Fish Otolith Microchemistry as a Biomarker of Metal Pollution in the Estuarine Ecosystem

Abstract

Numerous metal pollutants naturally find their way into estuaries, where many of them build up in the bodies of fish. While otoliths can give a historical record of pollution exposure, metal concentrations in soft tissue and water samples require ongoing, long-term sampling procedures. Fish have otoliths, which are three pairs of ear bones called the sagitta, lapillus, and asteriscus. The chemical makeup of these otoliths can be a useful tool to determine the presence of hazardous substances in fish because the physiological activity of fish is controlled by a variety of environmental factors. The possible use of otoliths as inorganic tracers of metal contamination will be covered in this chapter.

1. Introduction

In aquatic environments, heavy metals stand out as the primary pollutants due to their widespread distribution and substantial presence. They tend to amass within the tissues of freshwater organisms, leading to fish poisoning and causing diverse effects across multiple biological levels, often resulting in pathological alterations. The majority of fish have otoliths, or “ear-stones,” which are calcium carbonate structures that aid in hearing and balance and are located below the brain. The otoliths float in fluid-filled sacs in the inner ear and are not connected to the skull or any other bone. The three pairs of otoliths most common to fish are the sagitta (the largest pair), lapillus, and asteriscus [1]. Only fish with cartilage (such as sharks, rays, and chimeras) and fish without jaws (such as lampreys and hagfish) lack otoliths. Black drum (Pogonias cromis) and Atlantic croaker (Micropogonias undulatus), two fish that inhabit artificial habitats and vocalize during mating, have exceptionally large otoliths. The relatively smaller otoliths of species that swim continually, including bluefin tuna (Thunnus thynnus) and alewives (Alosa pseudoharengus), may be an attempt to control excessive movement. The most popular applications for sagittal otoliths, which are hard, porous calcium carbonate structures in teleost fish inner ears, are population and microchemistry investigations. They play a role in fish hearing, motion, displacement, sensation, and linear acceleration [2]. Calcium carbonate crystallites implanted in a protein lattice mineralize to produce otoliths, which then continue to grow [2]. Metals are among the major chemical toxicants polluting the environment due to their prolonged persistence and complex interactions with organisms in aquatic ecosystems. Consequently, changes in metal levels can be reflected in aquatic organisms, serving as biological indicators of metal exposure [3]. The concentration of metals in fish tissues mirrors their exposure through water and/or food and serves as a means to evaluate the present condition of these organisms before the onset of adverse toxic effects. Nevertheless, excretion mechanisms modify the accumulated metal burden over time. As assessments of accumulation levels are typically sporadic, they cannot serve as markers for constructing comprehensive lifetime accumulation histories. However, the toxic impact of metals is notably pronounced during the early phases of fish development, significantly affecting various metabolic processes and leading to developmental delays, morphological and functional abnormalities, or mortality in the most vulnerable individuals. Substantial accumulation levels throughout an entire lifespan or during critical developmental stages can detrimentally impact the future of populations. If necessary, trace elements can be absorbed into either the organic or inorganic component of the developing otolith. The elemental incorporation of metal contaminants in the aragonite matrix of the otolith is not a simple function as the process is species-specific [3], driven by environmental conditions [4], and route of exposure [5]. Studies on the application of otoliths in the metal exposure assessment of aquatic ecosystems and their comparison to soft tissues as bioindicators are rare. Thus, it is very important to assess whether metal accumulation in hard tissues (scales, otoliths) reflects metal exposure in correspondence to soft tissues (liver, muscle) of fishes [5].

2. Metal pollutants in estuaries

Estuarine ecosystems serve as crucial transition zones between ocean and river ecosystems, boasting high productivity in aquatic environments. These areas are home to a diverse array of fish species, many of which hold significant commercial value. Estuaries play essential ecological roles, serving as nursery areas for juvenile fish, feeding grounds for resident and adult fish, and temporary environments for the reproduction and migration of various species [6].

Given their ecological significance, estuaries provide valuable goods and services for human activities. However, these ecosystems face growing anthropogenic pressures, particularly from pollution. Estuaries naturally act as sinks for a variety of metals, accumulating them at concentrations that, while potentially highly toxic, are generally non-lethal. This accumulation of metals poses a threat to the overall health of estuarine ecosystems [7].

Metal pollution poses a worldwide challenge in estuaries, stemming from historical contamination legacies and ongoing increases in metal emissions. Nevertheless, establishing water and sediment standards or implementing effective management strategies in brackish systems has proven challenging, primarily due to the complex, multidisciplinary nature of estuarine processes. As per the European Commission, achieving an integrated understanding of the fate and effects of contaminants across various compartments of these transitional environments (including estuarine sediment, water, and biota) is still necessary to more effectively establish, evaluate, and monitor the targeted good ecological status outlined by the Water Framework Directive [8]. Numerous environmental issues stemming from human activities like industrialization and urbanization are faced by this ecosystem. Physical, chemical, and biological processes all play significant roles in the metal biogeochemical cycle in this complex coastal environment. However, metal pollution in the coastal area has been brought on by anthropogenic factors, including urbanization and industrialization. Since most metals can poison organisms when present in amounts over threshold levels, this issue is getting more and more serious [9]. The sources include mining, the production of metal products, the disposal of solid waste, the burning of fossil fuels, and municipal and industrial waste effluent [10, 11, 12]. Particle size, amounts of particulate and dissolved organic matter, and many other variables all have an impact on the amount of exchangeable metal stored in the sediment. Thus, it indicated decreased cobalt and cadmium adsorption and enhanced zinc adsorption [13]. It is generally known that benthic fauna can serve as a sign of heavy metal contamination [14]. Several aquatic species including Tittorina sp., Patella sp., Mytilus edulis, Scrobicularia plana, Macoma sp., Nucella sp., and Nereis sp. are among the species and/or genus that are frequently employed. Heavy metal concentrations in all of these creatures could be thousands of times higher than they are in the water column. Therefore, heavy-metal concentrations can be easily and reliably determined as long as enough material can be gathered. However, seasonal fluctuations in metal content have been identified in several benthic mollusks [15]. The degree of toxicity of various metals in estuarine organisms varies depending on the chemical form of the metal in the solution [16]. The composition of both organic and inorganic ligands in the medium influences the free metal ion, which in turn affects the availability and toxicity of trace metals. As a result of complexation with chloride ions, it has been demonstrated that the concentration of free cadmium ions in solution varies inversely with salinity [17]. Other trace metals may behave similarly, making it likely that the estuarine biota’s access to them will frequently rely on the surrounding medium’s salinity. Unlike freshwater systems, where nutrients primarily limit primary production, the growth of phytoplankton in coastal zones is predominantly constrained by a complex interplay of factors, including nutrient loading, the system’s filtering capacity, contaminants, light availability, and ecological interactions. For example, exposure to less than 90 μg·L⁻¹ of Cu or Zn has been shown to result in growth reductions of up to 50% for cosmopolitan phytoplankton species. Laboratory experiments simulating metal exposure have indicated that phytoplankton may experience reduced cytokinesis, disturbances in photosynthesis, and increased cell size in response to various metals and species [8]. In contaminated estuaries, it is evident that cellular concentrations of metals may reach toxic levels, exerting control over growth and potentially leading to alterations in phytoplankton productivity and species composition [18]. Nevertheless, the quantitative significance of these effects under environmental conditions is yet to be fully elucidated.

Ultimately, the physical and chemical gradients within estuaries play a crucial role in shaping biogeochemistry and organism physiology, thereby influencing the exposure and toxicity of organisms. The significance of each gradient in determining the behavior and toxicity of metals can vary considerably and show seasonal variations. For example, the impact of salinity on partitioning coefficients may be significantly influenced by fluctuations in river flow, input of particulate and organic matter, and phytoplankton growth. Consequently, the behavior and effects of metals in estuaries are dynamic and depend on both environmental and biological factors. These dynamics must be taken into account when establishing site-specific water quality criteria and environmental quality targets [19].

3. Trace metal accumulation

To date, a comprehensive analysis has identified 51 elements in otoliths, encompassing major components such as calcium (Ca), carbon (C), oxygen (O), and nitrogen (N), as well as minor elements like sodium (Na), strontium (Sr), phosphorus (P), magnesium (Mg), potassium (K), chloride (Cl), and sulfur (S). Additionally, numerous other elements are present at trace levels. Otolith microchemistry has emerged as a sclerochronological biomarker for aquatic pollution by metals. The majority of these elements exist at micro and trace levels, necessitating thorough investigation for detection [20]. The incorporation of elements into the otolith can occur through one of three mechanisms: random entrapment in the crystal lattice, calcium substitution on the developing crystal surface, and binding to the organic matrix components. This suggests that the otolith’s chemical composition reflects certain physicochemical features of the surrounding water. Recently, the otolith microchemistry of trace elements has gained recognition as a valuable technique for providing insights into a fish’s life cycle and environmental history, particularly challenging information to obtain otherwise. However, its application has been limited in studies related to freshwater environments [21]. The concept of the bioaccumulation factor provides insight into the absorption of metals by fish from their environment. The variation in accumulation capacity in fish tissues is influenced by a combination of external and internal factors. The metal accumulation in fish tissues is species-specific, meaning that the concentration of certain metals may be higher in one tissue compared to another. Whereas the elemental signatures found in otoliths establish a baseline for quantifying pollution levels in the wetland [22].

The metal build-up has been studied using otolith chemical analysis. Many different trace metals, such as Sr., Mg, Mn, Fe, Cu, Co, Ni, Zn, Rb, Li, and rare earth elements, can be incorporated into the structure of carbonate minerals [23]. For many years, researchers have exploited the uptake of metals into the aragonite lattice of fish otoliths (ear bones) as a historical record of exposure to metals in polluted habitats. In the forensic chemical examination of crude oils, the relative abundance of two metals, in particular Ni and V, is utilized to help establish the origin of the oil. Ion size and crystallization temperature are two significant restrictions on trace metal substitution. The oxidation state of the elements, the alkalinity, and metabolism of the organism, the nutrition, and their quantities in the water could all affect ionic substitution in biogenic carbonates. The inner ears of teleost fish include calcified structures called otoliths. Otoliths are made up of calcium carbonate crystals, typically aragonite, that accumulate over the course of a person’s lifetime within a protein matrix. The organic or inorganic part of the otolith can contain trace amounts of several different elements during creation. It is believed that the fish’s environment has the biggest impact on the concentrations of these trace elements. According to their concentration in the surrounding water, certain elements, such as Sr., Ba, Hg, and Pb, seem to be deposited in otoliths [24]. Furthermore, changes in environmental contamination through time and space have been linked to the amounts of heavy metals in otoliths [25]. The element strontium, which frequently takes the place of calcium ions in carbonates, has by far received the most attention in relation to this phenomenon. Although a number of environmental and physiological factors influence how much strontium is incorporated into fish otoliths, the amount of strontium in the surrounding water is crucial. Zinc was found in some otoliths [26]; however, the precise distribution of zinc was unknown to them. The lithosphere, particularly the rocks encircling the water in which the fish lives, must be the primary source of zinc in the ecosystem. Zinc availability and uptake by fish, however, appears to be influenced by both the environment they are in and their physiological processes. Food appears to be the main source of zinc uptake in seawater. A similar analysis does not seem to have been done in freshwater. Additionally, the fish may very likely take up zinc by active ion transport through the gills. The heating regime, pH, alkalinity, and amount of particulate matter in the water are all dependent on the type of aquatic habitats, such as the availability of zinc, food-chain bio-magnification, and zinc flux. Analysis of zinc in addition to strontium may be able to provide temporally restricted information on habitat environment and fish migration if zinc exhibits a systematic distribution in otoliths along the life history transect [5].

4. Otolith microchemistry

Otolith microchemistry stands as a potent tool for investigating life stage dispersal, and regional connectivity, evaluating population structure, delineating estuarine nurseries, gauging connectivity between juvenile and adult populations and, to a lesser extent, serving as an indicator of environmental pollution. The continuous accretion of otoliths, coupled with their metabolic inertness and resistance to resorption, results in the permanent incorporation of various elements from the local environment into the crystalline matrix [27]. While otolith microchemistry can reflect a blend of local environmental chemistry and individual physiology, the resultant elemental composition acts as a distinctive fingerprint, functioning as a natural tag for distinguishing location and inferring ontogenetic changes, enabling the differentiation of otolith microstructures from various environmental conditions or locations. Consequently, marine organisms inhabiting impacted areas are likely to accumulate metals, including anthropogenic ones, from their surroundings and subsequently transfer or transport them into higher trophic levels of the food chain [28].

Otoliths, which are the calcium carbonate (CaCO₃) earstones of fish, offer a rich source of information about the life and environmental history of fish. However, the specific CaCO₃ polymorph forming the otolith represents a critical yet often overlooked aspect during otolith analysis. While otolith trace element chemistry data increasingly plays a crucial role in informing management decisions, recent research indicates that the CaCO₃ polymorphs—aragonite, vaterite, and calcite—can significantly influence the incorporation of trace elements in a non-trivial manner. While it has been traditionally believed that most fishes have otoliths composed of the aragonite CaCO₃ form, recent literature reports challenge this assumption, suggesting a more diverse landscape in otolith composition [29]. Studies on the microchemistry of otoliths in urban settings are particularly important since metal pollutants are more concentrated in urban areas than in natural ecosystems. Copper (Cu), zinc (Zn), and lead (Pb) are common metals found in metropolitan areas. Fish otoliths may include metals that have been deposited there from a variety of environmental compartments. They can be found in food sources, bonded to the sediment, present in sediment porewater, and also in the water column [30]. In order to estimate the possibility of a biological influence in aquatic environments, sediments are frequently evaluated as a line of evidence and known to be a repository of contaminants [31].

5. Conclusion

The fish otolith, commonly known as the earstone, has traditionally served as a timekeeping structure. However, recent years have seen a surge in interest in its use as a metabolically inert environmental recorder, driven in part by technological advancements. Various applications have emerged, including stock identification, determination of migration pathways, reconstruction of temperature and salinity history, age validation, detection of anadromy, utilization as a natural tag, and chemical mass marking. Some of these applications are challenging or even impossible to achieve using alternative techniques. Microsampling and the latest developments in beam-based probes now enable the coupling of elemental assays with daily or annual growth increments in the otolith, offering a detailed chronological record of the environment. Despite these advancements, there has been limited critical assessment of the assumptions underpinning environmental reconstructions or consideration of potential variations in elemental incorporation compared to other calcified structures. Drawing on insights from recent advances in geochemistry and paleoclimate research for comparison, it becomes evident that not all applications of otolith chemistry are firmly grounded, although some are poised to become powerful and perhaps routine tools for mainstream fish biologists.

Over the past three decades, otolith microchemistry has been a valuable tool in various studies, including those focused on the life history, migration patterns, and environmental ecology of commercially important fish stocks. Comparisons are made between the concentrations of elements and isotopes in otoliths and those in the water where the fish resides. The accumulation of heavy metals in fish otoliths is influenced by several factors, including the concentration of the specific heavy metal in the environment, its bioavailability, and the physiological state of the individual fish (which affects the exchange rate between external and internal environments). The chemical composition of otoliths is regulated by the physiological activity of fish, itself influenced by environmental conditions. The concentration of heavy metals in the calcified otoliths of fish serves as a tracer of environmental pollution exposures. Analyzing the microchemistry of fish otoliths provides a potent tool for monitoring pollution levels in aquatic ecosystems. These trace elements have the potential to serve as a biomonitoring tool, distinguishing between metals in different seasons. It is evident that the accumulation of heavy metals in fish otoliths depends on a variety of factors, including the concentration in the environment, bioavailability, the physiological state of the individual fish (affecting the exchange rate between the external and internal environments), the mechanisms of various species for detoxifying various metals, the growth rate of the individual fish (affecting the rate of accumulation of otolith material), and the affinity of certain heavy metals for particular fish species. In instances when metal pollution is low enough not to affect fish growth, it is more likely that the rate of metal accumulation will be higher since otoliths develop more quickly in individuals who grow more quickly. Other divalent cations, including Mg, Sr., Ba, Mn, CU, Zn, and Pb, as well as smaller monovalent cations like Li can alter the amounts of various elements across the otoliths of individual fish. However, it has also been proposed that bigger cations and anions, including Mg, can be integrated by being trapped within the crystal lattice as crystal inclusions.

Acknowledgments

The authors are thankful to ICAR-Central Institute of Fisheries Education, Mumbai, Inida-61, for providing all the necessary support to write this book chapter.

Conflict of interest

No conflict of interest.