Ellard R. Hunting

Invertebrate footprints on detritus processing, bacterial community structure, and spatiotemporal redox profiles

Abstract.  Detritus processing is driven by a complex interplay between macroinvertebrate and microbial activities. Bioturbation/feeding activities of invertebrates in sediments are known to influence decomposition rates. However, direct effects of invertebrates on bacterial communities and detritus processing remain ill-defined, mainly because identifying interactions between invertebrates and sediments is methodologically challenging. We incubated 5 macroinvertebrate species with various bioturbation/feeding traits separately in sediment-filled microcosms inoculated with bacterial communities for 5 d. At the end of the experiment, we assessed: 1) detritus processing (mass loss on ignition [LOI] and dissolved organic C accumulation in the overlying water [absorbance at 280 nm]), 2) bacterial community structure (intergenic spacer analysis [RISA]) and bacterial activity (electron transport system activity [ETSA]), and 3) development of redox potential (Eh) over time (with permanently installed microelectrodes). Invertebrates enhanced bacterial activity and detritus processing, and the magnitude depended on bioturbation/feeding traits. Bacterial community structure differed significantly between microcosms with burrowing invertebrates and microcosms with sediment-dwelling invertebrates. Eh profiles were similar among microcosms with invertebrates with similar bioturbation/feeding traits, but differed among microcosms with invertebrates with dissimilar bioturbation/feeding traits. Our results suggest that bioturbation by aquatic invertebrates mediates detritus processing, Eh dynamics, and structure of the microbial community. These findings highlight the significance of bioturbation and show the utility of spatiotemporal Eh dynamics as footprints reflecting functioning of benthic detrital food webs.

An improved datalogger and novel probes for continuous redox measurements in wetlands.

In soils and sediments, the redox potential (Eh) is an important parameter controlling the persistence of many organic and inorganic compounds. Especially in wetlands, fluctuations in redox potential values can be very large and depth dependent. For this reason, field deployable logging systems have previously been developed, yet these systems were limited in several aspects. Here we report the development of an improved multichannel datalogger (HYPNOS) and two novel probes for continuous monitoring of Eh profiles, and briefly illustrate the potential applications. The combination of a multichannel logger with different types of probes allows characterisation of spatial and temporal variability of redox potential in relation to environmental and ecological parameters, and we expect this will greatly enhance our knowledge of the functioning of wetlands.

UV radiation and organic matter composition shape bacterial functional diversity in sediments.

UV radiation and organic matter (OM) composition are known to influence the species composition of bacterioplankton communities. Potential effects of UV radiation on bacterial communities residing in sediments remain completely unexplored to date. However, it has been demonstrated that UV radiation can reach the bottom of shallow waters and wetlands and alter the OM composition of the sediment, suggesting that UV radiation may be more important for sediment bacteria than previously anticipated. It is hypothesized here that exposure of shallow OM-containing sediments to UV radiation induces OM source-dependant shifts in the functional composition of sediment bacterial communities. This study therefore investigated the combined influence of both UV radiation and OM composition on bacterial functional diversity in laboratory sediments. Two different OM sources, labile and recalcitrant OM, were used and metabolic diversity was measured with Biolog GN. Radiation exerted strong negative effects on the metabolic diversity in the treatments containing recalcitrant OM, more than in treatments containing labile OM. The functional composition of the bacterial community also differed significantly between the treatments. Our findings demonstrate that a combined effect of UV radiation and OM composition shapes the functional composition of microbial communities developing in sediments, hinting that UV radiation may act as an important sorting mechanism for bacterial communities and driver for bacterial functioning in shallow waters and wetlands.

Contribution of bacteria to redox potential (E h) measurements in sediments

Increasing evidence suggests that bacteria are capable of creating specific redox conditions which are visible as species-specific continuous redox potential (Eh) measurements. It has been demonstrated that continuous measurements of Eh are valuable for tracking bacterial metabolic activities of bacterial species in liquid cultures. However, it remains uncertain whether this phenomenon is widespread among bacterial species and whether Eh measurements reflect similar mechanisms in more complex systems such as soils and sediments. The purpose of this study is therefore to evaluate whether bacteria that naturally occur in sediments have the capacity to control Eh and assess the relative partitioning of biological processes involved in Eh in natural sediments. To this end, continuous Eh measurements are linked to growth of bacteria in liquid cultures and bacterial metabolic activity in aquatic sediment microcosms containing the bioturbator Tubifex spp., in which we evaluate bacterial partitioning in microcosms treated with the bacteriocide formalin. The tested bacterial species (Micrococcus luteus, Paracoccus pantotrophus and Aminobacter aminovarans) appeared to have specific stable Eh signals during linear-exponential growth phase, suggesting that these species are capable of exerting an extracellular control on Eh measurements, thereby supporting the notion that species-specific Eh signals may be widespread among bacterial species. Formalin treatment reduced temporal variability of Eh in sediment microcosms. This outcome suggests that bacterial metabolism and inherent relative contributions of members of bacterial community principally determine development of Eh in sediment systems and that quantitation of sediment electrochemical properties may offer a potential indicator that characterizes bacterial processes.

DECOTAB: a multipurpose standard substrate to assess effects of litter quality on microbial decomposition and invertebrate consumption

Abstract.  Currently available tools for studying plant litter decomposition and invertebrate consumption in aquatic ecosystems have at least 2 major limitations: 1) the difficulty of manipulating litter chemical composition to provide mechanistic insights into attributes of litter quality controlling decomposition rate, and 2) lack of a standardized litter that hampers comparisons of results among studies. These limitations point to a need for a standard litter surrogate with adjustable chemical composition. We propose using a decomposition and consumption tablet (DECOTAB) consisting of cellulose powder embedded in an agar matrix to evaluate decomposition and consumption rates in aquatic environments. We describe the preparation of DECOTABs and demonstrate some applications in laboratory microcosms and outdoor mesocosms. A leaf shredder, the isopod Asellus aquaticus, and a collector-gatherer, the nonbiting midge larva Chironomus riparius, readily consumed DECOTABs, leading to massive mass loss of the tablets within 21 d (∼90%). The isopod also consumed DECOTABs amended with extracts of riparian plants and soil to create a chemically complex source of organic matter. Our results highlight the potential utility of DECOTABs to assess invertebrate contributions to organic matter decomposition in aquatic systems. In the absence of invertebrates, exposure of basic and complex DECOTABs to microorganisms resulted in significant mass loss within 21 d (10–25%), and addition of an antibiotic and fungicide suppressed microbial decomposition, suggesting that the tablets are useful for studying microbial processes. Complex tablets decomposed faster than the basic tablets, a result illustrating the importance of chemical composition of organic material for microbial decomposers. DECOTABs are a novel, versatile tool for addressing long-standing questions in aquatic ecology and environmental assessment.

Predictability of bacterial activity and denitrification in aquatic sediments with continuous measurements of redox potential.

Redox potential has been adopted as a qualitative parameter for interpreting solubility changes of nutrients and contaminants and the biological activity within wetland systems for several decades. The majority of studies considering the redox geochemistry in sediments used measurements of bulked material and single point measurement of biogeochemical parameters for interpretation, yet it remains questionable whether this information is reliable for environments that are very dynamic, such as wetlands. In this study it is evaluated whether variations in redox potential reflect dynamics of denitrification and overall bacterial respiration using continuous measurements of redox potential in time-series experiments in laboratory microcosms, in which the biogeochemical variation was enhanced by bioturbation. The results presented here suggest that measurements of redox potential have predictive potential in approximating rates of denitrification and overall bacterial respiration in aquatic sediments. The data clearly suggest that, while sediment bulk measurements and measurements of single profiles of redox potential, denitrification and bacterial activity often fail to provide ecological relevant information in dynamic systems, measurements of spatial and temporal redox potential profiles provide a useful parameter that reflects biogeochemical processes and functioning of sediments.

Mangrove-sponge associations: a possible role for tannins

A positive correlation between sponge coverage and tannin concentrations in prop roots of Rhizophora mangle L. has previously been reported. However, the ecological role of tannins within the mangrove sponge association remains speculative. This study investigated whether tannins play a role in sponge recruitment and assessed tannin and polyphenol production in R. mangle roots in response to sponge colonization. We demonstrated in a field experiment using artificial substrates with different tannin concentrations that tannins are positively involved in larval recruitment of the sponge Tedania ignis and that roots significantly enhanced tannin and polyphenolic content in response to natural and experimental sponge fouling. Differential recruitment in response to tannins may have been the result of a behavioral response in sponge larvae. It is also possible that tannins affected the structure of the fouling microbial biofilm on the artificial substrate, or tannins affected the post-settlement dynamics of sponge recruits. Elevations in concentrations of tannins and polyphenolic compounds upon coverage with sponges, combined with differential recruitment of T. ignis in response to differences in tannin concentrations, may indicate a positive feedback in recruitment. This may in part explain the typical heterogeneity in sponge coverage and community composition among roots.

Resource niche overlap promotes stability of bacterial community metabolism in experimental microcosms

Decomposition of organic matter is an important ecosystem process governed in part by bacteria. The process of decomposition is expected to benefit from interspecific bacterial interactions such as resource partitioning and facilitation. However, the relative importance of resource niche breadth (metabolic diversity) and resource niche overlap (functional redundancy) on decomposition and the temporal stability of ecosystem processes received little scientific attention. Therefore, this study aims to evaluate the effect of an increase in bacterial community resemblance on both decomposition and the stability of bacterial metabolism in aquatic sediments. To this end, we performed laboratory microcosm experiments in which we examined the influence of bacterial consortia differing in number and composition of species on bacterial activity (Electron Transport System Activity, ETSA), dissolved organic carbon production and wavelet transformed measurements of redox potential (Eh). Single substrate affinities of the individual bacterial species were determined in order to calculate the metabolic diversity of the microbial community. Results presented here indicate that bacterial activity and organic matter decomposition increase with widening of the resource niche breadth, and that metabolic stability increases with increasing overlap in bacterial resource niches, hinting that resource niche overlap can promote the stability of bacterial community metabolism.

Silver Nanoparticles, Ions, and Shape Governing Soil Microbial Functional Diversity: Nano Shapes Micro.

Silver nanoparticles (AgNPs) affect microbial metabolic processes at single cell level or lab-culture strains. However, the impact of different AgNPs properties such as the particle, ion release, and shape on functional responses of natural soil microbial communities remain poorly understood. Therefore, we assessed the relative importance of particles and ions of AgNPs in bacterial toxicity and how the functional diversity of soil microbial communities were impacted by AgNPs shapes (i.e., plates, spheres, and rods) in laboratory incubations. Our results showed that the relative contribution of AgNPs(particle) increased with increasing exposure concentrations (accounted for about 60–68% of the total toxicity at the highest exposure level). In addition, the functional composition of the microbial community differed significantly according to different AgNPs shapes. The various properties of AgNPs thus can significantly and differentially affect the functional composition of microbial communities and associated ecosystem processes depending on the level of environmental exposure.

Postregistration monitoring of pesticides is urgently required to protect ecosystems

Current admission policies for pesticides follow a controlled experimental tiered risk assessment approach, giving results that are difficult to extrapolate to a real-world situation. Later analyses of compounds such as DDT and neonicotinoid pesticides clearly show that the actual chemical impacts frequently affect many more components of an ecosystem than a priori suggested by risk assessment. Therefore, to manage the actual risks for ecosystems imposed by manufactured compounds, it is proposed that current admission policies for chemicals be enriched by using postregistration monitoring. Such monitoring is essential to identify unexpected direct and indirect impacts on organisms by accounting for multiple propagation routes and exposures. Implementation of postregistration monitoring could build on existing monitoring networks. This approach would tackle the current policy impasse of compartment-based regulations versus exposure-based regulations, and, more importantly, would provide a safety lock for risk assessment across compartments and more likely ensure the protection of our natural environment. Environ Toxicol Chem 2017;36:860-865.