塔里木盆地晚泥盆世东河塘组河口湾相遗迹化石

河口湾具有特定的环境条件和沉积组成,其遗迹化石具有半咸水沉积的遗迹群落特征。描述和分析塔里木盆地塔中4井区上泥盆统东河塘组河口湾沉积中的遗迹化石和生物扰动构造后,发现3类遗迹组构,其中Ophiomorpha遗迹组构发育在纯净砂岩中,与河口湾潮汐砂坝有关;Skolithos遗迹组构发育在薄层砂岩中,与河口湾砂坪有关;Palaeophycus遗迹组构发育在泥岩中,与河口湾泥坪、砂泥坪有关。     

Ichnodiversity and ichnodisparity: significance and caveats

Ichnodiversity has been used as a proxy for environmental stress and stability in facies interpretations and to reconstruct evolutionary radiations and colonization histories in evolutionary palaeoecology. The three components of global diversity are exported from ecology and adapted for ichnology. Alpha ichnodiversity is used for palaeoenvironmental characterization, being assessed for individual facies. Beta ichnodiversity is commonly overlooked, although it may provide information about degree of similarity between ichnofaunas formed along environmental gradients. Gamma ichnodiversity may provide clues to detect ichnofossil provincialism. The concept of disparity may also prove to be significant in ichnological studies. Whereas ichnodiversity refers to ichnotaxonomic richness, ichnodisparity provides a measure of the variability of morphological plans in biogenic structures. Changes in global ichnodiversity do not necessarily parallel changes in ichnodisparity. For example, while the Cambrian explosion involved a dramatic increase in both, the Ordovician radiation essentially reflects an increase only in the former. Ichnodiversity and ichnodisparity should be used with caution because they are both affected by taphonomic processes. High diversity of superficial to shallow‐tier trace fossils may result from enhanced preservation due a poorly developed mixed layer, rather than a true reflection of ecosystem performance, as shown by underexploited infaunal ecospace after biotic crises (e.g. end‐Permian mass extinction).     

pmoA-based analysis of methanotrophs in a littoral lake sediment reveals a diverse and stable community in a dynamic environment

Diversity and community structure of aerobic methane-oxidizing bacteria in the littoral sediment of Lake Constance was investigated by cloning analysis and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of the pmoA gene. Phylogenetic analysis revealed a high diversity of type I and type II methanotrophs in the oxygenated uppermost centimeter of the sediment. T-RFLP profiles indicated a high similarity between the active methanotrophic community in the oxic layer and the inactive community in an anoxic sediment layer at a 10-cm depth. There were also no major changes in community structure between littoral sediment cores sampled in summer and winter. By contrast, the fingerprint patterns showed substantial differences between the methanotrophic communities of littoral and profundal sediments.     

pmoA-Based Analysis of Methanotrophs in a Littoral Lake Sediment Reveals a Diverse and Stable Community in a Dynamic Environment

Diversity and community structure of aerobic methane-oxidizing bacteria in the littoral sediment of Lake Constance was investigated by cloning analysis and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of the pmoA gene. Phylogenetic analysis revealed a high diversity of type I and type II methanotrophs in the oxygenated uppermost centimeter of the sediment. T-RFLP profiles indicated a high similarity between the active methanotrophic community in the oxic layer and the inactive community in an anoxic sediment layer at a 10-cm depth. There were also no major changes in community structure between littoral sediment cores sampled in summer and winter. By contrast, the fingerprint patterns showed substantial differences between the methanotrophic communities of littoral and profundal sediments.     

Influences of infaunal burrows on the community structure and activity of ammonia-oxidizing bacteria in intertidal sediments

Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O(2) concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O(2) transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH(4)(+) consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH(4)(+) and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O(2) transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH(4)(+) consumption capacity. Consequently, the infaunal burrow became an important site for NH(4)(+) consumption in the intertidal sediment.     

Ediacaran–Cambrian bioturbation did not extensively oxygenate sediments in shallow marine ecosystems

The radiation of bioturbation during the Ediacaran–Cambrian transition has long been hypothesized to have oxygenated sediments, triggering an expansion of the habitable benthic zone and promoting increased infaunal tiering in early Paleozoic benthic communities. However, the effects of bioturbation on sediment oxygen are underexplored with respect to the importance of biomixing and bioirrigation, two bioturbation processes which can have opposite effects on sediment redox chemistry. We categorized trace fossils from the Ediacaran and Terreneuvian as biomixing or bioirrigation fossils and integrated sedimentological proxies for bioturbation intensity with biogeochemical modeling to simulate oxygen penetration depths through the Ediacaran–Cambrian transition. Ultimately, we find that despite dramatic increases in ichnodiversity in the Terreneuvian, biomixing remains the dominant bioturbation behavior, and in contrast to traditional assumptions, Ediacaran–Cambrian bioturbation was unlikely to have resulted in extensive oxygenation of shallow marine sediments globally.     

Depth-discrete metagenomics reveals the roles of microbes in biogeochemical cycling in the tropical freshwater Lake Tanganyika

Lake Tanganyika (LT) is the largest tropical freshwater lake, and the largest body of anoxic freshwater on Earth’s surface. LT’s mixed oxygenated surface waters float atop a permanently anoxic layer and host rich animal biodiversity. However, little is known about microorganisms inhabiting LT’s 1470 meter deep water column and their contributions to nutrient cycling, which affect ecosystem-level function and productivity. Here, we applied genome-resolved metagenomics and environmental analyses to link specific taxa to key biogeochemical processes across a vertical depth gradient in LT. We reconstructed 523 unique metagenome-assembled genomes (MAGs) from 34 bacterial and archaeal phyla, including many rarely observed in freshwater lakes. We identified sharp contrasts in community composition and metabolic potential with an abundance of typical freshwater taxa in oxygenated mixed upper layers, and Archaea and uncultured Candidate Phyla in deep anoxic waters. Genomic capacity for nitrogen and sulfur cycling was abundant in MAGs recovered from anoxic waters, highlighting microbial contributions to the productive surface layers via recycling of upwelled nutrients, and greenhouse gases such as nitrous oxide. Overall, our study provides a blueprint for incorporation of aquatic microbial genomics in the representation of tropical freshwater lakes, especially in the context of ongoing climate change, which is predicted to bring increased stratification and anoxia to freshwater lakes.Subject terms: Biogeochemistry, Limnology, Microbial ecology, Freshwater ecology     

The Importance of Vertical and Horizontal Dimensions of the Sediment Matrix in Structuring Nematodes Across Spatial Scales

Intensive surveys have been conducted to unravel spatial patterns of benthic infauna communities. Although it has been recognized that benthic organisms are spatially structured along the horizontal and vertical dimensions of the sediment, little is known on how these two dimensions interact with each other. In this study we investigated the interdependence between the vertical and horizontal dimensions in structuring marine nematodes assemblages. We tested whether the similarity in nematode species composition along the horizontal dimension was dependent on the vertical layer of the sediment. To test this hypothesis, three-cm interval sediment samples (15 cm depth) were taken independently from two bedforms in three estuaries. Results indicated that assemblages living in the top layers are more abundant, species rich and less variable, in terms of species presence/absence and relative abundances, than assemblages living in the deeper layers. Results showed that redox potential explained the greatest amount (12%) of variability in species composition, more than depth or particle size. The fauna inhabiting the more oxygenated layers were more homogeneous across the horizontal scales than those from the reduced layers. In contrast to previous studies, which suggested that reduced layers are characterized by a specific set of tolerant species, the present study showed that species assemblages in the deeper layers are more causal (characterized mainly by vagrant species). The proposed mechanism is that at the superficial oxygenated layers, species have higher chances of being resuspended and displaced over longer distances by passive transport, while at the deeper anoxic layers they are restricted to active dispersal from the above and nearby sediments. Such restriction in the dispersal potential together with the unfavorable environmental conditions leads to randomness in the presence of species resulting in the high variability between assemblages along the horizontal dimension.     

AUTECOLOGY AND THE FILLING OF ECOSPACE: KEY METAZOAN RADIATIONS

Abstract:  All possible combinations of six tiering positions in relation to the substratum/water interface, six motility levels and six feeding strategies define a complete theoretical ecospace of 216 potential modes of life for marine animals. The number of modes of life actually utilized specifies realized ecospace. Owing to constraints of effectiveness and efficiency the modern marine fauna utilizes only about half the potential number of modes of life, two-thirds of which (62 of 92) are utilized by animals with readily preserved, mineralized hard parts. Realized ecospace has increased markedly since the early evolution of animal ecosystems. The Ediacaran fauna utilized at most 12 modes of life, with just two practised by skeletal organisms. A total of 30 modes of life are recorded in the Early and Middle Cambrian, 19 of which were utilized by skeletal organisms. The other 11 are documented from soft-bodied animals preserved in the Chengjiang and Burgess Shale Konservat-Lagerstätten. The number of modes of life utilized by skeletal organisms increased by more than 50 per cent during the Ordovician radiation to a Late Ordovician total of 30. Between the Late Ordovician and the Recent the number of utilized modes of life has doubled again. The autecological and taxonomic diversity histories of the marine metazoa appear to be broadly parallel, and future studies of theoretical ecospace utilization should provide more detailed tests of pattern and process in the ecological history of the metazoa.     

The evolutionary ecology of biotic association in a megadiverse bivalve superfamily: sponsorship required for permanent residency in sediment

Background

Marine lineage diversification is shaped by the interaction of biotic and abiotic factors but our understanding of their relative roles is underdeveloped. The megadiverse bivalve superfamily Galeommatoidea represents a promising study system to address this issue. It is composed of small-bodied clams that are either free-living or have commensal associations with invertebrate hosts. To test if the evolution of this lifestyle dichotomy is correlated with specific ecologies, we have performed a statistical analysis on the lifestyle and habitat preference of 121 species based on 90 source documents.

Methodology/Principal Findings

Galeommatoidea has significant diversity in the two primary benthic habitats: hard- and soft-bottoms. Hard-bottom dwellers are overwhelmingly free-living, typically hidden within crevices of rocks/coral heads/encrusting epifauna. In contrast, species in soft-bottom habitats are almost exclusively infaunal commensals. These infaunal biotic associations may involve direct attachment to a host, or clustering around its tube/burrow, but all commensals locate within the oxygenated sediment envelope produced by the host’s bioturbation.

Conclusions/Significance

The formation of commensal associations by galeommatoidean clams is robustly correlated with an abiotic environmental setting: living in sediments (). Sediment-dwelling bivalves are exposed to intense predation pressure that drops markedly with depth of burial. Commensal galeommatoideans routinely attain depth refuges many times their body lengths, independent of siphonal investment, by virtue of their host’s burrowing and bioturbation. In effect, they use their much larger hosts as giant auto-irrigating siphon substitutes. The evolution of biotic associations with infaunal bioturbating hosts may have been a prerequisite for the diversification of Galeommatoidea in sediments and has likely been a key factor in the success of this exceptionally diverse bivalve superfamily.     

Macrobenthic Distribution within the Sediment along an Estuarine Salinity Gradient

The distribution of the macrobenthic infaunal community within the upper 25 cm of the sediment was studied at 16 stations in the lower Chesapeake Bay. Stations were located from the tidal freshwater to the polyhaline zone of major tributaries (James, York and Rappahannock Rivers) and in the polyhaline portion of the lower bay mainstem. Profiles for total number of individuals, total ash-free dry weight biomass and species encountered with depth were calculated. Except for the deep dwelling bivalve, Macoma balthica, tributary macrobenthic communities had a shallow depth distribution compared to the mainstem sites which were found in generally coarser sediments in the higher salinity region of the estuary.     

Composition, abundance and stratification of soft-bottom macrobenthos from selected areas of the Ross Sea shelf (Antarctica)

Composition, abundance and stratification of soft-bottom macrobenthos were studied at three selected sites on the Ross Sea shelf (Antarctica) with different geomorphology and sedimentation regimes. Sites A (southwest Ross Sea, 810 m depth) and B (Joides basin, 580 m depth) were characterized by biogenic mud and clay sediments, whereas site C (Mawson bank, 450 m depth) featured sandy sediments mixed with a conspicuous biogenic component characterized by shells and tests of calcareous invertebrates (mainly barnacles of the genus Bathylasma). The macrofauna of sites A and B was mainly composed of infaunal polychaetes and bivalves. The assemblages comprised both surface and sub-surface deposit feeders, including some conveyor-belt polychaetes (Maldanidae and Capitellidae) that are responsible for high sediment mixing and bioturbation. The macrobenthos of site C was dominated by crustaceans, polychaetes and echinoderms (ophiuroids), and mainly by filter feeders and epifaunal or interstitial forms. Abundances were higher (up to 1040 ind. m⁻²) at site B than at sites A and C (430 and 516 ind. m⁻², respectively). At sites A and B the benthos was mainly concentrated in the upper 5 cm of the sediment, and abundances declined sharply in the deeper sediment layers. These results indicate a high degree of consistency between sediment features and benthic community structure, which are both strongly related to local hydrography and bottom dynamics. Sites A and B represent areas where the organic input to the seafloor by vertical sedimentation from the upper water column is high. Site C, however, is characterized by high sediment instability and food particles advecting mainly horizontally. The community is more physically controlled by unpredictable, and probably frequent, disturbance events (e.g., bottom turbid currents, sediment reworking and displacement). Individuals were relatively small, indicating that probably they are not able to grow up to the adult size and reproduce. The community may be represented by “pseudopopulations” depending on the settlement of larvae invading from neighbouring areas. Accepted: 23 October 1998     

Influences of Infaunal Burrows on the Community Structure and Activity of Ammonia-Oxidizing Bacteria in Intertidal Sediments

Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O₂ concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O₂ transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH₄⁺ consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH₄⁺ and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O₂ transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH₄⁺ consumption capacity. Consequently, the infaunal burrow became an important site for NH₄⁺ consumption in the intertidal sediment.     

Infaunal communities and tiering in Early Palaeozoic nearshore clastic environments: trace-fossil evidence from the Cambro-Ordovician of New South Wales

Understanding of the palaeoecology of Early Palaeozoic shallow-marine communities in sandy habitats is incomplete. Reasons for this include the poor preservation of body fossils and the dominance of Skolithos piperock in sandstones of that age. Cambrian and Ordovician deposits preserving diverse nearshore trace-fossil assemblages are therefore of critical importance for assessing the early evolution of marine ecosystems. The basal part of the Cambro-Ordovician Bynguano Formation of the Mootwingee area (New South Wales, Australia) is a series of interbedded sandstones and mudstones, which were deposited under nearshore conditions. These strata provide an early example of the Arenicolites ichnofacies. Two ichnocoenoses are distinguished in the sandstones of this unit. A 'predepositional' ichnocoenosis, which reflects the benthic community prior to episodes of sand deposition, includes dense aggregations of Rusophycus with rare Planolites. The 'postdepositional' ichnocoenosis is more diverse and includes Thalassinoides, Arenicolites (various types), Monocraterion, Skolithos, Trichichnus , and epichnial grooves. The tiered structure developed in this ichnocoenosis is preserved as a 'frozen tiered profile' and is characterised by a Thalassinoides tier, 10–30 cm in depth, which is cross-cut by Skolithos and Arenicolitesin the middle tier, and by Arenicolites and Trichichnusin the shallowest tier. The pattern of tiering indicates that a complex ecosystem of opportunistic organisms, capable of exploiting shifting substrates, had evolved by the earliest Ordovician. □ Predepositional, postdepositional frozen tier profile, ichnocoenosis, nearshore clastics, RUSOPHYCUS, Cambro-Ordovician, Bynguano Formation, Mootwingee, New South Wales.     

放牧强度对高寒嵩草草甸土壤养分特性的影响

植物-土壤系统是草地生态和生产服务价值实现的基础,放牧是草地植物群落演替的重要因素。植物、土壤亚系统对放牧的敏感性是评价草地稳定性和提高草地恢复力的重要依据。以不同放牧强度下的高寒嵩草(Kobresia)草甸为研究对象,探讨土壤养分特征对放牧强度的响应及作用位点,结果表明:改变放牧强度可以明显改变植物群落数量特征,但没有明显改变土壤层次分类特征,说明土壤养分特征对一定范围内放牧强度具有自我稳定维持功能;但放牧干扰强度不同时,土壤剖面过渡层养分含量存在差异,说明长期放牧强度的差异会对土壤剖面养分性质产生影响,且这种影响起源于土壤剖面过渡层。在放牧高寒嵩草草甸植物-土壤系统中土壤剖面养分特征较植物群落数量特征更稳定;土壤剖面过渡层养分特征是土壤亚系统中对放牧的敏感因素;而放牧引起土壤剖面养分特征的改变主要表现在各过渡层上,并构成土壤发生层迁移的风险,因此推测,更为持久和更高强度的放牧干扰将最终改变土壤剖面特征及养分性质。     

Effects of shallow and deep sediment disturbance on whole-stream metabolism in experimental sand-bed flumes

Water flow causes complex patterns of sediment disturbance in sand-bed streams, but effects on stream metabolism resulting from different depths of sediment scour and fill are poorly known. We assessed such effects by manually disturbing sandy sediments of 16 experimental outdoor flumes to two different depths (1 and 4 cm) during an early and a more advanced stage of stream community succession. To separate effects on heterotrophic and autotrophic metabolism, half of the flumes were permanently covered. At the early successional stage, sediment disturbance did not affect net community production (NCP), while sediment mixing reduced production independent of disturbance depth in the later stage. Microbial respiration, in contrast, was significantly stimulated when sediment was mixed to greater depth. These results suggest that disturbing sediments during early successional stages has no effect on whole-stream metabolism, whereas at later stages, deep sediment disturbance can lead to a transitory shift toward heterotrophy. The recovery time of NCP from perturbation was independent of disturbance depth. Similar trajectories observed after deep and shallow sediment disturbance indicate that delayed recovery was not simply due to mixing algae into deeper sediment layers but primarily a result of disrupting the fine structure of the surface sediment.     

Meiofaunal colonization of azoic estuarine sediment in Louisiana: Mechanisms of dispersal1

Two mechanisms of muddy-bottom meiofaunal dispersal, waterborne suspended transport and holobenthic infaunal immigration, were compared as to their rate and effectiveness in mediating community reestablishment after small-scale defaunation. Colonizing meiofauna were quantitatively sampled in winter and summer from 16 replicates of two azoic sediment chamber designs on 2 and 29 days postplacement. The chambers were ≈ 3750 cm³; one design allowed colonization via suspended movement through an open top, while the other design permitted entry only by infaunal crawling through subsurface open sides. After 48 h, mean harpacticoid copepod and naupliar densities in sediment chambers open to colonization exclusively by meiofauna in suspended transport were not significantly different from background sediment densities. Sediment chambers allowing colonization exclusively via infaunal immigration through the sediment, however, contained copepod and naupliar densities that were significantly less than densities in background sediments and suspension-colonized chambers. In contrast, nematode densities in both suspension- and infaunally colonized chambers were significantly less than in background sediments, but densities were not significantly different between the chamber treatments. Thus for a small-scale defaunation, copepods most rapidly and completely recolonize sediments via suspended transport. Nematode dispersal occurs equally well via suspended or infaunal movement; however nematodes never seemed to utilize the chambers fully because densities did not reach background levels even after 29 days.     

Vertical segregation and seasonal changes in fish depth distributions between lakes of contrasting trophic status

High frequency (200kHz) echosounding was used to determine vertical structuring and seasonal changes in the depth distributions of limnetic fish in three New Zealand lakes. Juvenile (f.l. 30–50 mm) and large-sized (f.l. 50–80 mm) smelt Retropinna retropinna were segregated into discrete layers by depth in all three lakes throughout the year. Larval and post-larval bullies Gobiomorphus cotidianus (f.l. 8–20 mm) formed a further layer between the layers of small and iarge-sized smelt. Although the depths of the large-sized smelt and bully layers changed on a seasonal basis, vertical segregation between the three layers of fish persisted throughout the year, even when the lakes were homothermal. Seasonal movements of fish were disrupted by the autumnal deoxygenation of the hypolimnion in Lake Rotoiti. The schools of large-sized smelt were forced to ascend towards the layer of bullies, but segregation between these smelt and the bullies was maintained as the smelt occupied a much reduced depth range above the hypolimnion, and just below the bullies. The winter descent of bullies, which precedes their transition to a benthic existence failed to occur in Lake Rotoiti, even though the lake was fully mixed at this time.     

Arsenic(V) Reduction in Relation to Iron(III) Transformation and Molecular Characterization of the Structural and Functional Microbial Community in Sediments of a Basin-Fill Aquifer in Northern Utah

Basin-fill aquifers of the Southwestern United States are associated with elevated concentrations of arsenic (As) in groundwater. Many private domestic wells in the Cache Valley Basin, UT, have As concentrations in excess of the U.S. EPA drinking water limit. Thirteen sediment cores were collected from the center of the valley at the depth of the shallow groundwater and were sectioned into layers based on redoxmorphic features. Three of the layers, two from redox transition zones and one from a depletion zone, were used to establish microcosms. Microcosms were treated with groundwater (GW) or groundwater plus glucose (GW+G) to investigate the extent of As reduction in relation to iron (Fe) transformation and characterize the microbial community structure and function by sequencing 16S rRNA and arsenate dissimilatory reductase (arrA) genes. Under the carbon-limited conditions of the GW treatment, As reduction was independent of Fe reduction, despite the abundance of sequences related to Geobacter and Shewanella, genera that include a variety of dissimilatory iron-reducing bacteria. The addition of glucose, an electron donor and carbon source, caused substantial shifts toward domination of the bacterial community by Clostridium-related organisms, and As reduction was correlated with Fe reduction for the sediments from the redox transition zone. The arrA gene sequencing from microcosms at day 54 of incubation showed the presence of 14 unique phylotypes, none of which were related to any previously described arrA gene sequence, suggesting a unique community of dissimilatory arsenate-respiring bacteria in the Cache Valley Basin.