Garth Watson

The short-term effects of salinization on anaerobic nutrient cycling and microbial community structure in sediment from a freshwater wetland

Wetlands in many inland catchments are being subjected to increasing salinity. To expand our limited understanding of how increasing salinity will alter carbon and nutrient dynamics in freshwater sediments, we carried out microcosm experiments to examine the acute effects of increasing salinity on the anaerobic cycling of carbon, nutrients (N, P, and S), metals (Fe and Mn), and microbial community structure in sediments from a non-salt-impacted freshwater wetland. Sediments were collected from a wetland on the River Murray floodplain, south eastern Australia and incubated with NaCl concentrations ranging from 0 to 100 mmol L−1. Increasing NaCl concentration led to the immediate release of between about 80 and 190 μmol L−1 ammonium and 235 to 3300 μmol L−1 Fe(II) from the sediments, the amount released ‘increasing with NaCl concentration. Conversely, net phosphate release decreased with increasing NaCl concentration. The overall microbial community structure, determined from phospholipid fatty acid profiles, changed only at the highest NaCl loadings, with evidence of a decrease in microbial diversity. Bacterial community structure, determined by examining terminal restriction fragment length polymorphism (T-RFLP) of the bacterial 16S rRNA gene, showed little response to increasing NaCl concentration. Conversely, the archaeal (methanogen) population, determined by examining T-RFLP of the archaeal 16S rRNA gene, showed significant changes with increasing NaCl loading. This shift corresponded with a significant decrease in methane production from salt-impacted sediments and therefore shows a linkage between microbial community structure and an ecosystem process.

Impacts of inundation and drought on eukaryote biodiversity in semi-arid floodplain soils

Floodplain ecosystems are characterized by alternating wet and dry phases and periodic inundation defines their ecological character. Climate change, river regulation and the construction of levees have substantially altered natural flooding and drying regimes worldwide with uncertain effects on key biotic groups. In southern Australia, we hypothesized that soil eukaryotic communities in climate change affected areas of a semi‐arid floodplain would transition towards comprising mainly dry‐soil specialist species with increasing drought severity. Here, we used 18S rRNA amplicon pyrosequencing to measure the eukaryote community composition in soils that had been depleted of water to varying degrees to confirm that reproducible transitional changes occur in eukaryotic biodiversity on this floodplain. Interflood community structures (3 years post‐flood) were dominated by persistent rather than either aquatic or dry‐specialist organisms. Only 2% of taxa were unique to dry locations by 8 years post‐flood, and 10% were restricted to wet locations (inundated a year to 2 weeks post‐flood). Almost half (48%) of the total soil biota were detected in both these environments. The discovery of a large suite of organisms able to survive nearly a decade of drought, and up to a year submerged supports the concept of inherent resilience of Australian semi‐arid floodplain soil communities under increasing pressure from climatic induced changes in water availability.

Variability in sediment microbial communities in a semipermanent stream: impact of drought

Abstract Terminal-restriction fragment length polymorphism (T-RFLP) is a DNA-based technique used to examine microbial community structure. We used this technique to examine variability in microbial community structure in 3 pools and a riffle in a semipermanent stream that becomes a chain of pools during the summer. We examined microbial communities under 3 hydrological conditions: predrought (all sites inundated), drought (all sediments dried for ≥2 mo), and rewet (1 mo after inundation). We used nonmetric multidimensional scaling to ordinate microbial communities and analysis of similarity to test whether communities differed among sites and whether community structure within sites changed in response to drought and rewetting. Microbial communities within sites differed with respect to hydrological condition, with most within-site variability occurring within the riffle section of the stream. Sediment drying significantly changed the microbial community structure at all sites. One month after rewetting at all sites, the microbial communities had not returned to their predrought structures and were significantly different from the predrought and drought microbial communities. Within-site variability in microbial community structure was much lower after the drought than before the drought. T-RFLP proved to be a powerful method for resolving microbial community structure, and its application helped address our limited understanding of microbial community dynamics.

Spatial and temporal variability of nitrogen dynamics in an upland stream before and after a drought

In the current study, we explore the spatial and temporal variability of ammonia, nitrate and urea dynamics in an upland stream before and after a major drying event, using short-term nitrogen additions to benthic chambers. The potential for an initial flush of mineral nitrogen from re-wetted sediments following a prolonged period of drying was also assessed. The distribution of dissolved nitrogen species at four sites spaced along a 1-km reach of the stream were quite variable over time but, in general, not between sites. Conversely, sediment nitrogen dynamics were spatially variable. For example, in one instance, sediments from the uppermost site were a net sink for ammonia, whereas the sediments immediately downstream (separated from the first site by a small sand bar) were a net source of ammonia; with measured sediment fluxes up to ~2 µg N m−2 s−1. In general, the short-term addition of nitrate, ammonia or urea did not substantially affect the sediment nitrogen dynamics. After ~3 months of in situ drying, upon re-wetting, the sediments from all sites immediately produced pulses of ammonia, nitrate and, to a substantially lesser extent, urea. The rates of release of nitrogen were spatially variable, with up to an order of magnitude difference in the rate of release of ammonia from re-wetted sediments from the same small pool. Some differences were observed between nitrogen dynamics before and after drying but a causal linkage could not be established.

Sulfide formation in freshwater sediments, by sulfate-reducing microorganisms with diverse tolerance to salt.

Understanding how sulfate-reducing microbes in freshwater systems respond to added salt, and therefore sulfate, is becoming increasingly important in inland systems where the threat from salinisation is increasing. To address this knowledge gap, we carried out mesocosm studies to determine how the sulfate-reducing microbial community in sediments from a freshwater wetland would respond to salinisation. The levels of inorganic mineral sulfides produced after 6months incubation were measured to determine whether they were in sufficient quantity to be harmful if re-oxidized. Comparative sequence analysis of the dissimilatory sulfite reductase (DSR) gene was used to compare the sulfate-reducing community structure in mesocosms without salt and those incubated with moderate levels of salt. The amount of total S, acid volatile sulfide or chromium-reducible sulfide produced in sediments with 0, 1 or 5gL(-1) added salt were not significantly different. Sediments subjected to 15gL(-1) salt contained significantly higher total S and acid volatile sulfide, and levels were above trigger values for potential harm if re-oxidation occurred. The overall community structure of the sulfate-reducing microbiota (SRM) was explained by the level of salt added to sediments. However, a group of sulfate reducers were identified that occurred in both the high salt and freshwater treatments. These results demonstrate that freshwater sediments contain sulfate reducers with diverse abilities to respond to salt and can respond rapidly to increasing salinity, explaining the observation that harmful levels of acid volatile sulfides can form rapidly in sediments with no history of exposure to salt.

Decomposition of native leaf litter by aquatic hyphomycetes in an alpine stream

Despite the recognised significance of hyphomycetes in the degradation of leaf litter in streams, few studies have been carried out in alpine environments and none in Australian alpine streams. We hypothesised that the fungal communities responsible for leaf decomposition would change over immersion time, and would respond differently at different sites and on different types of vegetation. Leaf bags containing Epacris glacialis (F. Muell.), Eucalyptus pauciflora (Sieber ex. Spreng) and Eucalyptus delegatensis (R.T. Baker) were deployed at different sites in a stream in the Victorian Alpine National Park, south-eastern Australia. Leaf colonisation was delayed for 2 weeks and decay constants for E. pauciflora and E. delegatensis were 0.004–0.005 and 0.006 respectively. Maximum fungal biomass on leaves was similar to that in previous published studies, whereas sporulation rates were two or three orders of magnitude lower, indicating a reduced reproductive effort. Sporulation and DNA-based studies combined showed that fungal communities on the decomposing leaf material changed over time and exhibited significant preferences for leaf type and study site. We have shown that aquatic hyphomycetes can degrade physically tough leaves of Australian alpine plant species, potentially contributing to pathways for particulate carbon to enter alpine-stream food webs.

Changes in sediment microbial community structure within a large water-storage reservoir during an extreme drawdown event

Although drought and drying of waters occur globally, the effect of drying on sediment microbial communities underpinning aquatic biogeochemical processes is poorly understood. We used the molecular method of terminal-restriction fragment length polymorphism (T-RFLP) to assess changes in the microbial community structure of sediments undergoing different levels of inundation and drying within a reservoir during drawdown in a drought. Sediments with three hydrological conditions were investigated: dry sediments (no overlying water), littoral sediments (covered with 1–2 mm water) and inundated sediments (covered with >1 m water). Sampling was done in winter 2006 (August) and summer 2007 (January) in Lake Hume, Australia. The microbial communities differed significantly between the different levels of inundation at each sampling time. Community structure also changed significantly within each site between winter 2006 and summer 2007, possibly influenced by the change of season or protracted drying. Sites that were ‘littoral’ in winter 2006 became ‘dry’ in summer 2007, and became more similar to communities that were ‘dry’ at both sampling times. This suggested that the hydrological history of specific sites did not heavily influence the response of microbial communities to severe drying, and all communities undergoing ‘dry’ conditions within the summer 2007 sampling responded similarly.