Diatoms from the Troodos mountains

Troodos_snowscape_Apr19

Back in April, I wrote two posts about the algae from a stream draining a chromite mine in the Troodos mountains in Cyprus (see “Survival of the fittest (1)” and “Survival of the fittest (2)”).  I also planned to write a post about the diatoms growing in the stream but the slide I prepared has been sitting on my desk over the summer whilst I was distracted by other things.  However, I have just started looking at some samples from metal-enriched streams in the northern Pennines and, curious to see whether a Cypriot chromite mine had similar effects, I blew the dust off the slide and slipped it under my microscope.

The principal effect of toxic pollution is to reduce the number of species found and, in this respect, my sample from the outflow of the Hadjipavlou mine outflow was true to form, containing just eight species.  The most abundant of these was Meridion circulare, accounting for one in four of all the cells.  What is more, many of the cells were visibly distorted (see images a., c. and d., in particular, in the plate below).  This is quite a common phenomenon in metal-polluted streams (see “A twist in the tale”) though I have not seen it quite so obviously in Meridion circulare before. My own pet theory is that one of the enzymes involved in laying down the silica cell wall has a metal co-factor that is displaced by heavy metals.

Meridion_circulare_Hadjipavlou_Apr19

Meridion circulare from thepebbles from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre).   The photograph at the top of the post shows snow on the Troodos mountains near the mine.

The only other diatom that was at all common in the sample was Hantzschia amphioxys, which also occurred alongside a smaller population of Hantzschia abundans.  I’ve not come across Hantzschia in metal-enriched streams before: it is a species that is most often associated with habitats that are not permanently submerged.  That may be the case at Hadjipavlou but the water that flows from mines comes from groundwater rather than rainfall so would not be subject to the strong seasonal variations that we associate with Mediterranean streams.  It is hard to draw a firm conclusion from a single visit.   Unlike Meridion circulare, however, neither population of Hantzschia showed any obvious distortion, perhaps due to the Hantzschia cells being more heavily silicified than those of Meridion circulare.

The extent to which cellular distortions are obvious does vary between species, as can be seen in “A twist in the tale …”  which compared three different representatives of the same genus in a metal-polluted stream.  I chose the word “obvious” with care as I do think that these phenomena are more easily seen in long thin cells than in shorter ones.  In the same Pennine streams where distorted Fragilaria are common, for example, I can also see distorted cells of smaller diatoms such as Achnanthidium minutissimum.  But you need a keen eye to spot these reliably.   Some other people have used fluorescent stains to look at other cellular irregularities, such as the position of the nucleus and damage to the nuclear membrane, but these require specialist approaches whereas distortions to cell outlines can be spotted from a standard analysis.

Hantzschia_spp_Hajipavlou_Apr19

Hantzschia abundans (k., l.) and Hantzschia amphioxys (m. – p.) in the from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre). 

A few years ago I was involved in a study of diatoms from streams in Cyprus and I dug out some of these data in order to put the Hadjipavlou sample into context.  One immediate surprise was that many of the “reference” (i.e. pristine or near-pristine) samples in that survey also had relatively low diversity.   The 45 samples in this subset had, on average, nine species, and a mean Shannon diversity index of 1.7, compared to eight species and a Shannon diversity index of 1.42 for the Hadjipavlou sample.   I’ve never been a fan of diversity indices as measures of ecological quality (see “Baffled by the benthos (2) and links therein”) although I suspect that average diversity at Hadjipavlou measured over a period of time will always be low whereas average diversity at unimpacted sites is more likely to fluctuate. Equally, low diversity coupled with a second strand of evidence, such as distorted valves, is a useful sign to an ecologist that something untoward is happening.

diversity_indices

Number of taxa (left) and Shannon diversity (right) recorded in 45 samples from “reference” sites (i.e. minimal evidence of anthropogenic alteration) in Cyprus.  The arrows indicate the location of the Hadjipavlou stream within this dataset. 

The irony of writing about a heavily-polluted stream in the Troodos mountains is that the geological conditions which created the metal-rich veins hereabouts also create conditions for many plants endemic to Cyprus.   The serpentine and other ultramafic rocks create metal-rich soils within which few plants can survive (more about these here. I suspect that few of the plant enthusiasts drawn to Cyprus will ever cast more than a cursory glance at the green flocs adorning the abandoned mines of the Troodos mountains.

References

Licursi, M., & Gómez, N. (2013). Short-term toxicity of hexavalent-chromium to epipsammic diatoms of a microtidal estuary (Río de la Plata): Responses from the individual cell to the community structure. Aquatic Toxicology 134-135: 82-91.  https://doi.org/10.1016/j.aquatox.2013.03.007

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