Survival mechanisms in Antarctic lakes Johanna Laybourn-Parry
Philosophical transactions - Royal Society. Biological sciences,
07/2002, Volume:
357, Issue:
1423
Journal Article
Peer reviewed
Open access
In Antarctic lakes, organisms are confronted by continuous low temperatures as well as a poor light climate and nutrient limitation. Such extreme environments support truncated food webs with no ...fish, few metazoans and a dominance of microbial plankton. The key to success lies in entering the short Antarctic summer with actively growing populations. In many cases, the most successful organisms continue to function throughout the year. The few crustacean zooplankton remain active in the winter months, surviving on endogenous energy reserves and, in some cases, continuing development. Among the Protozoa, mixotrophy is an important nutritional strategy. In the extreme lakes of the McMurdo Dry Valleys, planktonic cryptophytes are forced to sustain a mixotrophic strategy and cannot survive by photosynthesis alone. The dependence on ingesting bacteria varies seasonally and with depth in the water column. In the Vestfold Hills, Pyramimonas, which dominates the plankton of some of the saline lakes, also resorts to mixotrophy, but does become entirely photosynthetic at mid-summer. Mixotrophic ciliates are also common and the entirely photosynthetic ciliate Mesodinium rubrum has a widespread distribution in the saline lakes of the Vestfold Hills, where it attains high concentrations. Bacteria continue to grow all year, showing cycles that appear to be related to the availability of dissolved organic carbon. In saline lakes, bacteria experience sub-zero temperatures for long periods of the year and have developed biochemical adaptations that include anti-freeze proteins, changes in the concentrations of polyunsaturated fatty acids in their membranes and suites of low-temperature enzymes.
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Grazing by the planktonic phytoflagellate, Pyramimonas gelidicola McFadden (Chlorophyta: Prasinophyta), and heterotrophic nanoflagellates (HNAN) in meromictic saline Ace Lake in the Vestfold Hills, ...eastern Antarctica was investigated in the austral summers of 1997 and 1999. Up to 47% of the P. gelidicola population ingested fluorescently labeled prey (FLP). Ingestion rates varied with depth. In January 1997 and November 1999, maximum P. gelidicola ingestion rates of 6.95 and 0.79 FLP.cell super(-1).h super(-1), respectively, were measured at the chemocline (6-8 m) where a deep chl maximum composed of phototrophic nanoflagellates (PNAN DCM), predominantly P. gelidicola, persisted all year. During the summers of 1997 and 1999, the grazing P. gelidicola community removed between 0.4% and approximately 16% of in situ bacterial biomass, equivalent to between 4% and>100% of in situ bacterial production. Because of their higher abundance, the community clearance rates of HNAN in Ace Lake generally exceeded those of P. gelidicola, but HNAN removed approximately only 3%-4% of bacterial biomass, equivalent to between 28% and 32% of bacterial production. Pyramimonas gelidicola growth rates were highest at the PNAN DCM concomitant with the highest ingestion rates. It is estimated that during the summer P. gelidicola can derive up to 30% of their daily carbon requirements from bacterivory at the PNAN DCM. This study confirms mixotrophy as an important strategy by which planktonic organisms can survive in extreme, polar, lacustrine ecosystems.
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The microbial communities and photosynthetic capacity of cryoconite holes on the Midre Lovénbreen Glacier at 79°N in Spitzbergen (Svalbard Archipelago) were investigated in July/August 2000 and July ...2001. The constituents of the microbial assemblages were more abundant in material on the cryoconite bottoms than in the overlying water. Bacterial concentrations ranged from 1.00 to 4.50×10^sup 4^ ml^sup -1^ in the water and from 4.67 to 7.07×10^sup 4^ ml^sup -1^ in the bottom material; virus-like particles (VLP) ranged from 3.97 to 12.70×10^sup 4^ml^sup -1^ in the water and from 27.5 to 37.59×10^sup 4^ ml^sup -1^ on the bottom. VLP: bacteria ratios ranged between 0.24 and 8.11, with highest ratios in the bottom assemblages. Heterotrophic nanoflagellate (HNAN) abundances were significantly lower than those of the autotrophic nanoflagellates (PNAN). Moreover, HNAN biomass was lower than bacterial biomass, indicating that the HNAN were exploiting other energy sources as well as bacteria, for example, VLP and dissolved organic carbon. The bottom material was dominated by cyanobacteria (mostly Phormidium sp.), while both the water and the bottom layer contained a small number of chlorophyte species (Chlorella sp., Cylindromonas sp. and Chlamydomonas nivalis). Ciliates were very sparse, only occurring on the bottom. On occasions, the glacier surface carried meltwater with well-developed biofilms, in which ciliates (Monodinium, Strombidium and Halteria) occurred. All of these species are found in nearby lakes. One to three rotifers were noted in the biofilm samples and in samples from three of the cryoconite holes. The assemblages of the cryoconite holes were comparable to the truncated food webs seen in Antarctic lakes, but were even more simplified and sparse in terms of biomass. Photosynthesis in the meltwater on the glacier surface ranged between 0.60 and 8.33 µg C l^sup -1^ h^sup -1^. Within the cryoconites, photosynthetic rates were usually highest on the bottom (0.63-156.99 µg C l^sup -1^ h^sup -1^), while in the overlying water, rates ranged between 0.34 and 10.56 µg C l^sup -1^ h^sup -1^. Given the density of cryoconite holes (circa 6% of the glacier surface, or 12 holes m^sup -2^), there was significant carbon fixation and nutrient cycling occurring on the glacier, associated with cryoconite communities.PUBLICATION ABSTRACT
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
1. The ingestion rates of planktonic, mixotrophic cryptophytes in two perennially ice‐covered Antarctic lakes in the McMurdo Dry Valleys, were investigated during the summer of 1997–1998.
2. In Lake ...Fryxell, which is meromictic, ingestion rates increased with depth in November and were highest in a cryptophyte maximum close to the chemocline. In Lake Hoare, which is unstratified and freshwater, there was no significant difference in ingestion rates with depth. In both lakes, the highest ingestion rates occurred in early summer, decreasing in December and January. Ingestion rates varied between 0.2 bacteria cell−1 h−1 and 3.6 bacteria cell−1 h−1.
3. During November, mixotrophic cryptophytes removed up to 13% of bacterial biomass day−1 and had a greater grazing impact than heterotrophic nanoflagellates (HNAN). As summer progressed, the grazing impact of cryptophytes and HNAN became similar.
4. The maximum depth of cryptophytes in Lake Fryxell was predated by a population of the ciliate Plagiocampa. Plagiocampa had an ingestion rate of 0.13–0.19 cryptophytes cell−1 h−1. The grazing impact on the cryptophyte community was insignificant. However, the ciliate appeared to be indulging in temporary mixotrophy, sequestering the cryptophytes for a number of weeks before digesting them.
5. It is suggested that mixotrophy is an important survival strategy in the extreme lake ecosystems of the McMurdo Dry Valleys.
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1. Virus‐like particle (VLP) abundances in nine freshwater to saline lakes in the Vestfold Hills, Eastern Antarctica (68° S) were determined in December 1999. In the ultra‐oligotrophic to ...oligotrophic freshwater lakes, VLP abundances ranged from 1.01 to 3.28 × 106 mL–1 in the top 6 m of the water column. In the saline lakes the range was between 6.76 and 36.5 × 106 mL–1. The lowest value was found in meromictic Ace Lake and the highest value in hypersaline Lake Williams. Virus to bacteria ratios (VBR) were lowest in the freshwater lakes and highest in the saline lakes, with a maximum of 23.4 in the former and 50.3 in the latter.
2. A range of morphologies among VLP was observed, including phages with short (Podoviridae) and long tails, icosahedric viruses of up to 300 nm and star‐like particles of about 80 nm diameter.
3. In these microbially dominated ecosystems there was no correlation between VLP and either bacterial numbers or chlorophyll a. There was a significant correlation between VLP abundances and dissolved organic carbon concentration (r=0.845, P < 0.01).
4. The data suggested that viruses probably attack a spectrum of bacteria and protozoan species. Virus‐like particle numbers in the freshwater lakes were lower than values reported for lower latitude systems. Those in the saline lakes were comparable with abundances reported from other Antarctic lakes, and were higher than most values published for lower latitude lakes and many marine systems. Across the salinity spectrum from freshwater through brackish to hypersaline, VLP concentrations increased roughly in relation to increasing trophy.
5. Given that Antarctic lakes have a plankton almost entirely made up of bacteria and protists, and that VLP abundances are high, it is likely that viruses play a pivotal role in carbon cycling in these extreme ecosystems.
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A model was developed to explore patterns of carbon flow through the pelagic food web of Lake Fryxell, Taylor Valley, Antarctica. The goals of this study were to quantify patterns of carbon flow and ...test hypotheses of top-down versus bottom-up controls on this system. The model included seven trophic groups: bacteria, photosynthetic nanoflagellates (PNAN), heterotrophic nanoflagellates (HNAN), ciliates (bacterial feeding and flagellate feeding), phytoplankton and rotifers (the top predator); all inputs were driven by predatory demands. This system has no insects, vertebrates, crustacea or allochthonous inputs from terrestrial plants. Autotrophs and bacteria contributed ca. 94% of total community biomass, with all other heterotrophic organisms representing <6% of the total. We defined a measure of trophic efficiency (
λ) as the ratio of carbon uptake by rotifers to total carbon uptake by all trophic groups.
λ was always <2% of total community carbon flow and seldom correlated to other flows of carbon through the system. Sensitivity analysis revealed that
λ was relatively insensitive to efficiency of carbon transfers between other components of the food web. In total, these results suggest little top-down influence of the top predator on system dynamics. Conversely, major fluctuations in total community biomass followed general patterns of seasonal and inter-annual availability of PAR, suggesting bottom-up control on this system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
MBL-MEL, a simple model of ecosystem biogeochemistry, is amended and applied to plant and soil C, ¹⁴N and ¹⁵N data for the summers of 2001-2003 from Brandalpynten, a maritime high Arctic site on ...Svalbard following the application of ¹⁵N (99 atom%) as ¹⁵NO₃-N at or ¹⁵NH₄-N at concentrations of 1 or 5 kg N ha-¹. Variants of this Parent model are also developed to incorporate: temperature dependence into equations describing nutrient fluxes (Temp model); cryptogams (Cryp model); both features combined (CrypTemp model). Goodness-of-fit (GOF) statistics suggest that the addition of temperature-dependence improves the utility of models with and without cryptogams: the residual weighted sums of squares per data point were 5.69, 3.91, 4.31 and 3.93 for the Parent, Temp, Cryp and CrypTemp models respectively. The application of model selection criteria confirm that the addition of temperature-dependence also improves model generalisability. Across all models, the principal discrepancies between observation and prediction are associated with the inorganic soil ¹⁵N pool. We conclude that models in which fluxes are described using simple equations that can be augmented to include additional complexity, are an ideal starting point from which the relationship between GOF and model generalisability can be assessed.
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Priscu describes the physical, chemical, and biological conditions in Lake Bonney, a prominent lake within the McMurdo Dry Valleys. Using this framework, he discusses microbial transformations of ...organic carbon within the water column.
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A large mixotrophic ciliate (∼ 200μ m long) of the genus Stentor is a common constituent of the protozooplankton of Australian lakes. We investigated the photosynthetic rates of populations of this ...ciliate from two lakes, one in the Australian Capital Territory and the other on the New South Wales/Victorian boder, in relation to photosynthesis by the whole phytoplankton community. The concentration of the ciliate varied between 192 and 4,267 cells liter-1 during the study period of May-January (the austral winter, spring, and autumn) and it contributed between 4.3 and 69.3% of total plankton photosynthesis. Individual photosynthetic rates ranged between 1.03± 0.8 and 3.98± 0.6 ng C cell-1 h-1 and individual Chl a content between 925± 62 to 1,461 ± 63 pg cell-1, giving assimilation numbers of 1.00-2.74. Light-response curves indicated that the ciliate achieved its highest rates of photosynthesis at high photon fluxes, typical of the surface waters. Vertical distribution patterns of Stentor in the water column of one of the lakes supported these physiological data. Southern hemisphere lakes seem to have a protozooplankton that may contain substantial numbers of a large ciliate capable of contributing a significant portion of carbon fixation in the plankton.
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