Biomonitoring: An appealing tool for assessment of metal pollution in the aquatic ecosystem

Abstract

Wide occurrence of aquatic metal pollution has caused much attention. Biomonitoring offers an appealing tool for the assessment of metal pollution in aquatic ecosystem. The bioindicators including algae, macrophyte, zooplankton, insect, bivalve mollusks, gastropod, fish, amphibian and others are enumerated and compared for their advantages and disadvantages in practical biomonitoring of aquatic metal pollution. The common biomonitoring techniques classified as bioaccumulation, biochemical alterations, morphological and behavior observation, population- and community-level approaches and modeling are discussed. The potential applications of biomonitoring are proposed to mainly include evaluation of actual aquatic metal pollution, bioremediation, toxicology prediction and researches on toxicological mechanism. Further perspectives are made for the biomonitoring of metal pollution in aquatic ecosystem.

Introduction

More and more attention has been drawn due to the wide occurrence of metal pollution in aquatic system. Some heavy metals may transform into the persistent metallic compounds with high toxicity, which can be bioaccumulated in the organisms, magnified in the food chain, thus threatening human health [1]. Various harmful effects including abnormal development of fetus, procreation failure, and immunodeficiency has exhibited due to aquatic metal exposure [2]. Monitoring and prevention of heavy metal pollution is one of the hot topics in environmental researches.

Heavy metals in aquatic system can be naturally produced by the slow leaching from soil/rock to water, which are usually at low levels, causing no serious deleterious effects on human health [2]. The development of industry and agriculture promotes the rapid increase of environmental metal pollution. Aquatic heavy metal pollution usually represents high levels of Hg, Cr, Pb, Cd, Cu, Zn, Ni etc. in water system [3], [4]. The anthropogenic activities such as discharge of heavy metal wastewater contribute to the predominant causation. The wastewater mainly origins from mining, mill run, metallurgy, plating, chemical plant, curry and paper making industry. Although some metallic compounds can be strongly absorbed onto the suspended particles and sediments, they are able to be released into the water under the suitable conditions such as pH values and Eh, leading to further contamination of aquatic metal [5]. Some heavy metal including Hg, Cr, Cd, Ni, Cu, Pb etc. introduced into environmental water system may pose high toxicities on the aquatic organisms [6]. As an example, cadmium is a priority environmental contaminant with consequences for human health and the maintenance of bio-diversity in affected ecosystems and the timeliness of a broader, ecosystem-based approach to cadmium research is highlighted based on the overview of recent developments in the field by Campbell [7].

Wide occurrence of metal pollution exists worldwide now, including China. For example, investigations on Yangtze River showed the occurrence of the various levels of heavy metal in alongshore-aquatic areas with the predominant elements of Zn, Pb, Cd, Cu, Cr. Some elements with high affinity to sulfur atoms such as Cd, Pb, Hg and Cu detected in Yangtze River might produce the potential toxicity [8]. Survey on the water quality in Shanghai City showed that Cd was the main pollutant, while Hg was at the second highest level. Determination of Cu, Pb, Zn and Cd in the surface sediments in Huangpu River indicated that the level of Pb in the mainstream was 2 folds higher than the national water quality standard. Serious pollution of Cu, Zn, Cd and Pb was found in 9 branch rivers, wherein 100, 75, and 62.5% of samples contained the high levels of Pb, Cd and Hg, respectively, which exceeded the corresponding national water quality standard values in Suzhou River [9]. Different levels of various metal pollutants are reported in many other inland and marine water systems in China [10].

Considering of the use of some rivers and lakes as water supplies, threaten are thus posed on human health via drinking water, polluted vegetable and foodstuff etc. besides the disruption of the natural environment.

Chemical analysis of the environment matrix such as water, sediment is the most direct approach to reveal the heavy metal pollution status in the environment, while it cannot afford the powerful evidence on the integrated influence and possible toxicity of such pollution on the organisms and ecosystem. Biomonitoring is a scientific technique for assessing environment including human exposures to natural and synthetic chemicals, based on sampling and analysis of an individual organism's tissues and fluids. This technique takes advantage of the knowledge that chemicals that have entered the organisms leave markers reflecting this exposure. The marker may be the chemical itself. It may also be a breakdown product of the chemical or some biological changes in the organisms that is a result of the action of the chemical on the individual. The results of these measurements provide information about the amounts of natural and man-made chemicals that have entered and remained in the organisms and the corresponding effects induced. Due to the consistency between the selected organisms and the corresponding living space, biomonitoring can directly offer the data on the potential effects and actual integrated toxicities of pollutants, reflecting the corresponding deleterious degree in the environment. Precaution may be drawn based on the sensitive biomonitoring of chronic effects induced at low dose of pollutants for long-term exposure. These characters endowed biomonitoring with attractive advantages of wide practicability, high sensitivity and high integration, which the conventional chemical analysis is lack of [11].

For the biomonitoring of aquatic pollution including heavy metal, the organisms in the given aquatic systems are sampled for the analysis of various biological responses to chemical exposures. Suitable bioindicators usually give great help to the biomonitoring. A perfect bioindicator is expected to have the following characters: (1) it can accumulate high levels of pollutants without death; (2) it lives in a sessile style, thus definitely representing the local pollution; (3) it has enough abundance and wide distribution for the repetitious sampling and comparison; (4) its life is long enough for the comparison between various ages; (5) it can afford suitable target tissue or cell for the further research at microcosmic level; (6) easy sampling and easy raising in the lab; (7) it keeps alive in water; (8) it occupy the important position in food chain; (9) well dose-effect relationship can be observed in it [12]. As it is too rigorous to find such bioindicator for biomonitoring, the candidate bioindicator with several characters is practicable according to the specific monitoring purpose. Abundant organisms living in water system such as plankton, sedentary benthos, fish and bacteria promise the feasibility of the biomonitoring methods. As water quality directly affects their population, species, abundance and living behavior, they may act as the bioindicators for the evaluation of water pollution.

The common biomonitoring methods for aquatic metal pollution include biota population, bacteria test, acute toxicity assay, chronic toxicity assay and residue analysis etc. The method of biota population is usually performed by counting the species and amounts of various organisms in the tested water system. Many bacteria live in surface water, ground water, and other natural environmental water, which offers the possibility for water quality assessment especially for hygiene using bacteria test. Fish and algae are usually used for the acute toxicity assay of pollutants such as heavy metal. The data on half lethal or effect concentration (LC₅₀ or EC₅₀) obtained from these assays can serve as the powerful evidence for the enactment of water quality standards for industrial wastewater discharge regarding various pollutants. It can also be used for the risk assessment of the pollution levels of the water bodies, estimation of water treatment performance etc. Researches on chronic toxicity of pollutants at low levels may range from molecular reaction to individual alterations, including genetic toxicity, embryo toxicity, histopathological alteration, physiological changes and behavior abnormality etc. Biomonitoring using chronic toxicity assay may sensitively indicate the pollution stress posed by the pollutants at sublethal levels. Residue analysis can afford the information on the accumulation, distribution and transfer properties of the pollutants in the target organisms by the chemical analysis due to the occurrence of bioaccumulation and biomaginification for many chemicals in aquatic organisms. Other methods like productivity determination can also reflect aquatic pollution by measuring the chlorophyll contents, photosynthesis, nitrogen fixation in aquatic plants [13].

When compared with the conventional chemical analysis of aquatic environmental matrix, i.e. water and sediment, biomonitoring exhibits obvious predominance as follows: biomonitoring (1) reveals the subtle biological changes of organisms affected by exogenous chemicals, which is usually missed by the conventional chemical analysis; (2) reveals the integrated effects of the complex pollutants on the organisms in the environment; (3) has high sensitivity due to the rapid responses induced in the organisms exposed to pollutants, which helps to the declare of the precaution; (4) realizes the monitoring of the pollutants at low levels which were below the detection limits of the instrumental analytical techniques due to the occurrence of the chronic toxicities of the pollutants in the organisms under long-term exposure; (5) allows widely sampling even at remote areas; (6) avoids the limits of the convention chemical analysis such as continuous sampling, needs of expensive instruments. As an appealing tool, biomonitoring exerts unparalleled functions in the evaluation of environmental pollution, especially for the metal pollution in aquatic ecosystem.