Fish dislike heavy metal
How trout cope with toxic pollution
The remarkable tolerance of some fish to high concentrations of damaging pollutants has been a source of intrigue for biologists. Does this acquired ability point to evolution, or has some sort of innate response system been activated? Environmental scientists are keen to discover more so that their management of aquatic ecosystems is more effective,1 but there are some interesting lessons for all of us.
Cornwall, on the south-western tip of England, is famed for its historic tin and copper mines, once some of the richest in the world. In fact, by 1870, with 2,000 mines operating, it led the world in tin production. However, mining for these metals was already happening before the beginning of Roman occupation and probably centuries before Christ. For instance, Sicilian historian Diodorus Siculus2 (90–30 BC) wrote: “The natives [of Belerion3] work the tin, treating the bed which bears it in an ingenious manner. The bed, being like rock, contains earthy seam, and in them the workers quarry the ore, which they melt down and cleanse of its impurities…”4 Over the centuries, indeed millennia, mining for these heavy metals—plus iron, arsenic and smaller quantities of other minerals5 —greatly contaminated the Cornish groundwater and rivers. While all metal mining had ceased by 2007, there is a lasting heavy metal legacy.
A metallic assault
As in human bodies, and in common with other backboned creatures, a fish’s liver and kidneys detoxify and clean the blood. For this reason, these organs are obvious ones to study for the presence of any toxins. Scientists from the University of Exeter (UK) compared these and other tissues in brown trout (Salmo trutta) from two rivers in south-west England, the relatively clean River Teign (Devon) and the heavily contaminated River Hayle (Cornwall)—still affected by past industrialisation.6 The results were striking. Compared to the Teign trout, those from the Hayle were coping with much higher levels of heavy metals7—19 times higher in the kidney, 34 times higher in the liver (averaged across all the metals measured)! Since the gills were highly contaminated (63 times higher) but not the gut, they established that most of the poisonous build-up was from the water rather than their food. The researchers were surprised at the trouts’ “extraordinary ability to cope” with this heavy metal assault.1
Heavy metal poisoning can be serious. In ourselves, it can damage a comprehensive range of organs and tissues and is seemingly linked to autism.8 I am old enough to remember my parents’ warnings, as a young boy, about drinking water from upstairs bathrooms (far from the rising main) due to the risk of lead poisoning (plumbism). This can impair learning and seriously harm a person’s physical and mental health; of course, most lead piping has long since been replaced by copper or plastic.
Metal contamination also causes serious harm to aquatic wildlife globally; unfortunately, most fish don’t cope as well as these Cornish trout.
So, just how are the Hayle trout managing their endurance feat? When the researchers took a closer look at their overall gene expression9 (comparing this with fish from clean water), they discovered that a type of well-known metal-binding protein, metallothionein, was responsible. This is one of a class of proteins found throughout the living world (the first discovered 50 years ago), fulfilling several important purposes, including heavy-metal protection.10 For instance, metallothioneins are produced in normal human liver and kidneys, and altered levels seem to be associated with certain cancers of these organs.11 Clearly, it serves a very important role. These hardy fish were producing metallothionein and other metal-binding proteins in much larger quantities than their clean-living cousins from the River Teign.
While some might hail this as an evolutionary adaption, the facts speak otherwise. Commendably, the researchers merely concluded, “This indicates that a natural response system is activated in the Hayle trout which enhances their ability to cope with the metals they take up” (my italics).1 This makes a lot of sense. At the molecular-biological level, a great deal is going on in order for these fish to flourish in conditions that would ordinarily be seriously detrimental; this is clear from the fact that the researchers found dozens of genetic and biochemical changes of all sorts in the metal-stressed fish.6 Sensors must detect, then respond to, the presence of the heavy metals. In turn, other mechanisms must trigger cells to increase the expression of specific genes, resulting in higher production of the right proteins. These metal-binding and detoxifying proteins must particularly be expressed in the organs and tissues of the fish’s body where they’re most needed. It doesn’t stop there. The large quantities of protein-bound heavy-metals must be sequestered or excreted so they cause no further harm—the programming to achieve this is also a vital part of the trout’s heavy-metal response system.
All of these many processes are accomplished by a host of custom-made, highly integrated proteins, each of which is a sophisticated molecular machine in its own right. The programming language responsible for such a response system must be impressive, to say the least. Numerous DNA instructions are needed to specify each complex component of the whole. Coded information of this sort requires a programmer.
The researchers were surely correct in describing the anti-metal-threat system as ‘natural’.12 Allowing that the Hayle trout population may be showing adaptation (that is, acquiring a metal-tolerance ability that was not there before), they suggest it may instead be the “result of an inherent genetic plasticity.”13
This is the most likely explanation because of the numerous changes seen in the Hayle trout and how quickly14 they have come about. There is growing awareness among evolutionists that certain changes observed in living things are much too rapid to be explained by natural selection of lucky, useful mutations as championed by the likes of Richard Dawkins; instead, they result from an in-built capacity.15
Ordinarily, fish cannot tolerate large concentrations of heavy metals. However, it seems that the Maker of these humble trout, knowing the sorts of stresses to which such fish would be subject in a post-Fall world, created them with a canny, innate system, ready to be activated in such situations. One might say that they’re ‘designed to adapt’, enabling them to thrive where other (metal-naïve) fish would suffer acute metal toxicity.
References and notes
- Santos, E. & Uren Webster, T., Secrets of Cornwall’s metal-tolerant trout, Planet Earth, Spring 2014, pp. 28, 29. Return to text.
- Much of his Bibliotheca historica (itself drawing on earlier works) still survives and covers such well-known classical themes as the Trojan War and the conquests of Alexander the Great. Return to text.
- Ancient name for Cornwall, or Kernow in the Cornish language. Return to text.
- From: Cornwall Council, Prehistory and Romano-British Period (ad 43–410), 2011; available at cornish-mining.org.uk, accessed 28 January 2015. Return to text.
- Including: lead, zinc, silver, uranium, antimony, bismuth, and cobalt. Return to text.
- Uren Webster, T.M. et al., Global Transcriptome Profiling reveals molecular mechanisms of metal tolerance in a chronically exposed wild population of Brown Trout, Environmental Science & Technology 47(15):8869–8877, 2013 | doi:ipdf/10.1021/es401380p. Return to text.
- They tested for zinc, copper, cadmium, iron, lead, nickel, and the mineral arsenic. Return to text.
- Adams, J.B. et al., Toxicological status of children with autism vs. neurotypical children and the association with autism severity, Biological Trace Element Research 151(2):171, 2012|doi:10.1007/s12011-012-9551-1 Return to text.
- The range of proteins produced by an organism depends on which genes are switched on (expressed). Other factors determine how long the production of a particular protein lasts, the quantity produced, and so on. Return to text.
- Ruttkay-Nedecky, B., et al., The role of metallothionein in oxidative stress, International Journal of Molecular Science 14(3):6044–66, 2013 | doi:10.3390/ijms14036044. Return to text.
- Huang, G.-W. & Yang, L.-Y., Metallothionein expression in hepatocellular carcinoma, World Journal of Gastroenterology 8(4):650–653, 2002; and Pal, D., et al., Metallothionein gene expression in renal cell carcinoma, Indian Journal of Urology 30(3):241–244, 2014 | doi:10.4103/0970-1591.134242. Return to text.
- They do say, “we do not know how or when this tolerance arose”, a sop to evolution; see ref. 1. Return to text.
- Ref. 6. Related to the term ‘phenotypic plasticity’, this means that a rapid and significant change in a creature’s physical makeup (phenotype) is, in turn, due to its genome also being ‘plastic’. That is, the DNA has pre-programmed flexibility, enabling a rapid adaptive response to the environment—but the instructions were there all along. Return to text.
- That is, over the course of several centuries, associated with the more intense mining of the industrial era—a virtual instant in evolutionary terms. Return to text.
- This is explained more fully in: Statham, D., Only the Bible explains the diversity of life, Creation 37(1):40–43, 2015. Return to text.