The gut microenvironment of sediment-dwelling Chironomus plumosus larvae as characterised with O2, pH, and redox microsensors

Microscale sampling of the coral gastrovascular cavity reveals a gut-like microbial community

Animal guts contain numerous microbes, which are critical for nutrient assimilation and pathogen defence. While corals and other Cnidaria lack a true differentiated gut, they possess semi-enclosed gastrovascular cavities (GVCs), where vital processes such as digestion, reproduction and symbiotic exchanges take place. The microbiome harboured in GVCs is therefore likely key to holobiont fitness, but remains severely understudied due to challenges of working in these small compartments. Here, we developed minimally invasive methodologies to sample the GVC of coral polyps and characterise the microbial communities harboured within. We used glass capillaries, low dead volume microneedles, or nylon microswabs to sample the gastrovascular microbiome of individual polyps from six species of corals, then applied low-input DNA extraction to characterise the microbial communities from these microliter volume samples. Microsensor measurements of GVCs revealed anoxic or hypoxic micro-niches, which persist even under prolonged illumination with saturating irradiance. These niches harboured microbial communities enriched in putatively microaerophilic or facultatively anaerobic taxa, such as Epsilonproteobacteria. Some core taxa found in the GVC of Lobophyllia hemprichii from the Great Barrier Reef were also detected in conspecific colonies held in aquaria, indicating that these associations are unlikely to be transient. Our findings suggest that the coral GVC is chemically and microbiologically similar to the gut of higher Metazoa. Given the importance of gut microbiomes in mediating animal health, harnessing the coral "gut microbiome" may foster novel active interventions aimed at increasing the resilience of coral reefs to the climate crisis.

Vibrio cholerae Invasion Dynamics of the Chironomid Host Are Strongly Influenced by Aquatic Cell Density and Can Vary by Strain

Cholera has been a human scourge since the early 1800s and remains a global public health challenge, caused by the toxigenic strains of the bacterium Vibrio cholerae. In its aquatic reservoirs, V. cholerae has been shown to live in association with various arthropod hosts, including the chironomids, a diverse insect family commonly found in wet and semiwet habitats. The association between V. cholerae and chironomids may shield the bacterium from environmental stressors and amplify its dissemination. However, the interaction dynamics between V. cholerae and chironomids remain largely unknown.  In this study, we developed freshwater microcosms with chironomid larvae to test the effects of cell density and strain on V. cholerae-chironomid interactions. Our results show that chironomid larvae can be exposed to V. cholerae up to a high inoculation dose (109 cells/mL) without observable detrimental effects. Meanwhile, interstrain variability in host invasion, including prevalence, bacterial load, and effects on host survival, was highly cell density-dependent. Microbiome analysis of the chironomid samples by 16S rRNA gene amplicon sequencing revealed a general effect of V. cholerae exposure on microbiome species evenness. Taken together, our results provide novel insights into V. cholerae invasion dynamics of the chironomid larvae with respect to various doses and strains. The findings suggest that aquatic cell density is a crucial driver of V. cholerae invasion success in chironomid larvae and pave the way for future work examining the effects of a broader dose range and environmental variables (e.g., temperature) on V. cholerae-chironomid interactions. IMPORTANCE Vibrio cholerae is the causative agent of cholera, a significant diarrheal disease affecting millions of people worldwide. Increasing evidence suggests that the environmental facets of the V. cholerae life cycle involve symbiotic associations with aquatic arthropods, which may facilitate its environmental persistence and dissemination. However, the dynamics of interactions between V. cholerae and aquatic arthropods remain unexplored. This study capitalized on using freshwater microcosms with chironomid larvae to investigate the effects of bacterial cell density and strain on V. cholerae-chironomid interactions. Our results suggest that aquatic cell density is the primary determinant of V. cholerae invasion success in chironomid larvae, while interstrain variability in invasion outcomes can be observed under specific cell density conditions. We also determined that V. cholerae exposure generally reduces species evenness of the chironomid-associated microbiome. Collectively, these findings provide novel insights into V. cholerae-arthropod interactions using a newly developed experimental host system.

Mechanisms influencing the impact of microplastics on freshwater benthic invertebrates: Uptake dynamics and adverse effects on Chironomus riparius

Chironomids inhabit freshwater benthic ecosystems which are prone to microplastic contamination. This work aimed at understanding the factors and mechanisms influencing microplastic uptake and related adverse effects on Chironomus riparius, by exploring an extensive project database, conducting a literature review, and performing an agent-based model to explore trends in data. Results reveal that high concentrations of small microplastics fill the gut of fourth instar C. riparius (99.7 %). Ingested microplastics had an average size of 38-61 μm, presenting slower elimination rates than undigested organic or mineral particles. Ingestion rates of microplastics depend mainly on encounter rates, and therefore on available concentrations, until reaching a plateau corresponding to the maximum gut volume. Short-term toxicity of microplastics seems to result from damage to gut epithelium, with inflammatory reactions, production of reactive oxygen species, and a negative energy balance exacerbated by the lack of food (organic matter). Long-term toxicity is characterized by a reduction in larval body length and increase in mean time to emergence, seemly from increased energy costs rather than a decrease in nutrient absorption. Wild chironomids already present microplastics in their guts and environmental concentrations in hotspots may already exceed no effect concentrations. Therefore, environmental exposure to microplastics may induce adverse effects to wild C. riparius in freshwater benthic ecosystems, which could compromise their ecologic role as deposit-feeders (e.g., reducing their nutrient cycling ability) and key-stone species in aquatic food webs.

The Effect of Chironomid Larvae on Nitrogen Cycling and Microbial Communities in Soft Sediments

The combination of biogeochemical methods and molecular techniques has the potential to uncover the black-box of the nitrogen (N) cycle in bioturbated sediments. Advanced biogeochemical methods allow the quantification of the process rates of different microbial processes, whereas molecular tools allow the analysis of microbial diversity (16S rRNA metabarcoding) and activity (marker genes and transcripts) in biogeochemical hot-spots such as the burrow wall or macrofauna guts. By combining biogeochemical and molecular techniques, we analyzed the role of tube-dwelling Chironomus plumosus (Insecta, Diptera) larvae on nitrification and nitrate reduction processes in a laboratory experiment with reconstructed sediments. We hypothesized that chironomid larvae stimulate these processes and host bacteria actively involved in N-cycling. Our results suggest that chironomid larvae significantly enhance the recycling of ammonium (80.5 ± 48.7 µmol m−2 h−1) and the production of dinitrogen (420.2 ± 21.4 µmol m−2 h−1) via coupled nitrification–denitrification and the consumption of water column nitrates. Besides creating oxygen microniches in ammonium-rich subsurface sediments via burrow digging and ventilation, chironomid larvae serve as hot-spots of microbial communities involved in N-cycling. The quantification of functional genes showed a significantly higher potential for microbial denitrification and nitrate ammonification in larvae as compared to surrounding sediments. Future studies may further scrutinize N transformation rates associated with intimate macrofaunal–bacteria associations.

Potential nitrous oxide production by marine shellfish in response to warming and nutrient enrichment

Bivalves facilitate microbial nitrogen cycling, which can produce nitrous oxide (N2O), a potent greenhouse gas. Potential N₂O production by three marine bivalves (Mytilus edulis, Mercenaria mercenaria and Crassostrea virginica) was measured in the laboratory including responses to nitrogen (N) loading and/or warming over short-terms (up to 14 or 28 days). N additions (targeting 100 μM-N ammonium nitrate) or warming (22 °C) individually and in combination were applied with experimental controls (20 μM-N, 19 °C). N₂O production rates were higher with N additions for all species, but warming lacked significant direct effects. Ammonium and nitrate concentrations varied but were consistent with nitrification as a potential N₂O source for all bivalves. Highest N₂O emissions (7.5 nmol N₂O g^-1 h^-1) were from M. edulis under hypoxic conditions coincident with a drop in pH. Macro-epifauna on M. edulis did not significantly alter N₂O production. Thus, under short-term hypoxic conditions, micro-organisms in M. edulis guts may be a particularly significant source of N₂O.

Freshwater copepod carcasses as pelagic microsites of dissimilatory nitrate reduction to ammonium

A considerable fraction of freshwater zooplankton was recently found to consist of dead specimens that sink to the lake bottom. Such carcasses host intense microbial activities that may promote oxygen depletion at the microscale. Therefore, we tested the hypothesis that sinking zooplankton carcasses are microsites of anaerobic nitrogen cycling that contribute to pelagic fixed-nitrogen loss even in the presence of ambient oxygen. Incubation experiments were performed with the ubiquitous copepods Eudiaptomus sp. and Megacyclops gigas at different ambient oxygen levels that sinking carcasses encounter during their descent in stratified lakes. ¹⁵N-stable-isotope incubations revealed intense carcass-associated anaerobic nitrogen cycling only at low ambient oxygen levels (<25% air saturation). Dissimilatory nitrate reduction to ammonium (DNRA) dominated over denitrification and thus the potential for fixed-nitrogen loss was low. Consistent with this partitioning of anaerobic nitrogen cycling, the relative abundance of the carcass-associated marker gene for DNRA (nrfA) was ∼20–400 times higher than that for denitrification (nirS). Additionally, the relative nrfA and nirS abundances were ∼90–180 times higher on copepod carcasses than in lake water. This functional distinctiveness of carcass-associated bacterial communities was further substantiated by 16S rDNA-based fingerprinting. We conclude that the unique bacterial communities and microenvironments provided by zooplankton carcasses influence pelagic nitrogen cycling in lakes, but mainly at seasonally low ambient O₂ levels in the bottom water.

Chromium bioavailability in aquatic systems impacted by tannery wastewaters. Part 1: Understanding chromium accumulation by indigenous chironomids

The tanning industry uses large quantities of Cr whose contribution to the contaminant burden of aquatic organisms is not yet fully understood. The present study investigated Cr bioaccumulation by indigenous chironomids in a freshwater ecosystem impacted by tannery effluents. Total Cr content in sediments and in chironomids was determined on several occasions. Chromium distribution among sediments and pore waters, and Cr speciation in overlying and pore waters were studied in detail to understand possible factors controlling Cr bioavailability to chironomids. Total chromium concentration ranged from 69 to over 3000 μg g^-1 dry weight in sediments and from negligible to over 300 μg g^-1 dry weight in chironomids (values corrected for sediment gut content). Filterable (<0.45 μm) Cr concentration in overlying waters and pore waters from the surface sediment layers (upper 2 cm) ranged from 3 to 120 μg L^-1, with Cr(VI) representing 0.5-28% of the total filterable Cr. Chromium profiles in pore waters as determined by diffusive equilibration in thin films (DET) and diffusive gradient in thin films (DGT) were comparable. DGT-labile Cr accounted for <2% of the total Cr measured by DET. Although Cr concentrations in sedimentary and aqueous matrices were not directly proportional to Cr levels measured in chironomids, the available findings suggested that Cr inputs from tanneries were bioavailable to resident chironomids. These observations are of particular importance considering that Cr(III), putatively of limited bioavailability and ecotoxicological concern, is the predominant redox form of Cr in bed sediments impacted by tannery discharges. The companion paper provides further insight into Cr bioavailability and effects in tannery impacted ecosystems using a combination of in situ and laboratory approaches.

Direct Nitrous Oxide Emission from the Aquacultured Pacific White Shrimp (Litopenaeus vannamei)

The Pacific white shrimp (Litopenaeus vannamei) is widely used in aquaculture, where it is reared at high stocking densities, temperatures, and nutrient concentrations. Here we report that adult L. vannamei shrimp emit the greenhouse gas nitrous oxide (N2O) at an average rate of 4.3 nmol N2O/individual × h, which is 1 to 2 orders of magnitude higher than previously measured N2O emission rates for free-living aquatic invertebrates. Dissection, incubation, and inhibitor experiments with specimens from a shrimp farm in Germany indicated that N2O is mainly produced in the animal's gut by microbial denitrification. Microsensor measurements demonstrated that the gut interior is anoxic and nearly neutral and thus is favorable for denitrification by ingested bacteria. Dinitrogen (N2) and N2O accounted for 64% and 36%, respectively, of the nitrogen gas flux from the gut, suggesting that the gut passage is too fast for complete denitrification to be fully established. Indeed, shifting the rearing water bacterial community, a diet component of shrimp, from oxic to anoxic conditions induced N2O accumulation that outlasted the gut passage time. Shrimp-associated N2O production was estimated to account for 6.5% of total N2O production in the shrimp farm studied here and to contribute to the very high N2O supersaturation measured in the rearing tanks (2,099%). Microbial N2O production directly associated with aquacultured animals should be implemented into life cycle assessments of seafood production.

IMPORTANCE

The most widely used shrimp species in global aquaculture, Litopenaeus vannamei, is shown to emit the potent greenhouse gas nitrous oxide (N2O) at a particularly high rate. Detailed experiments reveal that N2O is produced in the oxygen-depleted gut of the animal by bacteria that are part of the shrimp diet. Upon ingestion, these bacteria experience a shift from oxic to anoxic conditions and therefore switch their metabolism to the anaerobic denitrification process, which produces N2O as an intermediate and dinitrogen (N2) gas as an end product. The N2O/N2 production ratio is unusually high in the shrimp gut, because denitrification cannot be fully established during the short gut passage time of food-associated bacteria. Nitrous oxide emission directly mediated by L. vannamei contributes significantly to the overall N2O emission from aquaculture facilities.

Degradation of forchlorfenuron by nitrification and denitrification reactions in the gut and shell biofilm of Limnoperna fortunei

The capacity and mechanism of Limnoperna fortunei to reduce the concentration of forchlorfenuron [or 1-(2-chloropyridin-4-yl)-3-phenylurea (CPPU)] in water has been studied under laboratory conditions. Firstly, the evasive response of mussels to CPPU (10, 20, 40 and 60 mg L(-1)) was evaluated, and a toxicity test was carried out at these concentrations. Secondly, the effect of two different sizes of mussels on CPPU concentrations was investigated in a 24-day experiment. Thirdly, the role of intact mussels and valvae only were respectively evaluated in another 24-day experiment. The CPPU concentration decreased by about 40 % in the presence of large mussels and about 20 % in the presence of valvae only. Finally, nucleic acid extracts from the gut and biofilm microbial communities of L. fortunei were analyzed, and the number of copies of the bacterial genes amoA, nirK and nirS were determined. Based on these results, we propose possible mechanisms for CPPU degradation involving bacteria-associated nitrification and denitrification reactions. In summary, we found that the CPPU half-life depended on the presence of mussels, their size and their associated microorganisms.

Meiofauna increases bacterial denitrification in marine sediments

Denitrification is a critical process that can alleviate the effects of excessive nitrogen availability in aquatic ecosystems subject to eutrophication. An important part of denitrification occurs in benthic systems where bioturbation by meiofauna (invertebrates <1 mm) and its effect on element cycling are still not well understood. Here we study the quantitative impact of meiofauna populations of different abundance and diversity, in the presence and absence of macrofauna, on nitrate reduction, carbon mineralization and methane fluxes. In sediments with abundant and diverse meiofauna, denitrification is double that in sediments with low meiofauna, suggesting that meiofauna bioturbation has a stimulating effect on nitrifying and denitrifying bacteria. However, high meiofauna densities in the presence of bivalves do not stimulate denitrification, while dissimilatory nitrate reduction to ammonium rate and methane efflux are significantly enhanced. We demonstrate that the ecological interactions between meio-, macrofauna and bacteria are important in regulating nitrogen cycling in soft-sediment ecosystems.

Excessive nutrient loading is a threat to aquatic ecosystems; however, denitrification may be key in removing large amounts of reactive nitrogen and, therefore, mitigating consequent eutrophication. Here, the authors explore how meiofauna may impact the rate of denitrification in sediments.

Chironomus plumosus larvae increase fluxes of denitrification products and diversity of nitrate-reducing bacteria in freshwater sediment

Benthic invertebrates affect microbial processes and communities in freshwater sediment by enhancing sediment-water solute fluxes and by grazing on bacteria. Using microcosms, the effects of larvae of the widespread midge Chironomus plumosus on the efflux of denitrification products (N2O and N2+N2O) and the diversity and abundance of nitrate- and nitrous-oxide-reducing bacteria were investigated. Additionally, the diversity of actively nitrate- and nitrous-oxide-reducing bacteria was analyzed in the larval gut. The presence of larvae increased the total effluxes of N2O and N2+N2O up to 8.6- and 4.2-fold, respectively, which was mostly due to stimulation of sedimentary denitrification; incomplete denitrification in the guts accounted for up to 20% of the N2O efflux. Phylotype richness of the nitrate reductase gene narG was significantly higher in sediment with than without larvae. In the gut, 47 narG phylotypes were found expressed, which may contribute to higher phylotype richness in colonized sediment. In contrast, phylotype richness of the nitrous oxide reductase gene nosZ was unaffected by the presence of larvae and very few nosZ phylotypes were expressed in the gut. Gene abundance of neither narG, nor nosZ was different in sediments with and without larvae. Hence, C. plumosus increases activity and diversity, but not overall abundance of nitrate-reducing bacteria, probably by providing additional ecological niches in its burrow and gut.

Characterization of denitrifying granular sludge with and without the addition of external carbon source

In this study granular sludge taken from a denitrifying upflow sludge reactor was characterized. Denitrification rates were determined in batch tests with and without external carbon source addition and pH microprofiles of the granules were studied. The microbial community structure was also determined. The results showed that denitrification without carbon source addition occurs; however, the process rate was lower than with external carbon source. This suggests that bacteria use dead biomass and extracellular material in the granular sludge as a carbon source when readily available substrate has been exhausted and nitrate is still present. Microprofiles showed a slight pH decrease for denitrification without external carbon source addition, and an increase in pH when using nitrite as the electron acceptor. Microprofiles using acetate as the carbon source for denitrification showed a significant increase in pH. Clone sequences obtained were close to the species Vitellibacter sp., Denitromonas indolicum str. and Denitromonas aromaticaus sp.

Transcriptome responses of insect fat body cells to tissue culture environment

Tissue culture is performed to maintain isolated portions of multicellular organisms in an artificial milieu that is outside the individual organism and for considerable periods of time; cells derived from cultured explants are, in general, different from cells of the corresponding tissue in a living organism. The changes in cultured tissues that precede and often explain the subsequent cell proliferation of explant-derived cells have been partially studied, but little is known about the molecular and genomic basis of these changes. Comparative transcriptomics of intact and cultured (90 hours in MGM-450 insect medium) Bombyx mori tissues revealed that fewer genes represented a larger portion of the transcriptome of intact fat body tissues than of cultured fat body tissues. This analysis also indicated that expression of genes encoding sugar transporters and immune response proteins increased during culture and that expression of genes encoding lipoproteins and cuticle proteins decreased during culture. These results provide support for hypotheses that cultured tissues respond immunologically to surgery, adapt to the medium by accelerating sugar uptake, and terminate their identity as part of an intact organism by becoming independent of that organism.

Shell biofilm nitrification and gut denitrification contribute to emission of nitrous oxide by the invasive freshwater mussel Dreissena polymorpha (zebra mussel)

Nitrification in shell biofilms and denitrification in the gut of the animal accounted for N(2)O emission by Dreissena polymorpha (Bivalvia), as shown by gas chromatography and gene expression analysis. The mussel's ammonium excretion was sufficient to sustain N(2)O production and thus potentially uncouples invertebrate N(2)O production from environmental N concentrations.

Effect of nitrate on sulfur transformations in sulfidogenic sludge of a marine aquaculture biofilter

The effect of NO(3)(-) addition on dissimilatory SO(4)(2-) reduction and sulfide conversion in organic-rich sludge from the digestion basin of a recirculating marine aquaculture system was studied. SO(4)(2-) reduction could only explain a minor fraction (up to 4-9%) of the observed total sulfide production (up to 35 mmol L(-1) day(-1)), indicating that the main source of sulfide in the sludge was not SO(4)(2-) reduction, but desulfuration during the decomposition of organic matter. Although NO(3)(-) inhibited SO(4)(2-) reduction, but not desulfuration, the primary NO(3)(-) mitigation effect was the onset of NO(3)(-)-mediated sulfide oxidation (up to 75 mmol L(-1) day(-1)), partially to elemental sulfur (S(0)). Above NO(3)(-) concentrations of 0.6 mM in the bulk water, the net sulfide production and oxidation zones were moved deeper into flocs and sludge cores, which effectively prevented sulfide from entering the water column. However, the sulfide efflux from the sludge instantly recovered after NO(3)(-) depletion. Thus, the NO(3)(-) level in the water column controls the zonation and magnitude of sulfur transformations in the sludge. The effect of NO(3)(-) relies therefore on its sustained presence in the water column, which in turn depends on a well-functioning nitrification in the mariculture system.

Bioconcentration, bioaccumulation, and metabolism of pesticides in aquatic organisms

The ecotoxicological assessment of pesticide effects in the aquatic environment should normally be based on a deep knowledge of not only the concentration of pesticides and metabolites found but also on the influence of key abiotic and biotic processes that effect rates of dissipation. Although the bioconcentration and bioaccumulation potentials of pesticides in aquatic organisms are conveniently estimated from their hydrophobicity (represented by log K(ow), it is still indispensable to factor in the effects of key abiotic and biotic processes on such pesticides to gain a more precise understanding of how they may have in the natural environment. Relying only on pesticide hydrophobicity may produce an erroneous environmental impact assessment. Several factors affect rates of pesticide dissipation and accumulation in the aquatic environment. Such factors include the amount and type of sediment present in the water and type of diet available to water-dwelling organisms. The particular physiological behavior profiles of aquatic organisms in water, such as capacity for uptake, metabolism, and elimination, are also compelling factors, as is the chemistry of the water. When evaluating pesticide uptake and bioconcentration processes, it is important to know the amount and nature of bottom sediments present and the propensity that the stuffed aquatic organisms have to absorb and process xenobiotics. Extremely hydrophobic pesticides such as the organochlorines and pyrethroids are susceptible to adsorb strongly to dissolved organic matter associated with bottom sediment. Such absorption reduces the bioavailable fraction of pesticide dissolved in the water column and reduces the probable ecotoxicological impact on aquatic organisms living the water. In contrast, sediment dweller may suffer from higher levels of direct exposure to a pesticide, unless it is rapidly degraded in sediment. Metabolism is important to bioconcentration and bioaccumulation processes, as is detoxification and bioactivation. Hydrophobic pesticides that are expected to be highly stored in tissues would not be bioconcentrated if susceptible to biotic transformation by aquatic organisms to more rapidly metabolized to hydrophilic entities are generally less toxic. By analogy, pesticides that are metabolized to similar entities by aquatic species surely are les ecotoxicologically significant. One feature of fish and other aquatic species that makes them more relevant as targets of environmental studies and of regulation is that they may not only become contaminated by pesticides or other chemicals, but that they constitute and important part of the human diet. In this chapter, we provide an overview of the enzymes that are capable of metabolizing or otherwise assisting in the removal of xenobiotics from aquatic species. Many studies have been performed on the enzymes that are responsible for metabolizing xenobiotics. In addition to the use of conventional biochemical methods, such studies on enzymes are increasingly being conducted using immunochemical methods and amino acid or gene sequences analysis. Such studies have been performed in algae, in some aquatic macrophytes, and in bivalva, but less information is available for other aquatic species such as crustacea, annelids, aquatic insecta, and other species. Although their catabolizing activity is often lower than in mammals, oxidases, especially cytochrome P450 enzymes, play a central role in transforming pesticides in aquatic organisms. Primary metabolites, formed from such initial enzymatic action, are further conjugated with natural components such as carbohydrates, and this aids removal form the organisms. The pesticides that are susceptible to abiotic hydrolysis are generally also biotically degraded by various esterases to from hydrophilic conjugates. Reductive transformation is the main metabolic pathway for organochlorine pesticides, but less information on reductive enzymology processes is available. The information on aquatic species, other than fish, that pertains to bioconcentration factors, metabolism, and elimination is rather limited in the literature. The kinds of basic information that is unavailable but is needed on important aquatic species includes biochemistry, physiology, position in food web, habitat, life cycle, etc. such information is very important to obtaining improved ecotoxicology risk assessments for many pesticides and other chemicals. More research attention on the behavior of pesticides in, and affect on many standard aquatic test species (e.g., daphnids, chironomids, oligochaetes and some bivalves) would particularly be welcome. In addition to improving ecotoxicology risk assessments on target species, such information would also assist in better delineating affects on species at higher trophic levels that are predaceous on the target species. There is also need for designing and employing more realistic approaches to measure bioconcentration and bioaccumulation, and ecotoxicology effects of pesticides in natural environment. The currently employed steady-state laboratory exposure studies are insufficient to deal with the complexity of parameters that control the contrasts to the abiotic processes of pesticide investigated under the strictly controlled conditions, each process is significantly affected in the natural environment not only by the site-specific chemistry of water and sediment but also by climate. From this viewpoint, ecotoxicological assessment should be conducted, together with the detailed analyses of abiotic processes, when higher-tier mesocosm studies are performed. Moreover, in-depth investigation is needed to better understand the relationship between pesticide residues in organisms and associated ecotoxicological endpoints. The usual exposure assessment is based on apparent (nominal) concentrations fo pesticides, and the residues of pesticides or their metabolites in the organisms are not considered in to the context of ecotoxicological endpoints. Therefore, more metabolic and tissue distribution information for terminal pesticide residues is needed for aquatic species both in laboratory settings and in higher-tier (microcosm, mesocosm) studies.

Functional tomographic fluorescence imaging of pH microenvironments in microbial biofilms by use of silica nanoparticle sensors

Attached bacterial communities can generate three-dimensional (3D) physicochemical gradients that create microenvironments where local conditions are substantially different from those in the surrounding solution. Given their ubiquity in nature and their impacts on issues ranging from water quality to human health, better tools for understanding biofilms and the gradients they create are needed. Here we demonstrate the use of functional tomographic imaging via confocal fluorescence microscopy of ratiometric core-shell silica nanoparticle sensors (C dot sensors) to study the morphology and temporal evolution of pH microenvironments in axenic Escherichia coli PHL628 and mixed-culture wastewater biofilms. Testing of 70-, 30-, and 10-nm-diameter sensor particles reveals a critical size for homogeneous biofilm staining, with only the 10-nm-diameter particles capable of successfully generating high-resolution maps of biofilm pH and distinct local heterogeneities. Our measurements revealed pH values that ranged from 5 to >7, confirming the heterogeneity of the pH profiles within these biofilms. pH was also analyzed following glucose addition to both suspended and attached cultures. In both cases, the pH became more acidic, likely due to glucose metabolism causing the release of tricarboxylic acid cycle acids and CO(2). These studies demonstrate that the combination of 3D functional fluorescence imaging with well-designed nanoparticle sensors provides a powerful tool for in situ characterization of chemical microenvironments in complex biofilms.

Nitrous oxide emission by aquatic macrofauna

A large variety of aquatic animals was found to emit the potent greenhouse gas nitrous oxide when nitrate was present in the environment. The emission was ascribed to denitrification by ingested bacteria in the anoxic animal gut, and the exceptionally high N(2)O-to-N(2) production ratio suggested delayed induction of the last step of denitrification. Filter- and deposit-feeding animal species showed the highest rates of nitrous oxide emission and predators the lowest, probably reflecting the different amounts of denitrifying bacteria in the diet. We estimate that nitrous oxide emission by aquatic animals is quantitatively important in nitrate-rich aquatic environments like freshwater, coastal marine, and deep-sea ecosystems. The contribution of this source to overall nitrous oxide emission from aquatic environments might further increase because of the projected increase of nitrate availability in tropical regions and the numeric dominance of filter- and deposit-feeders in eutrophic ecosystems.

Ammonia-oxidizing Crenarchaeota and nitrification inside the tissue of a colonial ascidian

Marine Crenarchaeota represent an abundant component of the oceanic microbiota that play an important role in the global nitrogen cycle. Here we report the association of the colonial ascidian Cystodytes dellechiajei with putative ammonia-oxidizing Crenarchaeota that could actively be involved in nitrification inside the animal tissue. As shown by 16S rRNA gene analysis, the ascidian-associated Crenarchaeota were phylogenetically related to Nitrosopumilus maritimus, the first marine archaeon isolated in pure culture that grows chemolithoautotrophically oxidizing ammonia to nitrite aerobically. Catalysed reporter deposition (CARD)-FISH revealed that the Crenarchaeota were specifically located inside the tunic tissue of the colony, where moreover the expression of amoA gene was detected. The amoA gene encodes the alpha-subunit of ammonia monooxygenase, which is involved in the first step of nitrification, the oxidation of ammonia to nitrite. Sequencing of amoA gene showed that they were phylogenetically related to amoA genes of N. maritimus and other putative ammonia-oxidizing marine Crenarchaeota. In order to track the suspected nitrification activity inside the ascidian colony under in vivo conditions, microsensor profiles were measured through the tunic tissue. Net NO(x) production was detected in the tunic layer 1200-1800 microm with rates of 58-90 nmol cm(-3) h(-1). Oxygen and pH microsensor profiles showed that the layer of net NO(x) production coincided with O(2) concentrations of 103-116 microM and pH value of 5.2. Together, molecular and microsensor data indicate that Crenarchaeota could oxidize ammonia to nitrite aerobically, and thus be involved in nitrification inside the ascidian tissue.

Methane-derived carbon flows through methane-oxidizing bacteria to higher trophic levels in aquatic systems

Recent investigations have shown that biogenic methane can be a carbon source for macro invertebrates in freshwater food webs. Stable carbon isotopic signatures, used to infer an organism's food source, indicated that methane can play a major role in the nutrition of chironomid larvae. However, the pathway of methane-derived carbon into invertebrate biomass is still not confirmed. It has been proposed that chironomid larvae ingest methane-oxidizing bacteria (MOB), but this has not been experimentally demonstrated to date. Using (13)C-labelled methane we could show for the first time that chironomid larvae assimilate methane-derived carbon through MOB. Chironomid larval biomass was significantly (13)C-enriched after dwelling for 10 days in lake sediment enriched with labelled methane. Moreover, phospholipid fatty acids diagnostic for MOB were detected in larval tissue and were significantly (13)C-enriched, which encompasses the (13)C-uptake predicted for a methane-based nutrition. Additionally, chironomid larvae fed on sediment and water-column derived MOB biomass.