Thursday, August 1, 2013

Heavy Metals in Fish: Toxicity and Tolerance

Today I found an interesting paper that fits right in to my new job in the field of aquatic ecotoxicology. As the name suggests, this field is a combination of ecology and toxicology that deals with the nature, effects, and interactions of harmful substances in the environment. In my case, it is aquatic, freshwater systems in particular. The paper I came across looks at the effects of metal contamination and tolerance in freshwater fish.

Metal contamination is something that occurs worldwide. A number of industrial metals (particularly copper, cadmium and nickel) have been well studied in freshwater systems. These studies have used gradients in contamination to demonstrate correlations between chronic metal exposure and physiological changes that occur as a result of the toxicity. These changes can include alterations in various metabolic processes as well as impaired growth and reproduction. This study focuses on how wild brown trout (Salmo trutta) respond when exposed to a water-borne mixture of metals.

The researchers looked at the brown trout that inhabit the River Hayle in Southwest England. Historically, this area has been mined, peaking during the 1800s. The drainage from these mining operations contaminates the river with a mixture of metals, and the middle region of the River Hayle is known to have extremely high metal concentrations. So high that few fish or invertebrates are able to live there. However, brown trout migrate between the upper and lower sections, including this area. Trout found in the lower regions of the river have been shown to have acute metal toxicity, including total zinc, copper and iron which average 639, 42 and 200 ug/L respectively. Despite these high levels, the fish are able to sustain a population with no evidence of reduced genetic diversity. The aim of this study is to figure out how this tolerance of metals is possible.

To answer this question, the researchers used an integrative approach, combining genomics with the analysis of metal accumulation in tissues. They collected embryo and adult fish from the metal-polluted River Hayle and their control, the River Teign. In the adults, the researchers sampled portions of gill, gut, kidney and liver tissues and processed them to measure the concentrations of seven metals: copper (Cu). lead (Pb), zinc (Zn), arsenic (As), cadmium (Cd), iron (Fe), and nickel (Ni). Since there is relatively little gene sequence information on brown trout, they then had to sequence, assemble and annotate transcriptome (the set of all RNA molecules [mRNA, rRNA, tRNA, and non-coding RNA]). I think you'll thank me for not going in to how they do that (if you are interested in these methods, the paper lays them out nicely), but suffice it to say it is laborious but informative. Then they performed a functional analysis for differentially expressed genes from each tissue.

When the researchers compared the metal concentrations they found all seven metals to be significantly higher in the Hayle trout than the Teign trout. Across all metals the fold change was highest in the gills (62.6-fold change) followed by the liver (33.7-fold change) then the kidney (18.5-fold change). They found no significant differences in the gut. This suggests that the gills are the primary uptake route for these metals. That makes sense considering the large surface area in direct contact with the water and the abundance of uptake carriers and transporters for these metals. After the metals are taken in by the gills, they are transported in the bloodstream to the rest of the body, accumulating in the liver and kidney. As these organs are responsible for processing, detoxification, storage and excretion it is easy to see why accumulation might happen here.

In both rivers, zinc was the most abundant metal in the gill, gut, and kidney, while copper was found to be highest in the liver. They also found zinc and copper to be the ones that increased to the greatest extent in the gills, liver and kidney. That's logical when you consider that these two metals were the ones elevated to the greatest extent between the two rivers (60- and 40-fold, respectively). They also found evidence that may link the uptake, storage and metabolism of iron, cadmium, and arsenic.

In order to identify potential mechanisms of toxicity and/or tolerance to these metals, gene expression patterns for the four selected tissues were examined in fish from both rivers. A total of 998 transcripts were differentially exposed in at least one tissue. You should expect the activity of the components involved in the body's metal homeostasis system (that ensures an adequate supply of essential [trace] metals) to change with increased metal exposure. And indeed, the researchers found at least one MT (glutathione and metallothioneins; act as buffers for metal ions entering cells and have an affinity for most metals), particularly metallothionein b, to be the most strongly up-regulated genes in the Hayle trout. This suggests that the trout's metal tolerance mechanism may be as a result of the sequestration of metals by MT. And although zinc and copper were found to be in the highest concentrations in tissues, only the zinc transporter gene was differentially expressed (down-regulated in the kidney). However, they did find changes in iron-metabolism related genes.  Since metals also disrupt the balance of ions in the body causing oxidative damage, the researchers also looked at the ion homeostasis system. They found differential expression of enzymes and a number of other genes encoding proteins that are important in maintaining ion balance.

All of this put together gives some interesting mechanisms of metal toxicity, demonstrating that these fish have developed strategies for dealing with the pollution in their environment.

ResearchBlogging.orgUren Webster, T. M., Bury, N.R., van Aerle, R., & Santos, E.M. (2013). Global transcriptome profiling reveals molecular mechanisms of metal tolerance in a chronically exposed wild population of brown trout Environmental Science & Technology DOI: 10.1021/es401380p

(image via Biopix)
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