I visited Scotland vicariously last week (meaning that I did not actually cross the border but a little bit of Scotland made its way south to me). In this case, my wife had been reconnoitring potential sites for a field course along the Fife coast and had visited the sand dunes at Tentsmuir National Nature Reserve to see if the plant succession there would make a suitable exercise for her students. Behind the sand dunes there was a low lying area of saltmarsh and, within that, large areas of algal mats. I’m guessing that, having brought me a bottle of Highland Park whisky as a reward for not killing her houseplants during her previous trip to Scotland, she thought that my liver deserved a break.
Tentsmuir is a dynamic ecosystem, with sand dunes on the coastal side and, in places, a complete succession from colonising grasses on the seaward side to mature forest on the land. However, there are also slacks behind the dunes which are periodically inundated by seawater, leading to the development of saltmarshes. The periodic wetting and drying of saltmarshes is ideal for filamentous algae and these, in turn, create a mesh of interlocking filaments that binds the sand grains and traps organic matter. Over the course of many tidal cycles, conditions become suitable for higher plants such as glasswort and sea asters. I have a soft spot for saltmarshes and sand dunes as these are the habitats where I made some of my first forays as an ecologist (see “How to be an ecologist #4”); however, I have never looked in detail at the algal mats before. So, I poured myself a glass of Highland Park, turned on my microscope and teased out a few of the filaments from my present.
Algal mats from saltmarsh at Tentsmuir National Nature Reserve. The left hand picture shows a plant of Salicornia europaea agg. (Common Glasswort, or “samphire”) surrounded by algal mats (photo: Heather Kelly). The right hand picture demonstrates how the mat retains its integrity after being removed from the saltmarsh.
The mat, in this case, seemed to be made up predominately of two types of alga: the yellow-green alga Vaucheria and the Cyanobacterium Microcoleus chtonoplastes. I often see Vaucheria in freshwaters so it was the Microcoleus that attracted my attention. It belongs to the same family as the Phormidium that we met in Mallerstang (see “The stresses of summertime …”) and we can see several of the same features: rows of almost identical cells and, in particular, no “heterocysts”, specialised cells that are responsible for nitrogen fixation. Technically, the chain of cyanobacterial cells is referred to as a “trichome” and these are enclosed in a “sheath” (seen most clearly in genera such as Scytonema: see “Tales from the splash zone …”). In Phormidium there is a single trichome per sheath but each sheath of Microcoleus contains several, often twisted around each other to form rope-like bundles.
Microcoleus cf chthonoplastes from the saltmarsh at Tentsmuir National Nature Reserve, August 2017. Scale bar: 10 micrometres (= 100th of a millimetre).
Although I mentioned that Microcoleus lacked heterocysts, this does not mean that it is not capable of nitrogen fixation. The reason that cyanobacterial cells need heterocysts is that the nitrogenase enzyme only works in anerobic conditions. The oxygen that is produced as a result of photosynthesis is, therefore, a toxin that needs to be kept away from nitrogenase. Heterocysts have thick cell walls and less chlorophyll as means of keeping the nitrogenase in an oxygen-free environment. However, some non-heterocystous cyanobacteria, including Microcoleus, are able to fix nitrogen at night (when the photosynthetic apparatus is not pumping out oxygen) . As there seems to be no protection for the enzyme inside the cells, it is possible that the daily destruction of enzyme is offset by renewed synthesis when light levels fall and there is no more oxygen being produced internally. Nitrogen-fixation is already an expensive process for cells, requiring a large amount of energy, and this will increase the cost further. However, in the case of the saltmarsh at Tentsmuir, there is a large amount of habitat available and few other organisms capable of exploiting it, so perhaps this is an investment worth making?
The benefits of that investment then “trickle down” (or up, depending on your point of view) through the ecosystem. The cyanobacteria “fix” carbon and nitrogen and, in effect, create the soil within which other organisms thrive. Janet Sprent, of the University of Dundee, calculated that, assuming nitrogen to be the limiting nutrient, then the fixation by Microcoleus and other cyanobacteria in such habitats could probably support the biomass of higher plants that is usually observed. They are, in other words, a self-perpetuating “green manure” that creates a habitat within which other organisms can thrive. In turn, by binding sand, they help to stabilise coastal features and, in turn, protect other coastal habitats and the communities that live amongst these.
Malin, G. & Pearson, H.W. (1988). Aerobic nitrogen fixation in aggregate-forming cultures of the nonheterocystous Cyanobacterium Microcoleus chthonoplustes. Journal of General Microbiology 134: 1755-1763.
Omoregie, E.O., Crumbliss, L.L., Bebout, B.M. & Zehr, J.P. (2004). Determination of nitrogen-fixing phylotypes in Lyngbya sp. and Microcoleus chthonoplastes cyanobacterial mats from Guerrero Negro, Baja California, Mexico. Applied and Environmental Microbiology 70: 2119-2128.
Sprent, J.I. (1993). The role of nitrogen fixation in primary succession on land. pp. 209-219. In: Primary Succession on Land (edited by J. Miles & D.W.H. Walton), Blackwell Scientific Publications, Oxford.
Sroga, G.E. (1997). Regulation of nitrogen fixation by different nitrogen sources in the filamentous non-heterocystous cyanobacterium Microcoleus sp. FEMS Microbiology Letters 153: 11-15.