Dynamic features of ecophysiological response of freshwater clam to arsenic revealed by BLM-based toxicological model

A review of freshwater benthic clams (Corbicula fluminea): Accumulation capacity, underlying physiological mechanisms and environmental applications

Asian clams (Corbicula fluminea) have been extensively applied in biomonitoring and other environmental fields based on their high enrichment capacity and rapid response to pollutants. This review first summarizes the kinetic process of metals and organic pollutants enriched by C. fluminea and discusses the environmental behavior and application. The accumulation ability of Cu, Zn, and Mn were significantly higher than that of other metals, which were attributed to their high uptake rate constant and low elimination rate constant. The visceral mass was found to be the major burden tissue. However, large knowledge gaps existed regarding the accumulation capacity of C. fluminea for organic pollutants and nanoparticles. Moreover, physiological mechanisms underlying the accumulation of environmental pollutants were proposed. C. fluminea can improve the niche of benthic algae by ingesting pelagic algae, mitigating water eutrophication. It can also remove pathogens and parasites based on the biological assimilation of nonspecific immunity, interrupting disease transmission. The novel insight into the application of C. fluminea in wastewater treatment further broadens the range of pest management strategies and offers the feasibility of blocking the spread of invasive bivalves.

A global synthesis of ecosystem services provided and disrupted by freshwater bivalve molluscs

Identification of ecosystem services, i.e. the contributions that ecosystems make to human well-being, has proven instrumental in galvanising public and political support for safeguarding biodiversity and its benefits to people. Here we synthesise the global evidence on ecosystem services provided and disrupted by freshwater bivalves, a heterogenous group of >1200 species, including some of the most threatened (in Unionida) and invasive (e.g. Dreissena polymorpha) taxa globally. Our systematic literature review resulted in a data set of 904 records from 69 countries relating to 24 classes of provisioning (N = 189), cultural (N = 491) and regulating (N = 224) services following the Common International Classification of Ecosystem Services (CICES). Prominent ecosystem services included (i) the provisioning of food, materials and medicinal products, (ii) knowledge acquisition (e.g. on water quality, past environments and historical societies), ornamental and other cultural contributions, and (iii) the filtration, sequestration, storage and/or transformation of biological and physico-chemical water properties. About 9% of records provided evidence for the disruption rather than provision of ecosystem services. Synergies and trade-offs of ecosystem services were observed. For instance, water filtration by freshwater bivalves can be beneficial for the cultural service 'biomonitoring', while negatively or positively affecting food consumption or human recreation. Our evidence base spanned a total of 91 genera and 191 species, dominated by Unionida (55% of records, 76% of species), Veneroida (21 and 9%, respectively; mainly Corbicula spp.) and Myoida (20 and 4%, respectively; mainly Dreissena spp.). About one third of records, predominantly from Europe and the Americas, related to species that were non-native to the country of study. The majority of records originated from Asia (35%), with available evidence for 23 CICES classes, as well as Europe (29%) and North America (23%), where research was largely focused on 'biomonitoring'. Whilst the earliest record (from 1949) originated from North America, since 2000, annual output of records has increased rapidly in Asia and Europe. Future research should focus on filling gaps in knowledge in lesser-studied regions, including Africa and South America, and should look to provide a quantitative valuation of the socio-economic costs and benefits of ecosystem services shaped by freshwater bivalves.

Influences of size-fractionated humic acids on arsenite and arsenate complexation and toxicity to Daphnia magna

The intrinsic physicochemical properties of dissolved organic matter (DOM) may affect the mobility and toxicity of arsenic in aquatic environments. In the present study, the humic acid (HA) was ultra-filtered into five fractions according to molecular weight, and their physicochemical properties were characterized. Complexation of HA fractions with arsenite and arsenate was first determined by differential pulse polarography (DPP). The influences of HA fractions on arsenic toxicity were then examined using Daphnia magna as a model organism. As(V) had a higher affinity with HA than As(III), and their complexation was dependent on the total acidity and fluorescence characteristics of DOM. We demonstrated that the acidity and fluorescence also better explained the As toxicity to daphnids than UV absorbance and hydraulic diameter. Arsenic speciation determined by DPP significantly affected the toxicity of arsenite and arsenate. The results extended the free-ion activity model application to the case of arsenic. The present study clearly indicated that DOM with different molecular weights has distinct physicochemical properties, and could influence the speciation and toxicity of As to different extent.

A generic biotic ligand model quantifying the development in time of Ni toxicity to Enchytraeus crypticus

Biotic ligand models (BLMs) predicting metal toxicity for a fixed exposure time are available, but it is uncertain how to extrapolate predictions to a dynamic environment with time-variable exposure. Three BLM-based models were developed to describe change of Ni toxicity to Enchytraeus crypticus in time. These models assumed that: (a) biotic ligand binding constants of Ni and competing cations (K(NiBL) and K(CBL)) and the fraction of biotic ligands occupied by Ni to produce 50% mortality (f50) are fixed with time, (b) K(NiBL) and K(CBL) remain constant while f50 varies with time, and (c) K(NiBL), K(CBL), and f50 are all time-dependent. Model (a) successfully described the 7-d toxicity of Ni but failed in explaining Ni toxicity at longer exposure times. Both models (b) and (c) well described Ni toxicity, within a factor of 2, at varying solution chemistries and different exposure times. This shows that the acute BLM cannot directly be applied for predicting chronic metal toxicity and that some BLM parameters may vary with time. Our findings provide plausible explanations for differences in mechanisms of acute and chronic toxicity, offering a framework for incorporating toxicokinetic and toxicodynamic processes in describing Ni toxicity in time.

Toxicokinetics/toxicodynamics with damage feedback improves risk assessment for tilapia and freshwater clam exposed to arsenic

It has been proposed that irreversible responses of organisms exposed to contaminants are due to a systems-level feedback. Here we tested this hypothesis by reanalyzing the published data on toxicokinetics and survival probability based on a systems-level threshold damage model (TDM) incorporating with a positive damage feedback to explore the steady-state response and dynamic behavior of damage for tilapia and freshwater clam exposed to waterborne arsenic (As). We found that ultrasensitivity appeared in As-tilapia and freshwater clam systems with Hill coefficient n ≥ 4, indicating that the positive damage feedback mechanism has been triggered. We confirmed that damage can trigger a positive feedback loop that together with As stressor increases irreversibility. This study also showed that TDM with positive feedback gave a much better predictability than that of TDM at As concentrations ranging from 100 to 500 mg l(-1) for freshwater clam, whereas for tilapia, two models had nearly same performance on predictability. We suggested that mortality-time profile derived Hill coefficient could be used as a new risk indicator to assess the survival probability for species exposed to waterborne metals. We anticipated that the proposed toxicokinetics/toxicodynamics with a positive damage feedback may facilitate our understanding and manipulation of complex mechanisms of metal susceptibility among species and improve current risk assessment strategies.