Development and validation of biomarkers in a finfish species from southern Australian contaminated waters

In ecotoxicology, biomarkers are measurable biological responses to xenobiotics which are present in the environment and in organisms. Such biological responses are limited to sub-organismic levels including organ or tissue level, cellular or sub-cellular level and molecular level. These lower levels of biological responses precede the higher level of biological organization changes such as population, community and ecosystem. As such, the most important function of biomarkers is to provide early warning signals of significant biological effects. Appropriate organisms should be employed to detect such biomarker signals. In an aquatic ecosystem, bottom dwelling fish are commonly used for biomarker studies. Southern sand flathead (Platycephalus bassensis), is an endemic finfish species widely distributed along the southern Australian coastal waters. This species has been used as an indicator species since 1970s as it is demersal, sedentary, carnivorous and abundant, all favourable traits for environmental ecotoxicology research. However little is known about the effects of anthropogenic pollutants on this species so far and very few biomarker-related studies have been conducted on this ecologically important species. The aim of this thesis was to develop and validate the molecular and histological biomarkers in sand flathead from southern Australian contaminated waters. The development of biomarkers was conducted in sand flathead from a historical metal polluted estuary (Derwent estuary, Tasmania). In terms of validation, the findings from the Derwent estuary were further examined in sand flathead from a more complex marine environment (Port Phillip Bay, Victoria). This research had four research chapters to address this question. 1) Assess the relationships between thyroid-related genes transcripts and trace metals in sand flathead from the Derwent estuary. 2) Investigate gill histology and hepatic expression of metal homeostasis-related genes in sand flathead from the Derwent estuary. 3) Assess the liver histological biomarkers and usefulness of Diablo/SMAC homolog and GRP78 gene as biomarkers in sand flathead from the Derwent estuary. 4) Examine the applicability of all the candidate biomarker genes and liver histology in sand flathead from Port Phillip Bay. The data from field studies suggested that trace metals such as mercury and arsenic are associated with the transcripts of thyroid hormone related genes and environmental metal exposure influenced metals homeostasis related genes on transcriptional level, Apoptotic genes, Diablo/SMAC1 and Diablo/SMAC2 responded differently to environmental metal exposure and the ER stress marker gene GRP78 could be induced by metal stress. The results of gill histology indicated the both metal pollution and pathogen infection could contribute to gill lesions. A negative correlation observed between mercury residues and parasites number suggested pollutants may affect the abundance of gill parasites. The results for liver histology suggested chronic environmental metal exposures may contribute to the increased prevalence of inflammation and bile duct fibrosis. Port Phillip Bay is subject to high level of organic pollutants and heavy metal mainly due to anthropogenic activities and compared to the Derwent estuary is a more complex marine environment. Results for Port Phillip Bay suggested thyroid hormone receptor genes are the major target for pollutants; Corio Bay was the most affected sampling site as some genes such as transthyretin and ferroportin 1 transcripts were only regulated in fish from this area; Diablo/SMAC1 is a potential novel biomarker gene in female fish due to its high sensitivity response to environmental pollutant exposure. Combining the gene expression results from the Derwent estuary; these data implied different pollutants impact different genes of these pathways. In terms of assessment of effects of pollution, pathway based molecular biomarker genes could provide a more comprehensive and integrated picture than the traditional single biomarker gene. In terms of histological biomarkers, a higher prevalence of granuloma was observed in the fish with high level of total arsenic, suggesting environmental arsenic exposure may contribute to the formation of this lesion. In conclusion, this research focused on the development and validation of histological and molecular biomarkers in sand flathead. The gene transcripts involved in four different biological activities were newly identified. Two major organs, gill and liver, were used for histological assessment. The associations between the gene transcripts, pollution and parasites were observed in this work. Several gene transcripts, liver histological condition and absence of gill parasites potentially serve as biomarkers for future environmental monitoring. The results in this thesis have made a substantial contribution to understanding of interactions between pollutants and fish.