Hypoliths (cryptic microbial assemblages that develop on the undersides of translucent rocks) are significant contributors to regional C and N budgets in both hot and cold deserts. Previous studies in the Dry Valleys of Eastern Antarctica have reported three morphologically distinct hypolithic community types: cyanobacteria dominated (type I), fungus dominated (type II) and moss dominated (type III). Here we present terminal-restriction fragment length polymorphism analyses to elucidate the bacterial community structure in hypolithons and the surrounding soils. We show clear and robust distinction in bacterial composition between bulk surface soils and hypolithons. Moreover, the bacterial assemblages were similar in types II and III hypolithons and clearly distinct from those found in type I. Through 16S rRNA gene 454 pyrosequencing, we show that Proteobacteria dominated all three types of hypolithic communities. As expected, Cyanobacteria were more abundant in type I hypolithons, whereas Actinobacteria were relatively more abundant in types II and III hypolithons, and were the dominant group in soils. Using a probabilistic dissimilarity metric and random sampling, we demonstrate that deterministic processes are more important in shaping the structure of the bacterial community found in types II and III hypolithons. Most notably, the data presented in this study suggest that hypolithic bacterial communities establish via a successional model, with the type I hypolithons acting as the basal development state.

Hypolithic microbial communities (i.e. cryptic microbial assemblages found on the undersides of translucent rocks) are major contributors of carbon input into the oligotrophic hyper-arid desert mineral soils of the Eastern Antarctic Dry Valleys. Here we demonstrate, for the first time, that hypolithic microbial communities possess both the genetic capacity for nitrogen fixation (i.e. the presence of nifH genes) and the ability to catalyse acetylene reduction, an accepted proxy for dinitrogen fixation. An estimate of the total contribution of these communities suggests that hypolithic communities are important contributors to fixed nitrogen budgets in Antarctic desert soils.

The Miers Valley within the McMurdo DryValleys of Antarctica supports abundant quartz and marble substrates for hypolithons-microbial colonists on the underside of these translucent rocks. Three physically distinct hypolithic community types have been identified: cyanobacteria dominated (Type I), fungus dominated (Type II) or moss dominated (Type III). The distribution of the three types was mapped across much of the *75 km 2 area of the upper Miers Valley and correlated this with the measurements of selected micro-environmental variables. Type I hypolithons were most common and occurred at all altitudes up to 824 m, whilst Type II and Type III hypolithons were less abundant and restricted to lower altitudes on the valley floor (\415 m and \257 m, respectively). Whilst all colonized quartz effectively filtered incident UVB irradiance, transmittance levels for UVA and PAR varied markedly and were significant in determining hypolith type. Notably, the Type I hypolithons occurred under rocks with a significantly lower transmittance of photosynthetically active radiation than Type II and III hypolithons. Altitude and aspect were also significant factors determining hypolith type, and a role for altitude-related abiotic variables in determining the distribution of Type I, II and III hypolithons is proposed.

Two sets of arsenic resistance genes were isolated from the highly arsenic-resistant Leptospirillum ferriphilum Fairview strain. One set is located on a transposon, TnLfArs, and is related to the previously identified TnAtcArs from Acidithiobacillus caldus isolated from the same arsenopyrite biooxidation tank as L. ferriphilum. TnLfArs conferred resistance to arsenite and arsenate and was transpositionally active in Escherichia coli. TnLfArs and TnAtcArs were sufficiently different for them not to have been transferred from one type of bacterium to the other in the biooxidation tank. The second set of arsenic resistance genes conferred very low levels of resistance in E. coli and appeared to be poorly expressed in both L. ferriphilum and E. coli.Processes for the biooxidation of gold-bearing arsenopyrite concentrates were developed in the 1980s and are now used in several countries (17). These are among the largest commercial fermentation processes known. Biooxidation is a mineral pretreatment process during which the molecular structure of the arsenopyrite mineral is broken down, exposing the gold and allowing its extraction by cyanide (7). During this process, large quantities of arsenic are released into continuous-flow aeration tanks in which the biooxidation takes place. The majority of arsenopyrite biooxidation processes operate at 40°C and are dominated by a mixture of the sulfur-oxidizing bacterium Acidithiobacillus caldus and the iron-oxidizing bacterium Leptospirillum ferriphilum. During the first few years of operation, the continuous-flow nature of the processes resulted in the selection of highly arsenic-resistant bacteria.Studies to investigate what genetic changes had taken place that accompanied this high level of arsenic resistance have been carried out with A. caldus. Highly arsenic-resistant strains of A. caldus have been found to contain an unusual Tn21-like ars operon that is not present in less-resistant strains (6,23,24). The 12-kb TnAtcArs is unusual in that the tnpA (transposase) and tnpR (resolvase) genes occur on opposite ends of the transposon, unlike other transposons of the Tn21and Tn3 family, where they form an adjacent unit. These two transposon genes flank a series of genes (arsRCDADA orf7 orf8B) associated with arsenic resistance that are themselves unusual. Genes for an ArsR (negative regulator) (20, 27) and ArsC (arsenate reductase) (10, 11) are followed by a tandem duplication of the genes for ArsD (a second repressor) (28) and ArsA (an ATPase that associates with ArsB and links arsenite export to ATP hydrolysis) (22). These genes are followed by genes encoding ORF7 (an NADH-like oxidoreductase), ORF8 (a cystathione-␤-synthase [CBS] domain-containing protein), and ArsB, the arsenite efflux pump. Deletion of one copy of arsDA readily occurs, but this deletion does not appear to affect resistance when cloned on a multicopy plasmid in Escherichia coli (24). Similarly, the inactivation or deletion of ORF7 and ORF8 did not affect arsenic resistance in E. coli. Some strains of high...

A transposon, TnAtcArs, that carries a set of arsenic-resistance genes was isolated from a strain of the moderately thermophilic, sulfur-oxidizing, biomining bacterium Acidithiobacillus caldus. This strain originated from a commercial plant used for the bio-oxidation of gold-bearing arsenopyrite concentrates. Continuous selection for arsenic resistance over many years had made the bacterium resistant to high concentrations of arsenic. Sequence analysis indicated that TnAtcArs is 12 444 bp in length and has 40 bp terminal inverted repeat sequences and divergently transcribed resolvase and transposase genes that are related to the Tn21-transposon subfamily. A series of genes consisting of arsR, two tandem copies of arsA and arsD, two ORFs (7 and 8) and arsB is situated between the resolvase and transposase genes. Although some commercial strains of At. caldus contained the arsDA duplication, when transformed into Escherichia coli, the arsDA duplication was unstable and was frequently lost during cultivation or if a plasmid containing TnAtcArs was conjugated into a recipient strain. TnAtcArs conferred resistance to arsenite and arsenate upon E. coli cells. Deletion of one copy of arsDA had no noticeable effect on resistance to arsenite or arsenate in E. coli. ORFs 7 and 8 had clear sequence similarity to an NADH oxidase and a CBS-domain-containing protein, respectively, but their deletion did not affect resistance to arsenite or arsenate in E. coli. TnAtcArs was actively transposed in E. coli, but no increase in transposition frequency in the presence of arsenic was detected. Northern hybridization and reporter gene studies indicated that although ArsR regulated the 10 kb operon containing the arsenic-resistance genes in response to arsenic, ArsR had no effect on the regulation of genes associated with transposition activity.