Molecular ecology of chasmoendolithic environments in Miers Valley, McMurdo Dry Valleys, Antarctica

Abstract

The McMurdo Dry Valleys comprise some 4,800km2 of ice-free terrain in east

Antarctica and this constitutes the coldest and most arid desert on Earth. The

ecosystem of the Dry Valleys is characterized by microbial processes since

environmental extremes severely limit higher plant and animal life. A major

international collaborative research effort co-ordinated by the International Center

for Terrestrial Antarctic Research (ICTAR), identified long-term study sites

representative of maritime and inland Dry Valleys environments. The maritime site,

Miers Valley, has been the subject of intensive multi-disciplinary study in recent

years, of which the work in this thesis is a part. Previous studies have identified

soil microbial communities and their putative functional roles, but lithic communities

have not been previously appreciated. This thesis reports aspects on the

biodiversity and ecology of lithic microbial communities in Miers Valley.

A survey of terrain revealed extensive weathered granite, but no porous

sandstone or limestone rocks more commonly associated with cryptoendolithic

communities (those colonizing pore spaces within rock substrates). Granite was

extensively colonized (30-100% of available substrate) by chasmoendolithic

microorganisms (colonizing cracks and fissures in weathered rock). Visual

examination of colonized rocks revealed a distinct zone of biomass 2-5mm below the

rock surface, and this was overlain by a weathered and friable matrix of rock.

Microscopy revealed a community dominated by diverse cyanobacterial

morphotypes, plus other unidentifiable microbes of varied morphology.

A quantitative approach to broad-scale community fingerprinting was adopted,

utilizing terminal restriction fragment length polymorphism (TRFLP) and sequence

based identifications of restriction fragments. The multi-domain approach

encompassed Archaea, Bacteria and Eukarya. The results revealed relatively low

species richness (0.6-1.8) for each domain with community richness estimates also

relatively low (<3). Nonetheless very clear and statistically supported patterns in

the occurrence of phylotypes within chasmolithic communities were related to aspect

(which strongly affects temperature and moisture availability in Dry Valleys

locations). The bacterial assemblages formed two groups (cold-dry south facing

slopes and valley floor moraine). The eukaryal assemblages also formed two

groups although here the moraine samples grouped with the warmer wetter north

facing slope and the cold-dry south facing slope assemblages formed a separate

group. The archaeal assemblages displayed no difference within different valley

terrain.

Extensive sequence based interrogation of community structure using clone

libraries revealed a community dominated by cyanobacteria, Actinobacteria,

Deinococci and putative lichens. These phyla are all known for their extreme

tolerance to desiccation and occurrence in arid landscapes. Phylogenetic analysis

revealed that these abundant taxa shared close affiliation with those from other

Antarctic refuge niches such as hypoliths and cryptoendoliths. The cyanobacteria

were mainly Oscillatoriales, but other genera such as Chroococcidiopsis and Nostoc

commonly recovered in hot desert lithic communities were generally absent. The

eukaryal community was dominated by chlorophyte algae, whilst the archaeal

phylotypes were a diverse collection spanning both euryachaeal and crenarchaeal

lineages.

Overall the data revealed the chasmoendolithic community in Miers Valley

was widespread and with relatively restricted diversity. The selection pressures

related to topology of the valley have resulted in different community structure

within the valley.