney is at least two orders of magnitude greater than reported for the ocean. The maximum yield ratio for N2O [ANP/(NO2(-) + NO3-)] in Lake Bonney is 26% (i.e. 1 atom of N appears in N2O for every 3.9 atoms appearing in oxidized N), which exceeds previous reports for pelagic systems, being similar to values from reduced sediments. Areal N2O flux from the lake to the atmosphere is >200 times the areal flux reported for oceanic systems; most of this gas apparently enters the atmosphere through a small moat that occupies approximately 3% of the surface of the lake and exists for approximately 10 weeks in summer.

Evidence for bacterial chemotaxis to cyanobacteria from a radioassay technique.

Lyngbya birgei and Aphanizomenon flos-aquae elicited a significant chemotactic attraction of Aeromonas hydrophila compared with controls lacking cyanobacteria. There was a positive exponential relationship between biomass (chlorophyll a) of L. birgei and A. flos-aquae and chemotactic attraction of A. hydrophila. The assay equipment was simple and reliable and could be used to study bacterial chemotaxis in other species in situ.

An in situ technique to measure bacterial chemotaxis in natural aquatic environments.

A simple and reliable technique to study bacterial chemotaxis in natural aquatic environments is reported. This technique uses the test chemicals in known volumes of semi-solid agar media placed in double layered, highly porous, polyester tubes. Following in situ incubation, bacteria attracted by the test chemicals are enumerated with fluorescence microscopy following acridine orange staining. Studies in an eutrophic reservoir showed that significant numbers of bacteria were attracted to D-glucose and glycine; no significant effects were observed with L-serine, sodium succinate, or sodium chloride.

Immunochemical localization of nitrogenase in marine trichodesmium aggregates: relationship to n(2) fixation potential.

Colonial aggregation among nonheterocystous filaments of the planktonic marine cyanobacterium Trichodesmium is known to enhance N(2) fixation, mediated by the O(2)-sensitive enzyme complex nitrogenase. Expression of nitrogenase appears linked to the formation of O(2)-depleted microzones within aggregated bacterium-associated colonies. While this implies a mechanism by which nonheterocystous N(2) fixation can take place in an oxygenated water column, both the location and regulation of the N(2)-fixing apparatus remain unknown. We used an antinitrogenase polyclonal antibody together with postsection immunocolloidal gold staining and transmission electron microscopy to show that (i) virtually all Trichodesmium cells within a colony possessed nitrogenase, (ii) nitrogenase showed no clear intracellular localization, and (iii) certain associated bacteria contained nitrogenase. Our findings emphasize the critical role coloniality plays in regulating nitrogenase expression in nature. We interpret the potential for a large share of Trichodesmium cells to fix N(2) as an opportunistic response to the dynamic nature of the sea state; during quiescent conditions, aggregation and consequent expression of nitrogenase can proceed rapidly.

Influence of physical disruption on growth of attached bacteria.

Attached bacterial populations cultured without an exogenous carbon source or grown in conjunction with attached diatoms incorporated [H]thymidine at a rate between four and five times lower than that of replicate bacterial populations which were dispersed before being assayed.

Stimulation of bacterial DNA synthesis by algal exudates in attached algal-bacterial consortia.

Algal-bacterial consortia attached to polystyrene surfaces were prepared in the laboratory by using the marine diatom Amphora coffeaeformis and the marine bacterium Vibrio proteolytica (the approved name of this bacterium is Vibrio proteolyticus [W. E. C. Moore, E. P. Cato, and L. V. H. Moore, Int. J. Syst. Bacteriol. 35:382-407, 1985]). The organisms were attached to the surfaces at cell densities of approximately 5 x 10 cells cm (diatoms) and 5 x 10 cells cm (bacteria). The algal-bacterial consortia consistently exhibited higher rates of [H]thymidine incorporation than did biofilms composed solely of bacteria. The rates of [H]thymidine incorporation by the algal-bacterial consortia were fourfold greater than the rates of incorporation by monobacterial biofilms 16 h after biofilm formation and were 16-fold greater 70 h after biofilm formation. Extracellular material released from the attached Amphora cells supported rates of bacterial activity (0.8 x 10 to 17.9 x 10 mol of [H]thymidine incorporated cell h) and growth (doubling time, 29.5 to 1.4 days) comparable to values reported for a wide variety of marine and freshwater ecosystems. In the presence of sessile diatom populations, DNA synthesis by attached V. proteolytica cells was light dependent and increased with increasing algal abundance. The metabolic activity of diatoms thus appears to be the rate-limiting process in biofilm development on illuminated surfaces under conditions of low bulk-water dissolved organic carbon.