My field notes for my most recent visit to Wastwater include a just-decipherable scrawl of “usual Stigonmema/Rivulariaeceae” against one of the samples that I scraped from a rock just below the water level. These are the dark patches, easily overlooked, found on only the largest, most stable boulders in the littoral zone of the least productive lakes in the country that I’ve written about before (see “Tales from the splash zone …”). Each time I visit, I diligently scrape a few specks into a sample bottle to check. And each time, I find much the same genera. Except last week I found something I had not seen here before.
It was a cyanobacterium consisting of several narrow green filaments (technically “trichomes”) of cells all intertwined within the same sheath. Watching closely, I could see the individual trichomes gliding back and forth within the sheath. I recognised the genus as Microcoleus (see “How to make an ecosystem”) but had not found it in freshwaters before. It is much more common in marine intertidal zones although a few species – including this one, M. lacustris – are found in freshwaters.
Microcoleus, like several of the other cyanobacteria I find in these habitats, is capable of fixing nitrogen although, unlike these, this does not take place in specialised cells called “heterocysts”. You can read more about this in my earlier post on this genus but, for now, we will think about the bigger picture. I’ve never seen a paper that sets out the role that all these unassuming mats and crusts play in ecosystems such as Wastwater so what follows is largely speculation. But, as Wastwater is a very nutrient-poor lake, these Cyanobacteria are a potentially useful source of carbon and nitrogen for other organisms. For various reasons, these organisms inhabit the littoral zone of nutrient-poor lakes, whereas we would expect to find their relatives in the plankton of nutrient-rich lakes (see “Both sides now …”). There may be some invertebrates that are able to graze on these tough, unappetising patches of Cyanobacteiria; however, I suspect that the main route by which these organisms drive the wider littoral ecosystem in Wastwater is simply by organic compounds leaking out of the cells. There has been a lot of work on this phenomenon in other ecosystems so it is probably a reasonable extrapolation to assume that this happens in Wastwater too. This organic carbon will then be absorbed directly by bacteria and some heterotrophic protists and these, in turn, will be eaten by larger organisms.
Quite how important this is in the grand scheme of things is hard to judge. As soon as there is a sniff of nitrate from other sources – runoff from agriculture for example – these sensitive cyanobacteria will be outcompeted by other algae. Even when the lake is as pristine as Wastwater or Ennerdale, however, these species only seem to thrive when there are large emergent boulders, and this is not the case for the whole perimeter of either lake. Hanging on in the splash zone may mean that they can grab nitrogen from the atmosphere, so they are acting as pumps, sucking nitrogen into the lake ecosystems. It may not be much, but not much added to the very little that is there already could be, proportionately, quite important.
Some other highlights from this week:
Wrote this whilst listening to: Letter to You, new album by Bruce Springsteen and the E Street Band.
Cultural highlights: Enjoyed the gothic romance of Rebecca, just released on Netflix, which included Pentangle’s Let No Man Steal Your Thyme on the soundtrack.
Currently reading: The Poisonwood Bible by Barbera Kingsolver.
Culinary highlight: Homemade pasties filled with kedgeree, inspired by Great British Bake-off..