Photoheterotrophic Microbes in the Arctic Ocean in Summer and Winter

Photoheterotrophic microbes, which are capable of utilizing dissolved organic materials and harvesting light energy, include coccoid cyanobacteria (Synechococcus and Prochlorococcus), aerobic anoxygenic phototrophic (AAP) bacteria, and proteorhodopsin (PR)-containing bacteria. Our knowledge of photoheterotrophic microbes is largely incomplete, especially for high-latitude waters such as the Arctic Ocean, where photoheterotrophs may have special ecological relationships and distinct biogeochemical impacts due to extremes in day length and seasonal ice cover. These microbes were examined by epifluorescence microscopy, flow cytometry, and quantitative PCR (QPCR) assays for PR and a gene diagnostic of AAP bacteria (pufM). The abundance of AAP bacteria and PR-containing bacteria decreased from summer to winter, in parallel with a threefold decrease in the total prokaryotic community. In contrast, the abundance of Synechococcus organisms did not decrease in winter, suggesting that their growth was supported by organic substrates. Results from QPCR assays revealed no substantial shifts in the community structure of AAP bacteria and PR-containing bacteria. However, Arctic PR genes were different from those found at lower latitudes, and surprisingly, they were not similar to those in Antarctic coastal waters. Photoheterotrophic microbes appear to compete successfully with strict heterotrophs during winter darkness below the ice, but AAP bacteria and PR-containing bacteria do not behave as superior competitors during the summer.Photoheterotrophy, which is the ability to utilize organic substrates and to harvest light energy, occurs in a broad range of microbes (14). Phototrophic microbes should be included in models of carbon cycling and food web dynamics, which now typically include only photoautotrophs, which produce organic carbon and oxygen, and heterotrophs, which consume organic matter and oxygen via aerobic respiration (55). Photoheterotrophy is potentially an important competitive adaptation, enabling microbes to survive adverse conditions or to outgrow competitors. Photoheterotrophic microbes include proteorhodopsin (PR)-containing bacteria, aerobic anoxygenic phototrophic (AAP) bacteria, and cyanobacteria.PR is a membrane protein that binds retinal and functions as a light-driven proton pump that can have several physiological functions, including ATP generation (15). The actual role of PR in the environment is uncertain, however. Light enhances the growth of some PR-containing bacteria, such as Dokdonia sp. (17), but has no effect on the growth of others, including Pelagibacter ubique (16) and the SAR92-like strain HTCC2207 (44). Similarly, Campbell et al. (4) found no significant correlation with light intensity for three of four PR gene types examined in the North Atlantic Ocean. Nevertheless, emerging biogeographic patterns of PR genes are providing clues about what controls the distribution and abundance of PR-containing photoheterotrophs in oceanic systems. One of the first oceanic environments to be examined for PR was the coastal waters near Palmer Station, Antarctica (2). Sequence analysis revealed that the Antarctic PRs differed from those isolated from Monterey Bay and surface waters in the central North Pacific (2). In spite of this early report, there has been no work on PR-containing bacteria in Arctic waters. PR-containing bacteria may have unique responses to the continuous summer light, winter darkness, and shading by seasonal ice cover that occur in high-latitude environments.The diversity and abundance of AAP bacteria have been examined by sequencing of the pufM gene (20, 51, 58), which is involved in bacteriochlorophyll (BChl a) synthesis, and by counting of BChl a-fluorescing cells by infrared fluorescence microscopy (14). Enumeration by infrared epifluorescence microscopy indicates that the abundance of AAP bacteria in environments such as the North Pacific Gyre and the Northeast Atlantic Ocean ranges from 1% to 10% (12, 13, 42) and can exceed 10% of the total prokaryotic community in estuaries (41, 50). AAP bacteria have been found in freshwater high-latitude waters (20, 35), but sequence analysis of pufM genes indicates that these AAP bacteria are distinct from those found in marine systems (50). The abundance of AAP bacteria decreases with latitude within the North Atlantic Ocean, from the central gyre to the waters near Greenland (13). Although these photoheterotrophic microbes are still present at 65°N, extrapolation of the trend suggests that AAP bacteria might be absent from the high-latitude waters of the Arctic Ocean.Polar waters appear to be an exception to the otherwise widespread distribution of coccoid cyanobacteria in the world oceans (33, 54). The abundance of Synechococcus and Prochlorococcus decreases with latitude, as exemplified by the 4-orders-of-magnitude decline in abundance between 44°S and 62°S in the South Atlantic Ocean (25). The abundance of Synechococcus also decreases with latitude in the North Atlantic Ocean, between the central gyre and the waters near Greenland, to a low level at 65°N (13). The strong correlation between abundance and temperature (25) suggests that coccoid cyanobacteria are not important at high latitudes, although there are scattered reports of Prochlorococcus in waters as far north as 60°N, near Iceland (27), and of Synechococcus in Antarctic coastal waters (53). However, more data are needed on the abundance of Synechococcus and Prochlorococcus in polar waters such as the Arctic Ocean.The goal of this study was to explore the abundance and diversity of photoheterotrophic microbes in the Arctic Ocean in order to develop a better picture of the biogeographic range of these biogeochemically important microbes and to gain insights into their ecology. Coastal waters of the Chukchi Sea and the Beaufort Sea were sampled in summer at the end of 24-h daylight and in winter following the period of 24-h darkness. The abundances of cyanobacteria, PR-containing bacteria, and AAP bacteria were monitored using flow cytometry, infrared epifluorescence microscopy, and real-time quantitative PCR (QPCR). These data provide a unique perspective on the potential impact of photoheterotrophic microbes on food webs and carbon cycling in this high-latitude aquatic system.     

Indirect Interactions in the High Arctic

Indirect interactions as mediated by higher and lower trophic levels have been advanced as key forces structuring herbivorous arthropod communities around the globe. Here, we present a first quantification of the interaction structure of a herbivore-centered food web from the High Arctic. Targeting the Lepidoptera of Northeast Greenland, we introduce generalized overlap indices as a novel tool for comparing different types of indirect interactions. First, we quantify the scope for top-down-up interactions as the probability that a herbivore attacking plant species i itself fed as a larva on species j. Second, we gauge this herbivore overlap against the potential for bottom-up-down interactions, quantified as the probability that a parasitoid attacking herbivore species i itself developed as a larva on species j. Third, we assess the impact of interactions with other food web modules, by extending the core web around the key herbivore Sympistis nigrita to other predator guilds (birds and spiders). We find the host specificity of both herbivores and parasitoids to be variable, with broad generalists occurring in both trophic layers. Indirect links through shared resources and through shared natural enemies both emerge as forces with a potential for shaping the herbivore community. The structure of the host-parasitoid submodule of the food web suggests scope for classic apparent competition. Yet, based on predation experiments, we estimate that birds kill as many (8%) larvae of S. nigrita as do parasitoids (8%), and that spiders kill many more (38%). Interactions between these predator guilds may result in further complexities. Our results caution against broad generalizations from studies of limited food web modules, and show the potential for interactions within and between guilds of extended webs. They also add a data point from the northernmost insect communities on Earth, and describe the baseline structure of a food web facing imminent climate change.     

Reduction of Mercury Chloride by Chlorella: Evidence for a Reducing Factor

Evidence is brought for the presence of low molecular weight, heat stable, mercury reducing factors in Chlorella cells. Some of the properties of these factors, which appears to be normal metabolities, are described. These factors are also present in the medium in which the cells grow. The reduced mercury, Hg⁰, is volatilized from the culture medium more rapidly than Hg⁺². The resultant decrease in the Hg concentration appears to be the main reason for the recovery of the cells from Hg stress. No resistance to Hg developed in cells emerging from the stress.     

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (~74–76°N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68–95% to chironomid diet at a polar desert site and 70–78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0–32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not support chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study provides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.     

Manganese Reduction by Microbes from Oxic Regions of the Lake Vanda (Antarctica) Water Column

Depth profiles of metals in Lake Vanda, a permanently ice-covered, stratified Antarctic lake, suggest the importance of particulate manganese oxides in the scavenging, transport, and release of metals. Since manganese oxides can be solubilized by manganese-reducing bacteria, microbially mediated manganese reduction was investigated in Lake Vanda. Microbes concentrated from oxic regions of the water column, encompassing a peak of soluble manganese [Mn(II)], reduced synthetic manganese oxides (MnO₂) when incubated aerobically. Pure cultures of manganese-reducing bacteria were readily isolated from waters collected near the oxic Mn(II) peak. Based on phylogenetic analysis of the 16S rRNA gene sequence, most of the isolated manganese reducers belong to the genus Carnobacterium. Cultures of a phylogenetically representative strain of Carnobacterium reduced synthetic MnO₂ in the presence of sodium azide, as was seen in field assays. Unlike anaerobes that utilize manganese oxides as terminal electron acceptors in respiration, isolates of the genus Carnobacterium reduced Mn(IV) via a diffusible compound under oxic conditions. The release of adsorbed trace metals accompanying the solubilization of manganese oxides may provide populations of Carnobacterium with a source of nutrients in this extremely oligotrophic environment.     

Nitrogen fixation by three species of leguminosae in the Canadian High Arctic Tundra

Abstract. Significant levels of nitrogenase activity (nitrogen fixation) were demonstrated in three species of Arctic legumes ( Oxytropis maydelliana, O. arctobia and Astragalus alpinus ) growing in high tundra at Sarcpa Lake, Melville Peninsula, N.W.T. Nitrogenase activity of intact plants was correlated with the number of nodules per plant, with field soil temperatures and limited by water shortage. Activity in freshly detached nodules showed a plateau of maximum activity between 10°C and 25°C and a near linear decline with temperature down to 0°C. Unusually, the segmented nodules of all three species are perennial in which growth and leghaemoglobin production resumes each spring from an overwintering apical meristem. Nodules are most numerous in the warmer soil stratum (2–10 cm. depth). Other studies indicate that the arctic rhizobia belong to a single cold-adapted species which has co-evolved with the legumes of tundra.     

Small-scale diel vertical migration of zooplankton in the High Arctic

Diel vertical migration (DVM) of zooplankton is considered less prominent at high latitudes where diel changes in irradiance are minimal during periods of midnight sun and polar night, leaving zooplankton without a temporal refuge and thus eliminating a key advantage of DVM. One of the shortcomings of previous DVM studies of zooplankton based on net sampling is that the depth resolution often has been too coarse to detect vertical migrations over short distances. We investigated DVM of zooplankton during August 2010 in drifting sea ice northeast of Svalbard (~81.5°N, ~30.5°E). Classical DVM behaviour (midnight rising, midday sinking) was observed between 20 and 80 m in young copepodite stages (CI–III) of Calanus finmarchicus and Calanus glacialis. The copepods Microcalanus spp., Pseudocalanus spp., Oithona atlantica, Oithona similis and Triconia borealis, alongside Eukrohnia hamata, Limacina helicina, and Fritillaria spp., all displayed signs of DVM. We conclude that zooplankton exhibit DVM in ice-covered waters over rather short distances to optimise food intake in the presence of predators.     

Bioremediation Assessment of Hydrocarbon-Contaminated Soils from the High Arctic

The bioremediation potential of hydrocarbon-contaminated soils from the most northerly inhabited station in the world, Canadian Forces Station - Alert, was assessed. Microbial enumeration, by both viable plate counts and direct counts, combined with molecular analysis (polymerase chain reaction and colony hybridization) for hydrocarbon catabolic genes (alkB, ndoB, xylE), demonstrated the presence of significant numbers of cold-adapted hydrocarbon-degrading microorganisms. The degradative activity of these populations was assessed by mineralization of ¹⁴Clabeled hexadecane (C16) at 5°C in untreated and treated soils. Although very low rates of C16 mineralization were observed in the untreated soils, nutrient supplementation with a fertilizer markedly increased C16 mineralization. Highly active cold-adapted hydrocarbon-degrading consortia were prepared from soil slurries, and their degradative potentials were monitored by biomass measurements and mineralization activity. Bio augmentation of the contaminated soils with consortia containing the greatest percentages of degradative bacteria resulted in the shortest C16 mineralization acclimation period. However, treatment with the consortia plus fertilizer did not appreciably increase C16 mineralization or reduce total petroleum hydrocarbon concentrations to a greater extent than did the fertilizer treatment alone. These results indicate that the soils possessed sufficient numbers of cold-adapted degradative bacteria, and that fertilizer application alone was sufficient to obtain elevated levels of degradative activity at low ambient summer temperatures.     

Small-Scale Wetland Restoration in the High Arctic: A Long-Term Perspective

Results are presented using vegetative shoots and bryophyte sods to restore floristically impoverished high arctic wet sedge-moss meadows that had suffered intense damage from vehicle activity during the period 1960–1967. Clonal transplants of Carex aquatilis var. stans, a native sedge, were planted with and without bryophyte sods in vehicle ruts in 1972. After nearly two decades, there was less Carex cover in the planted ruts with flowing water than in the contiguous controls. This pattern was slightly reversed in planted plots with standing water. Reinvasion of Eriophorum angustifolium occurred in treated ruts, but cover was less in both treatments than in controls in 1990. The unexpected recruitment of Eriophorum scheuchzeri from the seed bank in moss-sodded plots is discussed in terms of its local and regional importance. Total plant cover in restored ruts was nearly equal to that of controls, but biomass was somewhat less than that in control plots. Plots with bryophytes were environmentally distinct, due primarily to increases in organic mat depth relative to controls. After 18 years, restoration efforts resulted in increased plant cover in treated ruts compared to naturally recovering ruts.

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Phloretin-Induced Reduction in Dipole Potential of Sterol-Containing Bilayers

The phloretin-induced reduction in the dipole potential of planar lipid bilayers containing cholesterol, ergosterol, stigmasterol, 7-dehydrocholesterol and 5α-androstan-3β-ol was investigated. It is shown that effects depend on the type and concentration of membrane sterol. It is supposed that the effectiveness of phloretin in reducing the dipole potential of the bilayers that contain cholesterol, ergosterol and 7-dehydrocholesterol correlates with the ordering and condensing effects. The role of the concentration-dependent ability of different sterols to promote lateral heterogeneity in membranes is also discussed.     

Incubation scheduling by Northern Fulmars ( Fulmarus glacialis ) in the Canadian High Arctic

Northern Fulmars (Fulmarus glacialis) are a ubiquitous seabird of the North Atlantic and North Pacific oceans that breed in colonies surrounded by markedly different marine environmental conditions. I have studied the incubation behavior of fulmars at a remote colony in the Canadian High Arctic, where the birds had to cross 200 km of sea-ice during the early incubation period to reach feeding areas. These fulmars completed incubation in fewer shifts and had a longer mean incubation shift duration (5.3 days) than their counterparts breeding in the Boreal oceanographic zone. In particular, the mean duration of the first incubation shift by males in successful pairs (10.3 days) was 2 days longer than that for males in unsuccessful pairs and was longer than that reported at any other colony. This exceptionally long shift by the male may be required at this site to give females enough time to recoup energetic reserves after egg-laying, at a time when marine productivity in the Arctic is still seasonably low.     

High viral infection rates in Antarctic and Arctic bacterioplankton

The frequency of visibly phage-infected bacterial cells (FVIB) and the average number of phages per cell [i.e. burst size (BS)] were determined in Antarctic and Arctic ultra-oligotrophic freshwater environments. Water samples were collected from two Antarctic freshwater lakes and cryoconite holes from a glacier in the Arctic. Data from this bipolar study show the highest FVIB (average 26.1%, range 5.1% to 66.7%) and the lowest BS (average 4, range 2-15) ever reported in the literature. The bacterial density is low in these ultra-oligotrophic freshwater environments but a large proportion of the bacteria are visibly infected. Our results suggest that a constant virioplankton population can be maintained in these extreme environments even though host density is low and often slow growing.     

High benthic bacteria standing stock in deep Arctic basins

During the Arctic Expedition ARK 8/3 (August to October 1991) with RV Polarstern sediment samples from 13 staions with water depths of between 258 and 4,427 m were taken along a transect from the Barents Sea slope across the deep Arctic Eurasian Basins and the Gakkel Ridge to the Lomonosov Ridge to determine bacterial biomasses and organic carbon contents. Bacterial abundance dropped along the transect from 3.03 to 0.63×10⁸ cells/cm³, and correspondingly bacterial biomass decreased from 17.35 to 3.43 g C/cm³ sediment. Positve correlations were only found between total organic carbon concentrations of surface sediment layers and biomasses of small coccoid cells and small rods. The ridges and slopes seem to be sedimentation areas for the larger coccoid cells, presumably cyanobacteria.     

Microbial Functional Potential and Community Composition in Permafrost-Affected Soils of the NW Canadian Arctic

Permafrost-affected soils are among the most obvious ecosystems in which current microbial controls on organic matter decomposition are changing as a result of global warming. Warmer conditions in polygonal tundra will lead to a deepening of the seasonal active layer, provoking changes in microbial processes and possibly resulting in exacerbated carbon degradation under increasing anoxic conditions. To identify current microbial assemblages in carbon rich, water saturated permafrost environments, four polygonal tundra sites were investigated on Herschel Island and the Yukon Coast, Western Canadian Arctic. Ion Torrent sequencing of bacterial and archaeal 16S rRNA amplicons revealed the presence of all major microbial soil groups and indicated a local, vertical heterogeneity of the polygonal tundra soil community with increasing depth. Microbial diversity was found to be highest in the surface layers, decreasing towards the permafrost table. Quantitative PCR analysis of functional genes involved in carbon and nitrogen-cycling revealed a high functional potential in the surface layers, decreasing with increasing active layer depth. We observed that soil properties driving microbial diversity and functional potential varied in each study site. These results highlight the small-scale heterogeneity of geomorphologically comparable sites, greatly restricting generalizations about the fate of permafrost-affected environments in a warming Arctic.