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Introduction

Antarctica offers nearly no opportunity for colonization, as it is almost entirely covered by ice . However, in the ice-free areas of the continent, lakes can exist . These lakes vary in composition, ranging from freshwater to hyper saline, as well as being either permanently covered with ice, such as subglacial lakes, or depending on the season, temporarily covered with ice . Research of these lakes provides new findings for microorganisms that can persist in these extreme environments, and have yet to be discovered elsewhere.

Physical Environment

Types of Lakes

Both subglacial and open lakes exist in Antarctica. Subglacial lakes are liquid water bodies that can be found under glaciers, ice caps or ice sheets. (Priscu 2005). The largest of these lakes is Lake Vostok, which is covered by a 4km thick ice sheet (Priscu 2005). The ice cover restricts interaction between the lake and the exterior environment (Cowan and Tow 2004). However, open lakes are not ice-covered, and usually have high salt concentrations (Cowan and Tow 2004).

Soybeans prior to soy milk production

Salinity

There are a number of both fresh and saline water lakes in Antarctica. Saline lakes contain a total of 3% dissolved salts (Burton 1981). Steep gradients of salt concentration are present in these lakes, making them meromictic, resulting in different niches within the lake (Cowan and Tow 2004). As well, freshwater lakes tend to average between 1-2oC, while saline lakes average below 0oC (Laybourn-Parry 2002).

Light

Availability of light depends on the thickness of ice covering the lake, if present. Solar radiation can be transmitted through thin ice sheets, but ice sheets of around 4-6m thick prohibit light penetration (Cowan and Tow 2004). Low light levels correspond to low levels of photosynthesis, which may not be sufficient for microbial growth (Laybourn-Parry 2009).

Oxygen

Oxygen is carried into the lakes through melting glaciers (Wharton et al 1986). As well, photosynthesis by benthic mat microbes provides the system with dissolved O2 (Cowan and Tow 2004). O2 gradients form in the lakes, with very high concentrations near the surface (or under the ice cover, if present), and anoxic environments at the bottom (Cowan and Tow 2004).

Microbial Processes

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Raw almond in a glass

The main microbial processes in these systems involve the cycling of carbon and nitrogen, which are essential to the growth of organisms.

Carbon Cycle

Carbon is a building block for many molecules, and is found in all living organisms. In a lake’s anoxic environment, methanogens are able to use acetate, H2, and CO2 for fermentation, leading to the formation of methane (Wilkins et al 2012). Where oxygen is available, phytoplankton uptake carbon during photosynthesis, converting CO2 into organic carbon (Laybourn-Parry 2009). Organic carbon from decomposing plankton, or other benthic microorganisms, is usually segregated into sediment through the process of sedimentation(Wharton et al 1986). As well, viruses lyse host cells to return carbon to the carbon pool in its inorganic form, which then gets transferred to protozoa in the system (Laybourn-Parry 2009).

Nitrogen Cycle

Nitrogen is important for all organisms, as it is makes up nucleic and amino acids, which are essential for life. Fixed nitrogen enters lakes when glacial streams melt (Cowan and Tow 2004). This nitrogen then becomes available for cyanobacteria to use for nitrogen fixation (Cowan and Tow 2004). Different processes of the nitrogen cycle are separated into different areas of the lake. When ammonium diffuses from anoxic layers, nitrifying bacteria use that ammonium to produce nitrate at the border between oxic and anoxic water (Howard-William and Hawes 2007). When nitrate and nitrous oxide diffuse into the anoxic layers, denitrification occurs (Howard-William and Hawes 2007). Decomposition also occurs within the anoxic layer, which releases ammonium through sulphate reduction (Howard-William and Hawes 2007)

Key Microorganisms

Antarctic lakes do not have large food chains, as no fish are present, resulting in microbes dominating the system (Laybourn-Parry 2002).

Cyanobacteria

Cyanobacteria are the major components of benthic phototrophs, which take part in forming mats at the bottom of lakes that range in thickness from a few micrometeres to a few decimeteres (Sabbe et al 2004). A large number of cyanobacteria can make up these mats; especially members of Oscillatoriales, and all these bacteria take part in oxygenic photosynthesis, as well as nitrogen fixation in these lakes (Wilkins et al 2012). Microbial mats are beneficial to cyanobacteria, as it allows for close interactions between the organisms, helping (Wilkins et al 2012). This association allows for better growth of the community as a whole, compared to an isolated bacterium, as metabolic processes of different organisms are combined (Wilkins et al 2012).

Phytoplankton

File:Different kinds of soy milk.jpg

Different kinds of soy milk

Nutritionally, soy milk is quantitatively similar to cows milk. The major difference is the lower sugar content, which contributes to an overall lower level of carbohydrates.

Phytoplankton are adapted to survive in the Antarctic lake environments of low light and temperatures, and use photosynthesis for growth (Laybourn-Parry 2009). However, when photosynthesis is not enough to sustain the organism, so mixotrophy is used for survivial (Laybourn-Parry 2002). This method involves growing on mixed nutrients, and the use of both autotrophy and heterotrophy (Laybourn-Parry 2009). Along with the use of photosynthesis, the organism grows on bacteria and dissolved organic carbon (Laybourn-Parry 2002). The use of mixotrophy is essential for phytoplankton survival, especially in times or areas, of low sunlight (Laybourn-Parry 2002).

Viruses

There is an abundance of viruses in Antarctic Lakes, and these viruses are thought to influence the lake’s food chains by being one of the main methods of regulation of microbial organisms (Lopez-Bueno et al 2009; Kepner et al 1998). Some main viruses in these communities are single stranded DNA (ssDNA) viruses, which infect eukaryotes, and double stranded DNA (dsDNA) viruses, which infect algae (Lopez-Bueno et al 2009). However, there is a large diversity of viruses in these lakes, many of which that have yet to be identified (Lopez-Bueno et al 2009).

Current Research

1 Scientists are have recently researched the microbial diversity of a sediment sample from Lake Hodgson (Pearce et al 2013). It was found that 77% of the microorganism diversity could be identified with previously found microbes, while 23% could not be recognized (Pearce et al 2013). This indicates that a large number of microbes still remain unknown to scientists. Further research is necessary to continue to reveal more information on the diversity of microbes, and how they continue to live in the extreme Antarctic Lake conditions (Pearce et al 2013).

2 Research in Deep Lake has demonstrated that horizontal gene transfer occurs quite frequently between microorganisms (DaMaere et al. 2013). Large DNA segments are exchanged between different classes of microorganisms, leading to evolution (DeMaere et al 2013). This allows the microorganisms to avoid competition for resources, as the exchanged DNA helps with niche adaptation (DeMaere et al 2013).