Different natural organic matter isolates cause similar stress response patterns in the freshwater amphipod, Gammarus pulex

Background, aim, and scope Dissolved humic substances (HSs) are exogenous stressors to aquatic plants and animals which activate a variety of transcriptional and biochemical reactions or block photosynthesis. While there are consistent indications which structures may lead to reduced photosynthetic activity, there is much less clear information available on which HS structures or building blocks act as stressors in animals. Consequently, this work was designed to comparatively study the impact of natural organic matter (NOMs) from different sources on major anti-stress mechanisms in one single animal. We utilized major antioxidant responses and relative expression levels of stress proteins (small HSPs and HSP70) and expected that different HSs provoke different response patterns. Materials and methods We tested the freshwater amphipod Gammarus pulex which was collected from several shallow creeks in Northern Germany. All specimens were maintained in aerated 5-L tanks with filtered water from their natural environment at 10°C with prior acclimation. Animals were fed ad libitum with a commercial preparation once every second day. The exposure water was exchanged with the same frequency. NOMs were isolated from three different sources: two from small brown-water lakes in Northern Germany by reverse osmosis and the third one as an aqueous extract from a black layer of a Brazilian sandbar soil (State of Rio de Janeiro). The rationale was to apply NOMs of contrasting quality. Chemical fingerprint features of the NOMs were taken by high-performance size exclusion chromatography. As stress parameters in the animals, the activities of peroxidase and catalase were recorded quantitatively, and stress proteins, HSP70, as well as small α-crystalline HSPs were analyzed semiquantitatively. Results The three NOMs clearly differed in molecular masses, humic substance contents, the moieties of polysaccharides, and low-molecular-weight substances. With the exception of one short-term response, the peroxidase activity increased after 3 to 12 h exposure, whereas the catalase activity did not show any significant modulation. With one exception, the stress protein expression increased after 30 min exposure in a biphasic pattern, and the sHSPs responded less strongly than HSP70. Discussion Although the quality of the exposed NOMs differs significantly, a rather uniform response pattern appears in the animals. Obviously, the contrasting contents of HSs and polysaccharides did not affect the anti-stress response of the exposed gammarids which is in contrast to previous lifespan studies with Caenorhabditis elegans. Furthermore, all NOM sources led to increased contents of both HSP70 and sHSPs. To the best of our knowledge, this is the first protein study to show that also small HSPs are expressed when the animals are exposed against humic material. Conclusions Since the response patterns of the exposed gammarids, in contrast to the initial hypothesis, are rather uniform and since HSs are parts of life on Earth, we furthermore presume that they may have been a primordial exogenous trigger for the development of anti-stress systems in exposed organisms. Recommendations and perspectives Effect studies of chemical stresses on organisms should consider exposure to both natural triggers and xenobiotic compounds in low concentrations—in order to prospectively differentiate between these triggers and, subsequently, classify them

Biodegradation kinetics of dissolved organic matter chromatographic fractions in an intermittent river

Controls on the degradation of dissolved organic matter (DOM) are complex but key to understand the role of freshwaters in the carbon cycle. Both the origin and previous degradation history have been suggested to determine DOM reactivity, but it is still a major challenge to understand the links between DOM composition and biodegradation kinetics. An appropriate context to study these links are intermittent rivers, as summer drought naturally diversifies DOM sources and sinks. Here we investigated the biodegradation kinetics of DOM in the main aquatic environments present in a temporary river. During dark incubations we traced the dynamics of bulk DOM and its main chromatographic fractions defined using LC-OCD: high molecular weight substances (HMWS), low molecular weight substances (LMWS), and humic substances and building blocks. Bulk DOM decay patterns were successfully fitted to the reactivity continuum (RC) biodegradation model. The RC parameters depicted running waters as the sites presenting a more reactive DOM, and temporary pools, enriched in leaf litter, as the ones with slowest DOM decay. The decay patterns of each DOM fraction were consistent throughout sites. LMWS and HMWS decayed in all cases and could be modeled using the RC model. Notably, the dynamics of LMWS controlled the bulk DOM kinetics. We discuss the mechanistic basis for the chromatographic fractions' kinetics during biodegradation and the implications that preconditioning and summer drought can have for DOM biodegradation in intermittent rivers.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Comparison of Organic Matter Composition in Agricultural versus Forest Affected Headwaters with Special Emphasis on Organic Nitrogen

Agricultural management practices promote organic matter (OM) turnover and thus alter both the processing of dissolved organic matter (DOM) in soils and presumably also the export of DOM to headwater streams, which intimately connect the terrestrial with the aquatic environment. Size-exclusion chromatography, in combination with absorbance and emission matrix fluorometry, was applied to assess how agricultural land use alters the amount and composition of DOM, as well as dissolved organic nitrogen (DON) forms in headwater streams, including temporal variations, in a temperate region of NE Germany. By comparing six agriculturally and six forest-impacted headwater streams, we demonstrated that agriculture promotes increased DOC and DON concentrations, entailing an even more pronounced effect on DON. The major part of DOC and DON in agricultural and forest reference streams is exported in the form of humic-like material with high molecular weight, which indicates terrestrial, i.e., allochthonous sources. As an obvious difference in agricultural streams, the contribution of DOC and particularly DON occurring in the form of nonhumic high-molecular-weight, presumably proteinous material is clearly elevated. Altogether, DOM in agricultural headwaters is mainly complex-soil-derived and aromatic material with a low C:N ratio, which is more microbial processed than its counterpart from forest reference catchments. Our results emphasize the importance of agricultural land use on DOM loss from soils and identify agricultural soils as important DOC and particularly DON sources to headwater streams

Global effects of agriculture on fluvial dissolved organic matter

Agricultural land covers approximately 40% of Earth's land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks and resulting in higher ecosystem productivity and CO2 outgassing