Terrestrial algae have been studied at widespread Antarctic localities. However, their diversity is not fully known as often collections have not been made from all habitats and techniques have been inadequate for recognition of the total flora. Identifications can be unreliable and are often left at generic level. Despite this it seems that they largely comprise cosmopolitan species but at a reduced diversity relative to other regions. There is a small element of endemic species which, in general, differ only slightly from related species elsewhere. Lack of base-line taxonomic knowledge, detailed characterization of environmental factors, and application of multivariate analysis restricts our ability to define communities and interpret their distribution patterns. Examples are provided where understanding would be greatly improved by more detailed analyses. Dispersal barriers could be operating both to and within Antarctica. However, local dispersal by wind seems readily accomplished. Overcoming dispersal barriers to Antarctica, and between ice-free localities within Antarctica, could present a greater problem. Investigation of long-distance transport of propagules by wind, birds and humans is needed.

Abstract-Asteroid and comet impacts on Earth are commonly viewed as agents of ecosystem destruction, be it on local or global scales. However, for some microbial communities, impacts may represent an opportunity for habitat formation as some substrates are rendered more suitable for colonization when processed by impacts. We describe how heavily shocked gneissic crystalline basement rocks exposed at the Haughton impact structure, Devon Island, Nunavut, Arctic Canada, are hosts to endolithic photosynthetic microorganisms in significantly greater abundance than lessershocked or unshocked gneisses. Two factors contribute to this enhancement: (a) increased porosity due to impact fracturing and differential mineral vaporization, and (b) increased translucence due to the selective vaporization of opaque mineral phases. Using biological ultraviolet radiation dosimetry, and by measuring the concentrations of photoprotective compounds, we demonstrate that a covering of 0.8 rnrn of shocked gneiss can provide substantial protection from ultraviolet radiation, reducing the inactivation of Bacillus subtilis spores by 2 orders of magnitude. The colonisation of the shocked habitat represents a potential mechanism for pioneer microorganisms to invade an impact structure in the earliest stages of post-impact primary succession. The communities are analogous to the endolithic communities associated with sedimentary rocks in Antarctica, but because they occur in shocked crystalline rocks, they illustrate a mechanism for the creation of microbial habitats on planetary surfaces that do not have exposed sedimentary units. This might have been the case on early Earth. The data have implications for the microhabitats in which biological signatures might be sought on Mars.

The Ward Hunt Ice Shelf (83 degrees N, 74 degrees W) is the largest remaining section of thick (> 10 m) land-fast sea ice along the northern coastline of Ellesmere Island, Canada. Extensive meltwater lakes and streams occur on the surface of the ice and are colonized by photosynthetic microbial mat communities. This High Arctic cryo-ecosystem is similar in several of its physical, biological and geochemical features to the McMurdo Ice Shelf in Antarctica. The ice-mats in both polar regions are dominated by filamentous cyanobacteria but also contain diatoms, chlorophytes, flagellates, ciliates, nematodes, tardigrades and rotifers. The luxuriant Ward Hunt consortia also contain high concentrations (10(7)-10(8) cm-2) of viruses and heterotrophic bacteria. During periods of extensive ice cover, such as glaciations during the Proterozoic, cryotolerant mats of the type now found in these polar ice shelf ecosystems would have provided refugia for the survival, growth and evolution of a variety of organisms, including multicellular eukaryotes.

Trebouxiophytes of the genus Prasiola are well known in Antarctica, where they are among the most important primary producers. Although many aspects of their biology have been thoroughly investigated, the scarcity of molecular data has so far prevented an accurate assessment of their taxonomy and phylogenetic position. Using sequences of the chloroplast genes rbcL and psaB, we demonstrate the existence of three cryptic species that were previously confused under Prasiola crispa (Lightfoot) Kützing. Genuine P. crispa occurs in Antarctica; its presence was confirmed by comparison with the rbcL sequence of the type specimen (from the Isle of Skye, Scotland). Prasiola antarctica Kützing is resurrected as an independent species to designate algae with gross morphology identical to P. crispa but robustly placed in a separate lineage. The third species is represented by specimens identified as P. calophylla (Carmichael ex Greville) Kützing in previous studies, but clearly separated from European P. calophylla (type locality: Argyll, Scotland); this alga is described as P. glacialis sp. nov. The molecular data demonstrated the presence of P. crispa in Maritime and Continental Antarctica. P. antarctica was recorded from the Antarctic Peninsula and Shetland Islands, and P. glacialis from the Southern Ocean islands and coast. Such unexpected cryptic diversity highlights the need for a taxonomic reassessment of many published Antarctic records of P. crispa. The results also indicate that marine species of Prasiola form a well-supported monophyletic group, whereas the phylogenetic diversity of freshwater species is higher than previously suspected (at least three separate lineages within the genus include species living in this type of environments).

The variability in physical, chemicaland biologicalpropertieswas examinedforanumberof glacier melt streams in south Victoria Land, Antarctica. Streams flowed for between one and two months. Stream water temperatures (range =0-11 "C) varied over short (hr) time scales whilst discharges varied considerably between streams (range 0.001-15 mJ s-1) and over die1 cycles. Solar radiation and air temperature were major determinants of stream discharge. Variability in discharge was reflected in variability in nutrient chemistry and sediment load. Nitrogen and phosphorus varied considerably between streams; the meltwaters early in summer contained 10-20 times higher levels of dissolved X and P than later in the season. Within stream nutrient levels were modified by dense algal growths and penguin rookeries. Epilithic algal communities were made up predominantly of cyanophyceae which formed mats and crusts. Longitudinal and horizontal variability of species in the communities in selected streams is described. Analyses of algal cover and biomass (chlorophyll a ) show that substrate type and flow rates are of greater importance than nutrients in influencing algal abundance and biomass. I n some streams biomass values of over 20 !ig Ch. n cni-2 were recorded. murh of which remains viable but inactive over the antarctic winter.