The river Hayle is found in an area of the southwestern United Kingdom with a history of mining since Neolithic times. Drainage from these mines over such long periods of time, particularly during the 1800s when mining reached its peak in this area, has led to exceedingly high levels of metals in this river. Amazingly, this river has a viable population of brown trout (Salmo trutta) that can stand these very toxic conditions. But how can the brown trout tolerate such high levels of contamination? Tamsyn Uren Webster, Eduarda Santos and their UK colleagues decided to find out using cutting-edge molecular techniques, and have published their findings in a recent issue of Environmental Science and Technology.
First, the team wanted to find out how high the levels of various metals actually were in these fish. They compared zinc, copper, cadmium, arsenic, iron, lead and nickel levels in various tissues from brown trout from the river Hayle with those from fish living in the clean and nearby river Teign. They found that brown trout from the river Hayle had 30–60 times higher levels of metals in their gills, livers and kidneys. Moreover, brown trout from the river Hayle had some of the highest concentrations of copper and zinc ever recorded in fish. These results confirm that these fish are exposed to and absorb these metals and, more importantly, that they are truly tolerant of such high metal contamination.
Next, the team investigated which genes were involved in such high metal tolerance in the Hayle river brown trout. To do this they sequenced RNA transcripts from all expressed genes using cutting-edge technology called high-throughput sequencing. As the genome of brown trout has not been published, the researchers first had to use computer programs to put together the majority of the genome of brown trout from small sequences of RNA transcripts. This process is an impressive undertaking as it is similar to putting together a puzzle with 70 million pieces without being able to look at a picture of it or knowing what exactly the picture is.
With a large part of the genetic code of brown trout assembled, the researchers compared active genes between the trout from the Hayle and Teign rivers. The researchers predicted that any differences in which genes were active between the populations would be related to surviving in a metal-contaminated river. As suspected, the brown trout from the river Hayle did indeed turn on genes that would help them store and detoxify the toxic metals. Additionally, brown trout from the river Hayle also activated genes that can help them compensate for disturbances to key ions, such as calcium, sodium and chloride, as ion imbalance is a life-threatening problem in this toxic environment.
Brown trout in the river Hayle are champions of metal tolerance perhaps because they have had the chance to adapt to this environment over several generations. Regardless, Uren Webster, Santos and their colleagues have unravelled some of the secrets to surviving in such a polluted environment and they give us hope that some fish can endure large environmental disturbances.
