Biodiversity at the MCM Long-term Ecological Research Site

Authors: Sarah Spaulding

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Abstract

Less than 2% of Antarctica is ice-free, and the McMurdo Dry Valleys are the largest of these ice-free regions. The dry valleys are located within the Transantarctic Mountains, which act as barriers to the flow of ice from the polar plateau and have been predominately free of ice for the past 4 million years. With precipitation less than 10 cm yr-1 and mean monthly temperatures that range from 0-35 degrees C, the dry valleys are the most arid, coldest deserts on earth. Within these cold mountainous deserts are isolated zones where liquid water may be present. Alpine glaciers release meltwater during the short austral summer that flows by meltwater streams into closed basin lakes. The surrounding soils are arid, lacking vascular plants and vertebrates, and very few insects are present. These valleys represent an extreme end of the range of ecosystems on earth, with simplified communities of microinvertebrates, algae, and microbes.

BIODIVERSITY

Issues of biodiversity in the McMurdo Dry Valleys contribute a unique perspectives a part of the LTER network. In these simplified systems, organisms are often delimited by physical and chemical parameters to very narrow zones of existence. Biotic interactions are less important than in temperate environments, and controls on species level questions may be more amenable to understanding. However, addressing questions of biodiversity in the dry valley also brings up challenges. Organisms within the dry valleys belong to taxonomic groups that historically have been poorly studied. Few trained practitioners remain with the expertise to bring to these groups. Fortunately, because of their position as one of the extreme environments where life exists, dry valleys have attracted the interest of investigators that specialize in traditionally little known groups. Further, it is fundamentally important to understand that interpretations may differ depending on the species concept that is used for each taxonomic group of organisms. Microinvertebrates with complex life histories, filamentous cyanobacteria with morphological changes based on environmental conditions, diatoms with asexual stages of reproduction, or bacteria that are distinguished based on physiology may each suggest a different answer to the question, (What is a species?v

TYPES OF HABITATS

Habitats within the dry valleys can be defined as of cryptoendolithic, soil, stream, and lake. A general characteristic of each of these habitats is that they occur in narrow zones, delimited by sharp gradients in the abiotic environment. These zones may range over the size of millimeters, with endolithic organisms following a gradient of solar radiation and moisture, to centimeters in the range of nematode distribution in soil, to several meters in lake benthic microbial mats across a gradient of water depth reflecting differences in light, dissolved oxygen, salinity, and nutrients.

CRYPTOENDOLITHIC

Life in the dry valleys is often not visible to the naked or untrained eye. Even epilithic lichens, common in the arctic and maritime antarctic are very rare. A peculiar habitat within the dry valleys is that of microorganisms that live within a narrow zone under the surface of rocks, or endolithic organisms. Endoliths may occur in rock fissures and cracks (chasmoendoliths) or in the interstices of porous rocks. Endolithic communities are based on a photosynthetic primary producer, and therefore occur in translucent rocks.

Cryptoendolithic lichens are the predominant organisms colonizing the subsurface of sandstone. Typical growth can be seen by the sectioning of sandstone rock, where an upper black zone of 1 mm in thickness occurs overlying a white zone 2-4 mm thick, and a lowest green zone. Each of the zones is produced by filamentous fungi and unicellular chlorophytes (green algae), joined in symbiotic association. In contrast to the structural association in most lichens (which form a coherent pseudotissue, or plectenchyma), cryptoendolithic lichens are associated with the structure of the rock itself. Loose filaments and cell clusters grow between and around the crystals of the rock substrate, and are embedded in the rock matrix.

In rocks that are less porous that sandstone, lichens grow in vertical fissures of the rock (chasmoendolithic growth). Granites an granodiorites contain both lichens and coccoid cyanobacteria. Some outcrops of weathered marble are heavily colonized by 12-15 species of unicellular and filamentous green algae and cyanobacterium growing in association with colonial colorless bacteria. One of the characteristic taxa is the common soil xanthophyte, Heterococcus. But in these assemblages, both lichens and fungi are absent.

Determining species composition of cryptoendolithic organisms is not a simple task. Lichen taxonomy is based primarily on reproductive structures. Yet, cryptoendolithic lichens do not develop reproductive structures except when that are epilithic or chasmoendolithic. Three genera have been identified within the dry valleys: Buellia, Lecidea, and Acarospora. These three genera show a remarkable similarity in morphology, despite belonging to different families. Within the Antarctic, a total of 90 genera and 434 species have been reported (Dodge, 1973).

The cryptoendolithic ecosystem is based on the primary producers: cyanobacteria and phycobionts; mycobionts may be regarded as consumers, and colorless bacteria as decomposers. Estimates of total organic matter range for 46 to 177 grams/m2 of rock surface. Decomposition is slow, and the values contain both living and dead material and the high biomass is not indicative of productivity.

SOILS

Physical structure of the soils of the McMurdo Dry Valleys can be described as coarse grained, saline, and periglacially active (Ugolini, 1970; Campbell and Claridge, 1987). Although the soils are up to 5 million years old, profile structure is generally poorly developed. Biological activity, organic carbon content, and moisture are all low. Although soils give the overall appearance of uniformity, they are highly variable spatially and temporally in terms of soil properties, hydrologic regimes, biotic composition.

Isolated patches of more complex soil ecosystems are found in moist areas near glaciers where mosses, algae, protozoa, rotifers, tardigrades, nematodes, yeast, filamentous fungi, bacteria, mites, and collembolands are found (Vincent, 1988). The abundance and diversity of organisms from dry valley soils are related to microenvironment (Cameron et al., 1970). trophic linkages are based upon the phototrophic cyanobacteria, algae, lichens, and bryophytes.

ALGAE

Soil algae are not diverse, with less that 70 genera recorded in the soils throughout the region (reviewed in Vincent, 1988). Algal diversity varies greatly between dry soils and soils that are moist in the periphery of streams and lakes. Cyanobacteria represent both the greatest diversity and biomass of the primary producers. The nitrogen-fixing Nostoc commune is widespread, as well as filamentous Oscillatoriaceae. Cosmopolitan aerophilic diatoms (Luticola muticopsis, Luticola mutica, Hantzschia amphioxys) are also common. Although algal diversity has been reported to decrease with increasing latitude (Hirano, 1965), it may be attributed more to limited investigation. As more basic biological studies are conducted, more taxa are found.

BACTERIA

Bacteria in soils have been identified by culture enrichment techniques, revealing a wide range of morphological types in many sites in Antarctica. Bacteria isolated from dry valley soils include Corynebarterium, Arthrobacter, and Micrococcus (Cameron et al. 1972). Halotolerant taxa, such as Planococcus, are important in the saline soils of the dry valleys (Miller et al., 1983). These techniques are thought to underrepresent actual diversity, because enrichment culture techniques may select for unusual or metabolically unimportant species.

FUNGI

Fungi in antarctic soils include basidiomycetes, oomycetes, and ascomycetes, an important group containing yeast that often dominate the soil flora. Twenty-five percent of soils were found to contain yeast in sufficient quantity to be enumerated (Atlas et al., 1978).

PROTOZOA

Few investigation have been completed of the protozoa. For one group, species diversity of testate amoebae was found to decrease with increasing latitude (Smith, 1982).

NEMATODES

Eleven species of nematodes with 6 genera have been described from the antarctic (Maslen, 1979). The most abundant taxa are Scottnema lindsayae Timm and Eudorylaimus antarticus (Steiner) Yeates. Thirty four percent of soils were found to lack nematodes (Freckman and Virginia, 1991). However, when nematodes were present, densities of up to 4000 kg/1 dry soil were found, values that are comparable to densities in other desert soils. Distribution of nematodes in dry valley soils showed a high degree of spatial heterogeneity. Vertically, the greatest abundance of nematodes occurred between 2.5 and 10 cm (Powers et al., 1995).

STREAMS

Stream Biota are characterized by thick epilithic communities of cyanobacteria mats up to greater than 10 mm thick (Broady, 1982; Broady & Kibblewhite, 1991). The simple communities include bacteria, protozoa, fungi, rotifers, nematodes, and tardigrades. The stream communities are composed primarily of black, mucilaginous mats of Nostoc or orange rubber-like mats of Phormidium. Mats have been classified into types based on apparent color, which were also associated with hydrologic zones along the stream channels (Alger et al., in press). Black mats occurred in wetted zones and areas of emergence of subsurface seeps. They were low in diversity, being almost entirely composed of Nostoc. Green mats occurred on the underside of rocks in the main channel. They were also comprised almost entirely by a single taxa, Prasiola. Orange and red mats occurred in the main stream channel and had the greatest diversity of species, with greater than 30 species present.

Biomass is dominated by cyanobacterial filamentous taxa, a group that has a great deal of taxonomic uncertainty. Species are identified based on cell morphology, yet morphology mat depend on environmental conditions.

With low rates of decomposition and few consumer organisms, the algal standing crop can reach high levels (>20 microgram chl a cm-2 and >20 mg C cm-2), representing several seasons of growth (Vincent & Howard-Williams, 1986) However, photosynthetic rates are low, indicating that much of the algal material is in a vegetative resting state or senescent. Rates of primary production have been measured at less that 0.2 microgram C (mg C/h). Respiration was found to be a high percentage (92%) of gross photosynthesis, reflecting the densities of stream heterotrophs.

LAKES

Stromatolitic microbial mats occur in the periphery of dry valley lakes (Wharton et al., 1993; Wharton, 1994). Mats are composed of bacteria , cyanobacteria, and eukaryotic algae and are termed (stromatoliticv for their trapping and binding of sediment and precipitating of mineral, which form laminated organosedimentary structures. Phormidium frigidum Fritsch, a filamentous cyanobacterium, is the most abundant component of the benthic matrix. While cyanobacterial filaments form the greatest biomass, diatoms contribute the greatest number of taxa, with 50 taxa found in Lake Hoare alone (Spaulding et al., 1996). Fungi and protozoans in the microbial mats have been studied by Baublis et al. (1991) and Cathey et al. (1981).

Plankton

The plankton in perennially ice-covered dry valley lakes are low in diversity and biomass compared to temperate lakes. Water temperatures are near freezing, with low light and nutrient-limited conditions (Parker et al, 1982; Wharton et al, 1983; Vincent, 1988). Despite the cold, dark environment and remote geographic location, planktonic algae are made up of cosmopolitan species. Although chemical gradients and physical parameters are extreme compared to temperate lakes, the algal composition is not.

Fifty-six planktonic algal taxa have been collected in Lake Fryxell over a period of 5 years (1987-1992) (Spaulding et al, 1994). The majority of species are cyrptophyte and chlorophyte flagellates and filamentous cyanobacteria. Carbon fixation rates were measured up to 30 mg C m3 day-1, with biovolume concentrations of approximately 106 microns3 liter-1, concentrations which are comparable to other oligotrophic lakes around the world. Phytoplankton are confined to the range of depths between 4.5 and 10.5 m, a narrow region delineated by the bottom of the ice cover and the oxyline. Vertical profiles of phytoplankton shows that species were often strongly vertically segregated, and taxa occupied distinct positions of the oxitic zone.

The nutrient status of several of the dry valley lakes is low (Priscu, 1995). Photosynthesis was stimulated by phosphorus enrichment, with additional stimulation by phosphorus plus ammonium. Several of the lakes were found to be deficient in nitrate, with phytoplankton production supported by upward diffusion of nutrients.

Examination of the protistan plankton resulted in distinguishing 14 ciliate species in high abundance for such lakes of 7700 l-1 (Laybourn-Parry et al, 1996) and included two rotifer taxa, which were the first record of planktonic metazoans in the dry valleys region. Bacterial concentrations were in the range of 108 l-1, more typical of oligotrophic lakes.

SUMMARY

Organisms are constrained by abiotic factors in dry valley ecosystems. Physically and chemically defined zones are important in controlling species composition and abundance within each of the habitats described within the dry valleys ecosystem. These zones form sharp boundaries dividing the line of existence from abiosis. The McMurdo Dry Valleys are low in species diversity, and contain organisms traditionally recognized as taxonomically difficult. But because of the limited numbers and taxa, it may be a more tractable problem to realistically estimate species diversity and its relation to ecosystem processes than in many other LTER sites.

Atlas, R.M., M.E. DiMenna, & R.E. Cameron. 1978. Ecological investigations of yeast in antarctic soils. Antarctic Research Series 30: 27-34.

Baublis, J.A., R.A. Wharton, Jr. & P.A. Volz. 1991. Diversity of micro-fungi isolated in an antarctic dry valley. Journal Basic Microbiology 31:10-20.

Broady P.A. & Kibblewhite, A.L. 1991. Morphological characterization of Oscillatoriales (Cyanobacteria) from Ross Island and southern Victoria Land, Antarctica. Antarctic Science 3: 35-45.

Broady, P. 1982 Taxonomy and ecology of algae in a freshwater stream in Taylor Valley, Victoria Land, Antarctica. Archiv fur Hydrobiol. Verhand. 63: 331-339

Cameron, R.E., F.A. Morelli, & R.M. Johnson. 1972. Bacterial species in soil and air of the Antarctic continent. Antarctic Journal of the U.S., 7:187-189.

Cameron, R.E., J. King & C.N. David. 1970. Microbiology, ecology, and microclimatology of soil sites in dry valleys of southern Victoria Land, Antarctica. In: Holgate, M.W. (ed.), Antarctica Ecology, Volume 2, Academic Press, London

Campbell, I.B. & G.C. Claridge. 1987. Antarctica: Soils, Weathering Processes and Environment. Developments in Soil Science. Elsevier, New York.

Cathey, D.D., G.M. Simmons, B.C. Aprker & W.H. Yongue. 1981. The mircofauna of algal mats and artificial substrates in southern Victoria Land lakes of Antarctica. Hydrobiology 85:3-15

Dodge, C.W. 1973. Lichen Flora of the Antarctic Continent and adjacent Islands. Phoenix, Canaan, New Hampshire.

Freckman, D.W. & R.A. Virginia. 1990. Nematode ecology of the McMurdo Dry Valley ecosystems. Antarctic Journal of the U.S. 25:229-230.

Freckman, D.W. & R.A. Virginia. 1991. Nematodes in the McMurdo Dry Valleys of southern Victoria Land. Antarctic Journal of the U.S. 26:233-234.

Friedman, E.I. 1982. Endolithic microorganisms in the Antarctic cold desert. Science 215:1045-1053.

Laybourn-Parry, J., M. James, D.M. McKnight, J. Priscu, S. Spaulding & R. Shiel. 1996. The microbial plankton of Lake Fryxell, Southern Victoria Land, Antarctica. Polar Biology (in press).

Maslen, N.R. 1979. Additions to the nematode fauna of the antarctic region with keys to taxa. Brit. Antarctic Survey Bull. 49:207-229.

Miller, K.L., S.B. Leschine, & R.L. Huguenin. 1983. Halotolerance of microorganisms isolated from saline antarctic dry valley soils. Antarctic Journal of the U.S. 18: 222-223.

Powers, L.E., D.W. Freckman, & R.A. Virginia. 1995. Spatial distribution of nematodes in polar desert soils of Antarctica. Polar Biology 15:325-333.

Priscu, J. 1995. Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica. Freshwater Biology 34:215-227.

Smith, H.G. 1982. The terrestrial protozoan fauna of South Georgia. Polar Biology 1:173-179.

Spaulding, S.A., D.M. McKnight, R.L. Smith & R. Dufford. 1994. Phytoplankton population dynamics in perennially ice-covered Lake Fryxell, Antarctica.

Spaulding, S.A., D.M. McKnight, E.F. Stoermer & P.T. Doran. 1996. Diatoms in the sediments of Lake Hoare. Journal of Paleolimnology 17: 403-420.

Ugolini, F.C. 1970. Antarctic soils and their ecology. In: Holgate, M.W. (ed.), Antarctic Ecology, Volume 2, Academic Press, London.

Vincent, W.F. & C. Howard-Williams. 1986. Antarctic stream ecosystems: physiological ecology of a blue-green algal epilithon. Freshwater Biology 16:219-233.

Vincent, W.F. 1988 Microbial Ecosystems of Antarctica. Cambridge University Press, Cambridge.

Wharton, R.A., Jr., B.C. Parker, & G.M. Simmons, Jr. 1983. Distribution, species composition, and morphology of algal mats in antarctic dry valley lakes. Phycologia 23: 355-365.

Wharton, R.A., Jr. 1994 Stromatolitic mats in antarctic lakes. In:J. Bertrand-Sarfati & C. Monty (eds), Phanerozoic Stromatolites 2:53.