Review Article

Ecotoxicology and Monitoring of Toxic Pollutants in the Marine Environment- A Review  

M. Mohan , Jyothy S , Navya Cherian , Toms Augustine , K. Sreedharan , V. G. Gopikrishna
School of Environmental Sciences, Mahatma Gandhi University, Kottayam-686560, India
Author    Correspondence author
International Journal of Marine Science, 2016, Vol. 6, No. 9   doi: 10.5376/ijms.2016.06.0009
Received: 11 Nov., 2015    Accepted: 24 Mar., 2016    Published: 28 Mar., 2016
© 2016 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Mohan M., Gopikrishna V.G., Cherian N., Jyothy S., Augustine T., and Sreedharan K., 2016, Monitoring of Toxic Pollutants in the Marine Environment- A Review, International Journal of Marine Science, 6(9): 1-20 (doi: 10.5376/ijms.2016.06.0009)


Since the turn of the century, the marine environment is getting adulterated day by day due to several anthropogenic activities which eventually leads to marine biodiversity loss. Land-based sources play a key role in polluting the marine environment. The pollutants once entered the marine ecosystem has got a chance to transform chemically or biologically, bioaccumulate or even biomagnify. Heavy metals, plastics, POPs, radioactive materials and other contaminants contribute much to the marine pollution. The resultant outcome of this voluminous oceanic pollution is a real threat to the entire biota. This chapter throws light on the various impacts of toxic pollutants on the marine ecosystems and the effective monitoring possibilities.

Marine environment; Ecotoxicology; Toxic pollutants; Heavy metals

1 Introduction
Humans depend on marine ecosystems for a number of valuable goods and services. But human activities has also altered the marine ecosystems through direct and indirect means leading to marine biodiversity loss which in turn increasingly impair the ocean's capacity to provide food, maintain water quality, and recover from perturbations (Worm et al., 2006). Until recently, it was widely assumed that no matter how much trash and chemicals humans dumped into oceans, the effects would be negligible. But recent studies have showed that human activities have severely affected the marine biodiversity, yet many upper trophic level species, including seabirds, marine mammals and large predatory fish, remain depleted owing to human activities (Lotze et al., 2006). The intensity of these human activities and their magnitude of impact on the ecological condition of marine communities vary across the globe (Halpern et al., 2008). Understanding the sources of pollution and their impacts on ecosystems is needed to improve and rationalize human activities and to develop appropriate mitigation measures and management strategies (Islam and Tanaka, 2004).
The majority of the contaminants entering the marine environment are from land-based sources-primarily from agricultural, urban and industrial sources. Land-based activities cause the runoff of pollutants and nutrients into coastal waters via rivers and estuaries causing deleterious effect on coastal and marine ecosystems (Syvitski et al., 2005). Atmospheric deposition, spills and dumping of dredging materials also contribute to marine pollution. Furthermore, ocean-based activities overexploit resources, spread invasive species and diseases, and change species composition (Crain et al., 2009). Indirect human effects on ocean chemistry can also occur, mainly through global warming and climate change, resulting in increasing sea surface temperatures and ocean acidification (Doney, 2010). The present paper attempts to provide a review on the major threats from the pollutants to marine ecosystems with special focus on their sources and their ecosystem-level impacts and how they can best be monitored.
2 Marine Ecotoxicology - Fate, Transport and impact of pollutants
The understanding of the impact of pollutants on environment is normally based on the toxicological studies with organisms that can readily be obtained, cultured, and tested, which can be characterized as environmental toxicology. Although those studies are useful to understand the effects of environmental contaminants on test organisms individually, the interactions of the species with each other and with the abiotic environment are not considered in those type of test. Hence a paradigm shift is occurring, the significance of ecology in toxicology is increasing and the integrated word is known as ecotoxicology (Chapman, 1995; Baird et al., 1996). Ecotoxicology understands the types of effects caused by chemicals, the biochemical and physiological processes responsible for those effects, the relative sensitivities of different types of organisms to chemical exposures, and the relative toxicities and fate of different chemicals and chemical classes in the environment (Chapman, 2002). Highly persistent pollutants released into the terrestrial or aquatic environment finally reach the marine ecosystem, where it can undergo transformation, transport and accumulation.
The chemical and physical properties especially the persistence in the environment is the major controlling factor for the transport of pollutants in the marine environment (Walker and Livingstone). Generally pollutants that readily shift their distribution between gas and condensed phase in response to temperature variations can travel long distance when compared with the involatile and water soluble contaminants. In the marine environment, circulations can take the pollutants from one region to other ones. The pollutants associated with particulate matter due to physical mixing can be vertically transported and settles at ocean sediments (Gioia et al., 2011). A part of the organic matter pools like phytoplankton, which accumulated the pollutants, especially the  hydrophobic ones, can also settles to the deep ocean and carries organic matter-bound pollutants (Dachs et al., 2002). The pollutants can chemically or biologically transform and bioaccumulate or biomagnify based on their bioavailability and lipophilic character. Some of the solid pollutants, e.g. plastics, can aggregate into big size particles. Ocean currents corrals trillions of decomposing plastic items and other trash into gigantic, swirling garbage patches. One such massive patch was discovered in North Pacific, known as the Great Pacific Garbage Patch (Kaiser, 2010). The schematic representation of general fate of toxic pollutants in marine ecosystem is shown in the Figure 1.


Figure 1 The general fate of pollutants in the marine environment


3 Pollutants and Their Toxic Impact on Marine Ecosystem

3.1 Heavy metals
The role of heavy metals in polluting the marine environment has gained a great deal of consideration these days. They accumulate in marine organisms and sediments and finally reach the humans through food chain. The major culprits in this area are Hg, Cd, Cr, Pb, As, Zn, and Cu. Out of these Hg and Cd snatches more attention due to their well-known toxic effects and biomagnification efficiency. Industrial discharge, agriculture run off, combustion, urban discharge, mining etc are the sources by which the heavy metals reach the marine environment (Bilandzic et al., 2011).
The major toxicity effects of heavy metals are following
- The blocking of essential functional groups of the biomolecules like proteins and enzymes
- The displacement of a metal ion from a biomolecule
- The inhibition of function of biomolecules by the modification of the structure
The heavy metals do not decompose naturally and in aquatic environment some of them get converted to more toxic forms thereby posing a real threat to marine organisms and human health. The best example is the conversion of mercury into methyl mercury (MHg). The concern about the metal transfer in the marine food web and its ecotoxicological studies stem from the Minamata incident which occurred in the 1950’s in Japan (Wang, 2002). Mercury is a global pollutant and it is highly persistent in the environment (Lacerda and Fitzgerald, 2001). Due to the very low solubility product of its compounds the major portion of mercury that reaches the coastal sea gets precipitated and accumulated in the sediments (Spada et al., 2012). Here it can stay for a longer period undergoing many transformations. The conversion of mercury to its organic form - methyl mercury under favourable conditions is more dangerous (Ullrich et al., 2001). The monomethyl form of mercury is the major part of mercury in fishes and shell fishes. Hence its consumption on regular basis may serious threat to humans (Giani et al., 2012, Agah et al., 2007). Cadmium accumulates mainly in the kidneys and liver of marine organisms (Ozden et al., 2010). The levels of mercury and cadmium in the fishes and mussels from various sites are shown in the Table 1.
The accumulation of metals varies according to the chemical form. Claisse et al. (2001) found that mussels accumulated higher Hg and MHg concentrations in their soft tissues than oysters, but they have less MHg than fish and hence apparently present a smaller risk to human consumers. Mussels generally accumulate more metals than any other organism. A review on metal accumulation in Mediterranean mussel Mytilus galloprovincialis, revealed that the concentrations of toxic metals were in the following order: As>Pb>Cd>Hg (Stankovic and Jovic, 2012).
3.2 Plastic Pollution
Plastics constitute the most significant part of marine litter deposits and all rubbish floating in the oceans. Marine litter consists of items that have been made or used by people and deliberately discarded into the sea or rivers or on beaches; brought indirectly to the sea with rivers, sewage, storm water or winds; accidentally lost, including material lost at sea in bad weather (fishing gear, cargo); or deliberately left by people on beaches and shores (UNEP, 2003). Monitoring the extent of plastic pollution in the marine environment at a global scale is complicated due to the large spatial and temporal heterogeneity in the amounts of plastic debris and also due to our limited understanding of the pathways followed by plastic debris and its long-term fate (Ryan et al., 2009).
Plastics are dumped in huge volumes in well-used beaches, lakes, navigation channels and other forms of water masses. Most plastics are less dense than water, and it enable them to float and readily be transported for long distances from source areas. Floating plastic debris have become a global problem now because they are carried across ocean basins, contaminating even the most remote islands and polar regions (Barnes, et al., 2009). The UN Environment Programme estimated that in 2006 that every square mile of ocean contains 46,000 pieces of floating plastic (UNEP, 2006).
Plastics do not degrade easily and thus poses a real threat to the marine world (Laist, 1997). Most plastics break down slowly through a combination of photo degradation, oxidation and mechanical abrasion (Andrady, 2003). Except for expanded polystyrene, plastics take much longer time to degrade in water than they do on land, mainly due to the reduced UV exposure and lower temperatures found in aquatic habitats (Gregory and Andrady, 2003). Thick plastic items persist for decades, even when subject to direct sunlight, and survive even longer when shielded from UV radiation under water or in sediments.


Table 1 Concentrations of Hg and Cd in marine organisms across the globe.

The most widely recognized problems caused by marine litter pollution are typically associated with entanglement, ingestion, suffocation and general debilitation (Gregory, 2009). According to the UN Environment Programme, plastic debris causes the deaths of more than a million seabirds every year, as well as more than 100,000 marine mammals (UNESCO, 2015). About 44% of all seabird species unfortunately ingest plastic. Sea turtles ingest plastic bags, fishing line and other plastics. A recent study revealed that about 267 species of marine organisms are badly affected by plastic pollution in one way or the other (Moore, 2008).
Another outcome of the plastic pollution is the invasion of alien species. Plastics are capable of carrying non-native, invasive pest species over long distances and thus increase the domain of certain marine organisms. There is also potential danger to marine ecosystems from the accumulation of plastic debris on the sea floor. The accumulation of such debris can inhibit gas exchange between the overlying waters and the pore waters of the sediments, and disrupt or badly affect benthic organisms.
Given the impacts of plastic litter, considerable effort should be made to remove waste plastic and other persistent debris from the marine environment. This removal can be conducted before it enters the sea, through litter collection and screening waste water systems (e.g. Marais and Armitage, 2004) or, thereafter, through periodic collections of litter from beaches (e.g. Ryan and Swanepoel 1996) or the seabed (e.g. Donohue et al. 2001). However, the most efficient and cost-effective solution is an “action at source”- approach creating awareness among people to reduce the release of plastics into the environment. The use of biodegradable plastics also will not help in reducing marine pollution. The plastics labelled as “biodegradable” will be used more by public and also the complete degradation of plastics by biological agents occurs very rarely even in the marine environment (UNEP 2015).
3.3 Microplastics and their impacts
Microplastics are the tiny fragments of plastics, fibres and granules in the environment and they exhibit a wide range of sizes varying from diameters, <10mm to <1mm (Barnes et al., 2009; Browne et al., 2010, Claessens et al., 2011). They can be also classified as primary (microscopic size) and secondary (products of breakdown of larger plastics debris) (Cole et al., 2011).
The small size enables pelagic and benthic marine organisms including sea birds to easily ingest microplastics and causes mechanical hazards (blocking the feeding appendages or by hindering the passage of food through the intestinal tract) especially to small marine organisms like zooplanktons, invertebrates and echinoderm larvae because they cannot differentiate it from their food (Moore, 2008; Tourinho et al., 2010; Barnes et al., 2009). Also microplastic can be easily absorbed into the body through the processes of translocation. The large surface- area- to- volume ratio of microplastics will cause for leaching of additives (e.g. Phthalates, Bisphenol A etc.) after the ingestion and finally interferes with many of the biological processes resulting in endocrine disruption, which in turn affects the mobility, reproduction and development, and can also result in carcinogenesis (Barnes et al., 2009; Lithner et al., 2011). The large surface area enables microplastics to act as vehicles in pollutant transport (Ashton et al., 2010; Cole et al., 2011).
3.4 POPs Pollution
Persistent organic pollutants (POPs) comprise both chlorinated as well as brominated environmental contaminants. Chlorinated organic pollutants include Polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) [e.g., DDTs, chlordanes, hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB)] and brominated ones include polybrominated diphenyl ethers (PBDEs). Organochlorine pesticides have been widely used throughout the globe both in agricultural sector and in control of vector borne diseases. For example, DDT was largely used against vectors and pests that cause tropical diseases, such as malaria and visceral leishmaniosis (van den Berg, 2009). However, POPs have the ability to bio-accumulate in organisms and in turn biomagnify through the food chain due to their hydrophobic nature and persistence in the environment (UNEP, 2003). Due to their very long persistence in the environment and impact on non-target organisms and biological accumulation via the food chain, the Stockholm Convention on Persistent Organic Pollutants in 2001 registered most of these substances on a priority list of pollutants and steps were taken to reduce their global production and usage.
Although the production and use of most of the POPs has been banned or restricted in many countries, various studies have shown that these contaminants are still present in coastal and marine environments; ie in water (Qiu et al., 2009) sediments (Galanopoulou et al., 2005), and biota (Potrykus et al., 2003, Alava et al., 2011). Many studies also revealed that seafood consumption is the main contributor to total dietary intakes of POPs in humans (Jiang et al., 2005; Moon et al., 2009).
POPs are easily adsorbed onto suspended particulate matter in both freshwater and marine situations, and may rapidly deposit to sediments. From such sinks, they can enter living organisms, via flux through the water phase, and eventual dissolution in tissue lipids (Hutzinger et al., 1974).
Studies on toxic effects of the insecticides lindane and chlorpyrifos, the herbicide diuron, the organometallic antifoulant tributyltin (TBT), and the surfactant sodium dodecyl sulfate (SDS) on Paracentrotus lividus (Echinodermata, Euechinoidea), Ciona intestinalis (Chordata, Ascidiacea), Maja squinado and Palaemon serratus (Arthropoda, Crustacea) showed that the early life stages (embryos and larvae) of marine invertebrates were more likely to be affected (Bellas et al., 2005). The organochlorine and PCB residues in marine biota from different regions are given in the table 2.
3.5 Acid Spill
Acids are transported in large quantities by ship every year. Approximately 800,000 tonnes of phosphoric acid,770,000 tons of sulphuric acid, and 650,000 tons of acetic acid are transported through European harbours (Marchand, 2003., HELCOM, 2002). While this, transportation accidents may occur and the consequent oceanic spills have a direct as well as immediate effects on the marine life. The decrease in pH and increase in temperature through the production of toxic gases cause much destruction to the marine world. When these acids interact with the sediments at the bottom, a secondary pollution will arise as a result of the release of the metals (Cabon et al., 2010). The metals are released into the marine environment at varying rates as they are present in sediments with different physico-chemical forms such as labile, carbonate, sulfate, sulfur, organic, etc. (Mohan et al., 2012; Hirose, 2006)


Table 2 Concentration of Organochlorine pesticides and polychlorinated biphenyls (PCBs) in marine Biota across the world


3.6 Oil spill

Oil spills are extremely dangerous to coastal and marine resources and it affect the feeding, growth, development and reproduction of living organisms. It directly affects the survival of marine organisms and indirectly affects these organisms by reducing the availability of prey. Seabirds are the ones which are mostly affected by the oil spill compared to other marine organisms. It is estimated that between 150,000 and 450,000 marine birds killed by routine releases of oil from tankers. It penetrates the plumage of seabirds or fur of marine mammals, affecting heat insulation and buoyancy (Dalton et al., 2010).

The deposition of oil in the shells or by ingestion of emulsified oil during feeding can be caused for tainting of shellfish. It also do harm to other marine organisms by long term exposure to the persistent and bioaccumulative components of oil via several indirect ecosystem processes (Velnado et al., 2010).
3.7 Radioactive materials
The world's oceans have been a sink for radioactive waste from the production of nuclear weapons and electric power (Wallberg and Moberg, 2002; Hirose, 2012). In recent years more studies have been carried out on the movement, distribution and possible concentration of radionuclides in the ocean environment (Fowler, 2011). Seawater and sediment are the most important sources of radionuclides to marine organisms (Khan and Wesley, 2012). The concentrations of radionuclide in marine biota can be determined by monitoring fishes since the levels increase in the marine food chain by bio concentration process. The two major consequences of radioactivity at the organism level are (a) toxic effect on living tissues due to the production of strong oxidizing agents by the ionization of atoms and molecules of living materials, (b) the mutation activity.
3.8 Upcoming pollutants
The new antifouling agents such as Irgarol and diuron used in small vessels instead of tributyl tin have showed some persistence in the marine environment (Konstantinou and Albanis, 2004). Other pollutants are brominated flame retardants, Nano particles, surfactants, perflourinated compounds and endocrine disrupters.
4 Degradation of Toxic Pollutants by Marine Microbes
Microbial communities are the essential but vulnerable part of all the ecosystems, including the deep oceans. Marine bacteria are often under extreme conditions. There are enormous studies on the ability of marine microbes to degrade hydrocarbons (Nikolopoulou and Kalogerakis, 2009, Yakimov et al., 2007, Pelletier et al., 2004). Many studies have been conducted to isolate and characterize polycyclic aromatic hydrocarbon (PAH) degrading bacteria in marine and estuarine ecosystems (Daane et al., 2001). Most of the earlier studies were concentrated on isolating maximum amount of pollutant degrading bacteria. In a study carried out by Bachoon et al. (2001), after one month of exposure, the bacterial community profile of the oil-impacted sediments significantly increased compared to the control sediment. However the findings of another research showed something different from the general belief that higher amounts of pollutants may enrich more degrading bacteria. Here the exposure time and PAH concentration caused a reduction of microbial diversity (Hong et al., 2009).
Two divergent views cited in the literature are that: (i) Micro organisms can use organic pollutants as their carbon source and thus increase their diversity (Feris et al., 2004) (ii) Organic pollutants pose serious threat to microorganisms and cause serious reduction in their abundance (Bachoon et al., 2001).
Hydrocarbon seepage into the benthos affects bacterial community structure as well as diversity (LaMontagne et al., 2004). The study overlooks the long term effects of oil from naturally occurring seeps on sediment bacteria (LaMontagne et al., 2004). A study conducted in Nigeria (Nwanyanwu, and Abu, 2010) revealed the effects of petroleum refinery waste water on marine bacteria. In all the bacterial strains tested, the dehydrogenase activity was progressively inhibited at petroleum concentrations greater than 12.5% (v/v). Discharging of improperly treated effluents would pose serious threat to metabolism of the bacterial strains in natural environments. The mangrove sediment microbial structures are susceptible to PAH contamination, and complex microbial community interactions occur in mangrove sediment (Zhou et al., 2009).
Xenobiotic pollutants act as important agents in the induction of lysogenic bacteria in the marine environment (Jiang and Paul, 1996). Sewage associated micro organisms grow and compete with indigenous marine microbial flora (Baross et al., 1975). In the above said study, sewage associated bacteria had shown the capacity to grow under the oceanic conditions equivalent to depth of 2500 m.
The Extracellular Polymeric Substance (EPS) produced under laboratory conditions by the strain isolated from a microbial mat showed very high binding capacity for copper and iron salts (Xavier et al., 2009). Though this finding was aimed at development of a low cost biosorbents, it could possibly be a threat to the marine microbes.
Marine and Environment substrata are often covered by microbial biofilms. The investigations of a study (Labare et al., 1997) from University of Maryland throws light to the toxic effects of bioconcentrated tributyl tin (TBT) on oyster larvae. The study clearly depicts the impacts of bioconcentration of TBT in bacterial biofilms, while the dissolved levels of TBT had no effect on the natural attachment and metamorphosis of the organism on bottom sediments. So the role marine bacterial biofilms should be seriously taken into consideration when evaluating the heavy metal toxicity in the marine environment.


Table 3 Biomarkers of marine toxic pollutants

5 Marine Pollution Monitoring
Environmental degradation of oceans and coastal areas should not be detrimental to human health, economic development, climate issues and biodiversity.
The latest studies showed that Hg and POPs are present even in the upper trophic level of marine mammals like polar bears (Ursus maritimus), Greenland sharks (Somniosus microcephalus) and seabirds (e.g. Letcher et al., 2010). Letcher et al. (2010) also observed the presence of new pollutants such as polyfluorinated compounds (PFCs) and brominated flame retardants (BFRs) in Arctic biota indicating the extend of marine pollution by toxic and bioaccumulative pollutants."
5.1 Chemical monitoring
Chemical monitoring includes the quantitative analysis of pollutants in water and sediments. Instrumental methods like Atomic absorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS) and Inductively coupled plasma mass spectrometry (ICPMS) can be used for the detection of metals. Also the organic toxic pollutants can be determined qualitatively and quantitatively by latest chromatographic techniques such as liquid chromatography Quadrupole time-of-flight (LC-QTOF), Gas chromatography–mass spectrometry (GC-MS) etc. The monitoring of concentration of pollutants in the abiotic environment may not be able to predict the actual effects on the biota. Hence it is very significant to conduct biomonitoring.
5.2 Biomonitoring
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 (Zhou et al, 2008). The biomonitoring methods commonly used for aquatic pollution include biota population, bacteria test, acute toxicity assay, chronic toxicity assay, residue analysis etc The in-situ biomonitoring has been reviewed by Hopkin (1993) as it follows:
a) Community effects: Presence or absence of species or changes in species composition in an ecosystem due to the effect of pollution.
The log normal distribution of individuals per benthic species in the sediment samples is the simplest method for analysing the impact of pollutants in marine ecosystem.
b) Bioconcentration of pollutants : determines the concentration of pollutants accumulated or concentrated in organisms.
Bioaccumulation studies are mainly focused on the (1) lower trophic level organisms like molluscs as they are filter feeders, food sources of vertebrates and (2) wide spread pollutants with longer residence time and lipophilic nature are accumulated easily. In higher mammals it occurs through their diet.
c) Effects of pollutants: Marine ecosystem is vast and hence it is very difficult to relate effects with specific pollutants. Addison (1996) suggested the use of three biochemical responses, measurement of energy partitioning in molluscs and analysis of benthic community structure to determine the impact of marine pollution. The three biochemical responses are monooxygenase induction, metallothionein induction and acetylcholine esterase inhibition.
d) Genetically based resistance: Assessing the resistance acquired by genetically different strains of species to pollutants.
5.3 Bioindicators – bioindicators of various types of pollutants
Algae, macrophyte, zooplankton, insect, bivalve molluscs, gastropod, fish and amphibian are all considered as bioindicators. High tolerance of pollutants without death, wide abundance, distribution, long lifecycle, importance of the organism in food chain and easy sampling are all considered to be the characters of a perfect bioindicator.
In spite of the tremendous use of marine fauna in biomonitoring programs, photosynthetic organisms like algae (seaweeds) have been increasingly used as biodetectors to monitor xenobiotics in marine environments (Barreiro et al., 2002; Conti and Cecchetti, 2003; Conti et al., 2007). Microalgae has been referred to as green liver of oceans as they play central role in monitoring xenobiotics and pollutant cycling across the globe (Nystroan et al., 2002) due to their substantial biomass and comparatively large surface-to-volume ratio (Okamoto and Colepicolo, 1998).Seaweed species were used as bioindicator for toxic trace elements (Serfor-Armah, et al., 2001).
5.4 Biomarkers – significance, identified biomarkers
Biomarker can be defined as the measurements of body fluids, cells, or tissues that indicate in biochemical or cellular terms the presence of contaminants or the magnitude of the host response (Bodin et al., 2004). Biomarkers offer an effective early warning system in biomonitoring of aquatic environments. Biochemical, cellular, physiological and behavioral variations in the tissue, body fluids or of whole marine organisms could be well defined by biomarkers (Lam and Gray, 2003; Galloway et al., 2002, 2004).
The effect of pollutants on the cellular biochemistry of microalgae and the biochemical mechanisms that they use to detoxify pollutants are being well researched around the world (Conti et al., 2007; Barros et al., 2005).
Lysosomal membrane stability (LMS) is considered as a very reliable biomarker of general stress in biomonitoring studies, as it is the main lysosomal response to a wide range of pollutants (Domouhtsidou and Dimitriadis, 2001). LMS test of digestive glands and Neutral Red Retention Assay (NRR) of the hemocyte lysosomes are evaluated to check the stability of lysosomal membrane, as both are influenced in marine bivalves by a wide range of stressors including temperature fluctuations (Petrovic et al., 2004; Bocchetti and Regoli, 2006; Moore et al., 2006; 2007). It has been reported that NRR measures the lysosomal content efflux into the cytosol which, in stressed mussels, reflects a physiological process after membrane damage and comparatively measures the capacity of cellular processes to adapt to stress conditions (Lowe and Pipe, 1994).
Acetylcholinesterase (AChE) activity has been measured by many researchers as exposure biochemical biomarker in the invertebrates in coastal waters and rivers (Moulton et al., 1996; Stien et al., 1998). Thus they have potential application in marine environment in screening the effects of pesticides and other pollutants in some vertebrate and invertebrate species. The responses of these biomarkers in digestive glands and hemocytes of horse-bearded mussel Modiolus barbatus during thermal stress was monitored (Vasileios et al, 2012). Results of several other works have also reported the connection between responses of biomarkers in marine molluscs to thermal stress (Domouhtsidou and Dimitriadis, 2001; Kagley et al., 2003; Petrovic et al., 2004; Bocchetti and Regoli, 2006; Moore et al., 2006a, 2007).
The use of biomarkers could provide useful information about the higher critical lethal ambient temperatures (Tcmax) initiating irreversible cellular damage in the tissues of marine bivalves during perturbation of its ecosystem because of global warming (Vasileios et al., 2012).
Another study in line with previous investigation,point outs the histochemical localization of N-acetyl-b-hexozaminidase (Hex), acid phosphatase (AcP) and b-glucuronidase (b-Gus) in the digestive gland of mussels Mytilus galloprovincialis (Raftopoulou and Dimitriadis, 2012) The results indicate appreciable alterations of the above parameters in large-sized mussels, supporting their greater influence by the environmental factors, in relation to small-sized ones. The application of cytochrome P 4501A, DNA integrity, Acetyl cholinesterase (AChE) and Metallothionein as molecular biomarkers for marine pollution monitoring are well documented in earlier studies (Sarkar et al., 2006). Some of the marine biomarkers identified by various researchers are given in the table 3.
Mussel/bivalves and other lower level organisms have shown molecular level variations against different pollutants. Most of the metals are affecting the acetylcholinesterase activity as well as Metallothionein induction of these organisms. Genotoxic substances like persistent organic pollutants affects the DNA integrity of the organisms.
5.5 Isotopes in marine pollution monitoring
Nuclear and isotopic techniques offers the diagnostic and dynamic information needed to identify the source of contamination, its history of accumulation, its environmental pathways and its impact on the marine environment.
Stable C, N and H isotope ratios and Carbon-14 isotopes are being used for the identification of sources of pollutants like PAHs, PCBs, Hydrocarbons/oil contamination and waste disposal from land (Mckinney, 2012; Carballeira, 2012). C14 can also be used for the isotope labelling studies to understand the fate of contaminants. Stable isotope ratio measurements of chlorine and oxygen have been applied for discrimination of different perchlorate (ClO4) sources in the environment. The stable carbon and nitrogen isotopes are mainly used to track biomagnification of persistent bioaccumulative toxic pollutants (PBTs) (Aubail et al., 2011; Cardona-Marek et al., 2009; Cabana and Rasmussen, 1994). This is due to difference in fractionation with trophic levels, these two elements give complementary information. Stable nitrogen isotope ratios (15N/14N) increase at every step in the food chain because stable N isotope concentration increase 3-4‰ per trophic level and thus indicating trophic level, while stable carbon isotope is enriched slightly by about 1‰ per trophic level and can provide information on the primary carbon sources into food webs (Michner and Schell, 1994). The isotope ratio mass spectrometry (IRMS) is used for the detection of isotope ratios.
Lavoie et al. (2010) studied the transfer of total mercury (THg) and methylmercury (MeHg) in a Gulf of St. Lawrence food web to the trophic structure, from primary consumers to seabirds, using stable nitrogen (δ15N) and carbon (δ13C) isotope analysis. He observed that biomagnification power were greater for pelagic and benthopelagic species compared to benthic species whereas the opposite trend was observed for levels at the base of the food chain.
Isotopes such as C-14, Cs137and Pb210 can be used to reconstruct the changes in pollution over time in the marine sediments, corals etc. which stores information on the pollution during the geological past. This will enable us to understand and compare the actual impacts of the pollutants.  
6 Conclusion
Marine environment is constantly being deteriorated due to human activity. Therefore, it is an emergency need to evaluate sensitive ecotoxicological endpoints and continuously monitoring pollutants, in order to detect the toxic effects of compounds in the complexity of marine environments. There are various monitoring programmes organised by each countries, a group of countries or an international organisation. The importance of understanding the marine environment and the xenobiotics that deteriorate its quality is very vital as more than 70% of the earth‘s surface is covered by the interconnected bodies of water.
The Authors acknowledges the positive comments and suggestions by anonymous reviewers.
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