I’ve been looking at shallow, calcareous lakes in two very different locations over recent weeks: on the Shetland Islands north of Scotland and in Greece. Climatically, they are different worlds but there are surprising similarities in the diatoms that I find in the two habitats: many species are common to both but, even when the species are not identical, genera that are not widespread in other habitats are well represented in these lakes in both the Shetlands and Greece. There must be something about these lakes that makes them attractive to a few genera over almost all other possible habitats despite the differences in climate.
One of the genera that falls into this category is Epithemia, which I also find in my local calcareous pond (see “Working their passage …”); another is Rhopalodia, a relative of Epithemia and a third is Mastogloia, the subject of this post. Mastogloia has a very unusual structure. If you focus carefully on the top of the valve, you see striae and a raphe; if you then adjust the focus very slightly a series of chambers (“partecta”) will come into view, arranged in a row along each side of the valve. They look a little like a row of cabins along the two sides of a boat.
When seen from the side, rather than from above, we see that Mastogloia cells are, typically, rather deep so, pushing our nautical metaphor just a little further, they are rather ungainly. Although they have raphes and are, in theory, capable of movement, these are not going to be found darting around like species of Nitzschiaand Navicula, constantly adjusting their position in relation to light and other resources. Instead, Mastogloia have a very different set of priorities, with the partecta playing a key role in enabling these.
We see the partecta as empty chambers because we usually look at diatoms after they have been treated with oxidising agents that remove all organic matter. When viewed live, however, Mastogloia are often seen surrounded by extracellular strands, capsules and tubes and it is assumed that these are secreted from pores which are, in effect, the “portholes” associated with the partecta. The empty “cabins” we see under the microscope are, in fact, busy little slime factories. A lot of different extracellular structures have been described, particularly from the many marine representatives of this genus, and most seem to be designed to keep the cell in one place, rather than to help it adjust position.
For the freshwater species, calcareous habitats offer some particular challenges to organisms: the porous nature of the rock means that there is often a high risk of drying out, high calcium carbonate concentrations lead to the precipitation of calcite. In the process, phosphorus is also removed from the water, trapped in the calcite crystals. Evelyn Glaiser and colleagues suggested that this combination of characteristics favoured organisms that produced a lot of extracellular polysaccharides. Firstly, the strands act to bind inorganic particles and the microbial life into dense aggregations and slows rates of desiccation. Second, this dense moist matrix will provide not just organic matter but also microbes that can break it down to recycle nutrients for the diatoms and other organisms to use. Third – not mentioned by Evelyn Glaiser and colleagues but touched upon in “A journey to the headwaters of the River Coquet” and other posts – the algae themselves may contribute to this recycling via enzymes present in their extracellular polysaccharides.
Mastogloia, in other words, is not just part of the structure of biofilms in in shallow calcareous lakes and ponds, it actively creates these. In the Florida Everglades it is a keystone species, around which thick microbial mats form and within which, in turn, other species, some unique to these ecosystems, are found. It does this through diverting energy and resources to produce extracellular polysaccharides and because the scant nutrients mean that more competitive algae are kept out. However, if this delicate balance is disrupted and nutrients become more widely available, then Mastogloia is out-competed, the mats lose the structural integrity that Mastogloiaimparts, and the biodiversity associated with them disappears. A classic ecological cascade: for want of a nail, the shoe is lost and so on …
Carter, J.R. & Bailey-Watts, A. (1981). A taxonomic study of diatoms from standing waters in Shetland. Nova Hedwigia 33: 513-629.
Glaiser, E., La Hée, J.M., Tobias, F.A.C. & Wachnicka, A.H. (2016). Mastogloia smithii var. lacustris Grun.: a structural engineer of calcareous mats in karstic subtropical wetlands. Proceedings of the Academy of Natural Sciences of Philadelphia 160: 99-112.
Hain, M.K., Winsborough, B.M., Davis, J.S. & Golubic, S. (1993). Extracellular structures produced by marine species of Mastogloia. Diatom Research 8: 73-88.
Some other highlights from this week:
Wrote this whilst listening to: The Go-Betweens, an Australian indie band from the early 1980s. There has been press coverage of a new book about one of their members and this reminded me of the one occasion I saw them whilst a student in London in 1982 or 1983. I remember that the venue was full of goths hoping to see another band on the bill, The Dancing Did, who did not show up for some reason. I found their sole album, And Did Those Feet, on Spotify too:
Cultural highlights: Watched the new film Ammonite about Mary Anning. It plays fast and loose with history (see “It all started here …”) but is well-acted. Charlotte Murchison, the character played by Saoirse Ronan, lived for a while in Barnard Castle, about 30 kilometres from Durham.
Currently reading: Persepolis by Marjane Satrapi, autobiographical graphic novel about growing up in Iran during and after the 1979 revolution.
Culinary highlight: Normandy-style galettes followed by a chocolate and ginger cheesecake.