Location
Below the surface of the Earth, deep within her crust, there is an abundance of microorganisms. Some scientists estimate that the biomass of these microbes living in the subsurface is equal to or greater than the biomass on the surface of the planet. However, at this stage we know very little about the microbes which define the subterranean biosphere, how they get enough energy to survive, how they influence geochemical fluxes promoted by ground water circulation, and how this biosphere interacts with the biosphere on the Earth’s surface. Some speculate that these life forms may be amongst the most primitive, the ones that may have been involved in the origin of life on earth, or, as they may exist on extraterrestrial worlds. In this subsurface biosphere, microorganisms are believed to derive much of their energy from inorganic chemical reactions as opposed to most of the planet’s organic carbon that is generated from photosynthesis. For all these reasons it is very important to study the microbiology of the deep biosphere, but this is a very complex and difficult undertaking: The deep biosphere is generally not accessible without major machinery and other biologically “dirty” technology that can introduce surface-derived microbes to depth, not to mention that 2/3 of the Earth’s crust is hidden beneath miles of ocean water.
Hydrothermal activity on Mt. Erebus offers some very interesting vantage points that allow us to study the microbial activity in the interior of the crust, with minimal or non-existent disturbance from human activities and without the impact of typical characteristics of Earth’s surface microbiology. Warm and moist air emanates from the volcano and melts caves in the overlying ice. When hot and moist air meets the extremely cold atmosphere, the water crystallizes into ice and forms prominent ice chimneys. The conditions in the caves and chimneys can vary from near freezing to sauna-like temperatures and from brightly lit to extreme darkness. All of them are devoid of organic matter as is so common in soils and elsewhere at the surface of the earth. All of these features are under positive pressure from gases inside the volcano and they are effectively isolated from Earth’s surface microbiology. We are studying these caves, because their microbial populations do not have access to organic matter and, in the case of the dark caves, they may provide a good and accessible analog of conditions found in deep subsurface crustal rock.
Our project is attempting to take advantage of these conditions in the ice caves to explore the potential for microbes to “eat rock”, that is, to harvest potential energy sources that may be harbored in volcanic glass. Volcanic glass may contain substantial amounts of reduced metals (iron and manganese) and sulfur, all of which may fuel energy-yielding reactions for microbes, in particular in the presence of atmospheric oxygen. Here in the caves, unlike the stratified water column in Lake Fryxell (see Daily Report 26), we are looking for aerobic processes in which the oxidation of iron (Fe), manganese (Mn) and sulfur (S) is coupled to the reduction of oxygen (O2). During these reactions Fe and Mn are oxidized to form insoluble Fe and Mn oxide minerals and S is oxidized to form sulfate which is generally soluble. All of these reactions are thermodynamically favorable and could provide energy to the microorganisms for their growth. 
When such inorganic energy sources are used for growth it is referred to as chemolithoautotrophy: energy is derived from chemical energy (chemo) as opposed to light (photo); the sources of electrons are inorganic chemicals (litho) in comparison to organic chemicals (organo); and cellular carbon is provided through the fixation of carbon dioxide (auto) just like in plants and unlike us humans which use organic (hetero) compounds. Thus a plant is a photolithoautotroph and a human in a chemoorganoheterotroph (where the suffix –troph refers to nourish or nourishment). So it is the chemolithoautotrophs that could be eating and growing on volcanic glasses and these same types of metabolisms, except perhaps those that grow anaerobically using other electron acceptors (review this concept in Daily Report 26), which may be important in subsurface environments.
During this season we visited 3 different caves with different temperature and light conditions: Harry’s Dream (a small cave with fairly high levels of light and 10°C); Hubert’s Nightmare (a moderately sized cave, almost completely dark and temperatures of 3.3°C two years ago and just above freezing at 0°C when we recovered our experiments this year); and Warren Cave (a large, dark cave where we sampled 18.5°C soils and measured air temperatures of 14.6°C). We recovered samples of rocks, rock surfaces and soils as well as rock substrates for microbial colonization that had been deployed 2 years ago. We also emplaced new colonization experiments that will be recovered in another two years. The experiments in these caves offer us some important insights into microbial processes at conditions as we expect them inside the Earth’s crust, and their positive pressure and the rare or non-existent human traffic allows us to study the inside of the crust with minimal or non-existing human contamination.
Now the work begins! We will use the samples recovered to try to culture chemolithoautotrophs as well as to extract the DNA and analyze the microbial communities using molecular approaches (e.g., amplification, sequencing and phylogenetic analysis of the ribosomal RNA genes) and light and electron microscopy. We are also attempting to analyze those microbes that are most active (growing the fastest) under the natural conditions of the caves using a technique called stable isotope probing (SIP) in which samples are incubated for a period of time with stable isotopes and then the DNA is extracted and the DNA that has the stable isotopes incorporated (i.e., DNA from growing cells) is analyzed. All of the work now planned is far more time consuming than the relatively brief time we have spent in the field—we hope to have some initial results within the next 3-4 months. Only time will tell whether the fruits of our labors in the extreme environments of Mt. Erebus will pay off.
Greetings from Hubert Staudigel (Hotel Sierra)
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