My curiosity about the habits of Navicula bottnica led me to the fishing village of Staithes, on the North Yorkshire coast, at the weekend, in search of a sample from the location I visited in 2012 (see “More diatoms that like cold weather …”). Staithes, the birthplace of local hero, Captain Cook, also has the added attraction of a pub in the harbour that sells wonderful crab sandwiches.
Staithes: left, a view of the village from the south and, right, sampling Staithes Beck.
I collected my samples in 2012 from a set of stepping stones across Staithes Beck, which flows through the village. At low tide, the stream here is virtually freshwater, but the diatoms that live on the stones have to be adapted to higher salinities when the tide is high. Even through the brown films looked identical to those that I see in rivers far from the sea (see “The ecology of cold days”), many of the diatoms are different. I did, however, see a number of cells of Navicula gregaria gliding around. The most abundant diatom was, however, larger than these and a subtly different shape to Navicula lanceolata, which tends to dominate these habitats in freshwaters. At very high magnification, I could just make out the dense radiate striae that characterise N. bottnica.
But no mucilage tubes. The Navicula cells were gliding around unencumbered by any such superstructures. They clearly have not read the literature: Andrej Witkowski’s Diatom Flora of Marine Coasts describes it as “probably cosmopolitan” (another way of saying “I don’t know where it isn’t found”), then “… tube dwelling species distributed along the coasts of the cooler seas of the northern hemisphere, more common in the brackish waters of the Baltic Sea.” I do often find diatoms that are described as “tube dwelling” that are free-living. I suspect that this is because I mostly look at habitats where a tube would be an impediment rather than an advantage. On these submerged stones, for example, I suspect that being able to constantly adjust their position relative to the light is more important than the benefits conferred by the relatively-static tube. On the other hand, Eileen Cox noted that tube-dwelling diatoms preferred hard to soft substrata in the Severn Estuary.
Navicula cf bottnica collected from upper surfaces of stones in the tidal section of Staithes Beck, Staithes, North Yorkshire, March 2015. The right hand view is a girdle view of a recently divided cell. Scale bar: 10 micrometres (=1/100th of a millimetre).
In case you have not seen a tube-dwelling diatom before, I have included a couple of Chris Carter’s photos of Encyonema (caespitosum?). The ability to form tubes seems to have evolved in several genera, although not necessarily in all representatives of a genus and not necessarily all the time. I have written about the tube-dwelling diatom Berkeleya rutilans (see “Diatoms from Pangong Tso”) and tubes are also common in the genus Frustulia as well as in several marine and intertidal genera.
Why produce tubes? I have never read any detailed accounts of the pros and cons of tube-formation, though I imagine that the mucilage must offer some protection against desiccation, high light and grazers and I have also heard speculation that the mucilage might act as a substrate for some extracellular enzymes. As I have grumbled before, diatomists are generally more interested in the minutiae of the silica frustule than the Meaning of Life.
Two photos of Encyonema (caespitosum?) growing in mucilage tubes. Both by Chris Carter.
Cox, E.J. (1977). The distribution of tube-dwelling diatom species in the Severn Estuary. Journal of the Marine Biological Association of the UK 57: 19-27.
Witkowski A. (2000). Diatom Flora of Marine Coasts I. Iconographica Diatomologica: Annotated Diatom Monographs (edited by H. Lange-Bertalot). Koeltz Scientific Books, Koenigstein.