I was tidying out some old files at the weekend and came across some articles I wrote for various newspapers and magazines between the mid-1980s and mid-1990s. One that particularly brought a smile to my face was a column for the Don’s Diary feature of the Times Higher Education Supplement in April 1995, describing my last week as an employee of Durham University before launching into life as a freelance scientist. At the time I thought that self-employment would be a relatively short-term episode in my life but here I am, nineteen years later, still going. Click THES_dons_diary_Apr95. if you want to read the whole piece.
One of the ironies of teaching a course on algal identification in the Lake District is that we actually take the students out of the Lake District on the first field trip in order to introduce them to the enormous variety of Cyanobacteria (blue-green algae). This is because the southern part of the Lake District, where the FBA is located, is situated on the Silurian Slates, which means that the streams, lakes and tarns have fairly soft water. Cyanobacteria, on the other hand, tend to be most abundant and diverse in hard water, so we drive about 40 minutes south and east from Windermere to a limestone escarpment called Whitbarrow, where there are a number of calcareous flushes and springs that are ideal for our purposes.
We always visit the floor of an abandoned quarry in this area which has several such flushes. The quarry owners had systematically removed the limestone until they had reached the Silurian Slate underneath. This, in turn, formed an impermeable layer that intercepted any water that had percolated through the limestone. The quarry floor was, typically, slippery with calcium-rich water that had seeped out from the surrounding limestone, as well as the mucilage that the algae produced. There were also, dotted around, several unprepossessing brown objects that, to the untrained eye, could easily be mistaken for the droppings of a small animal or bird (a Peregrine falcon was circling overhead during our visit). Once the students have the courage to pick these up, they see that they are composed of a firm jelly-like substance that is, we persuade them, actually an alga and should, therefore, be dropped into one of their specimen tubes to take back for closer investigation.
Animal, vegetable or mineral? Colonies of Nostoc commune on the floor of Whitbarrow Quarry, May 2014.
Once back in the FBA’s laboratory (complete with panoramic views of Windermere), we can dissect out small pieces of the jelly-like material and squash it onto a microscope slide. What they see when they peer down their microscopes is a plethora of chains of bead-like cells of a Cyanobacterium called Nostoc commune. Most of the cells have contents that have a granular appearance, with a background of bluish-green photosynthetic pigments. A few of the cells, however, are rounder and clearer: these are the “heterocysts”, cells that are especially adapted to “fix” atmospheric nitrogen and so help the organism survive in nutrient-poor habitats. The jelly-like matrix slows the rate at which water evaporates from the colonies, with the outer layers drying to form a tough, leathery skin around the colony.
Nostoc commune from Whitbarrow Quarry under the microscope. The cells are approximately 5 micrometres (1/200th of a millimetre) across.<br
There is a fascinating short paper by Malcolm Potts on the origin of the name “Nostoc”. Because Nostoc colonies often appeared very quickly following heavy rain (because the dried colonies absorb water quickly and swell in size), there was a belief in Medieval times that Nostoc fell from the sky. A German mystic and alchemist, Parselus, was the first to use the name “Nostoc”, claiming that it was “…excrement blown from the nostrils of some rheumatick planet. The name, indeed, is strongly suggestive of both the Old English word Nosthryl and the German term nasenloch both, as Potts politely explains, “…that part of the human anatomy intimately associated with extracellular polysaccharide.”
Potts, M. (1997). “Etymology of the Genus Name Nostoc (Cyanobacteria)” (pdf). International Journal of Systematic Bacteriology 47 (2): 584. doi:10.1099/00207713-47-2-584
I’m in the Lake District to teach a two-day course on algae for the Freshwater Biological Association. The weather forecast on the Breakfast News announced a heatwave but this seems to apply everywhere but here as you can see in the picture below. My co-tutor on this course is Allan Pentecost (see “Discovering a liking for lichens …”) and the picture below shows him introducing the participants to tufa in a small calcareous spring flowing off Whitbarrow. Notice how we all had blithely ignored the weather forecaster’s advice to pack sun cream
Allan Pentecost introducing participants on the FBA’s “Introduction to freshwater macroalgae” course to a calcareous stream at Whitbarrow.
The weather did steadily improve during the day, with the sun breaking through just as we all headed into the laboratory for an afternoon of microscopy. C’est la vie. It did mean that I had a long, warm evening to anticipate once the day had finished, and I took the opportunity to walk from my hotel in Near Sawrey to Hawkshead and back, with some spectacular views of the Cumbrian fells as a reward.
The view across West Een Tarn towards the Cumbrian Fells. The two peaks of the Langdales are on the right.
My trip to the River Coquet included a brief diversion to the Northumberland coast to help a colleague from the Royal Botanic Garden in Edinburgh collect some samples for a different study. Her interest was in the coastal diatom flora and, alongside sampling sand and mud, she also used a Pasteur pipetted to hoover up samples of the foam that accumulated around the low tide mark.
Diatom-bearing foam accumulating around a rock at low tide at Alnmouth, Northumberland, May 2014. Note the “selfie” of my thumb in the top left hand corner.
This foam proved to be rich in a diatom called Asterionellopsis glacialis, which we have met before in this blog (see “Back to Druridge Bay”). On this trip, I had found A. glacialis in brown-diatom rich patterns growing close to the low water level. We don’t know much about the ecology of Asterionellopsis glacialis, but there is a good study of another surf-zone diatom, Anaulus, which appears to move between the sand patches, foam and the inshore water over the course of a day. This is linked to a cycle of cell-division and mucus production – the latter occurring as the cells move back from the water to the sand and, it is suggested, the mucus may help the diatom cells to stick to the sand, and so helps them to maintain a population in one place rather than being washed away with the tide. As so often with diatoms, something as unprepossessing and easily-overlooked as foam on a beach proves to hide a complicated ecological story.
Asterionellopsis glacialis from the surf at Alnmouth, May 2014. The helical arrangement of cells can just be discerned, though it is hard to capture this crisply with the limited depth of field available under a high power microscope.
One other find on the Alnmouth sand were a few cells of Druridgea compressa, another constituent of the sand flora. I looked for this species last year (“In the footsteps of a Victorian microscopist …”) but without any luck. This, too, has been studied and shows a similar pattern, exuding sticky mucilage as they settle out from the sea water at low tide. The wave action then acts as a “whisk”, whipping these organic compounds into the froth that we saw at Alnmouth.
Life for a microscopic organism on a sandy beach is tough, with regular inundations from the tide and the constant movement of the substrata to add to the challenges faced by other organisms. Consequently the density of cells of all these organisms is very low, making it harder to find and study them. There are a few papers on the taxonomy and morphology of Druridgea compressa but, apart from the paper referenced below, we really do not know much about its ecology at all. Ironically, much the same can be said for Arthur Scott Donkin, the Victorian microscopist who first described Druridgea back in 1861. We have gleaned a few more details of his life since I wrote my post in May last year, but not much. Donkin, like Druridgea, remains elusive.
Two cells of Druridgea compressa from the sand at Alnmouth, May 2014.
Berryman, J. (2010). A contribution to the ecology of Druridgea compressa on Porthmeor Beach, Cornwall. Quekett Journal of Microscopy 41: 193-202.
McLachlan, A. and Brown, A.C., (2006). The Ecology of Sandy Shores (2nd edition). Elsevier, Burlington MS.
Peering down my microscope following my latest trip to the River Ehen, I saw the characteristic curved outlines of cells the diatom Hannaea arcus. It is a species that is most abundant in the spring time and, then, only in relatively unpolluted streams. What surprised me was that it was far less abundant in my sample this year than from samples collected at the same time last year. Of course, as I only visit once a month and this species only thrives for a few weeks, I may have missed the peak of its growth. Or some as-yet unknown combination of the organism’s life-cycle and local environmental fluctuations may have conspired to keep numbers lower than last year.
Live cells of Hannaea arcus from the River Ehen, near Ennerdale Village, May 2014. Scale bar: 10 micrometres (100th of a millimetre).
One other feature that struck me is that the diatoms I was looking at this year seemed to be shorter than those I saw last year. I dug out an old slide to check this and the difference is quite striking. Last year, I saw cells that were 100 micrometres or even longer in a few cases. This year, the longest I saw was 70 micrometres. Such fluctuations in size are common in diatoms and relate to the way the cells divide. The silica cell wall, the frustule, is in two parts, which overlap in the manner of the two halves of a Petri dish or old-fashioned date box. When the cell divides, each of these halves becomes the larger half of one of the two daughters so the average size of the population drops. This is repeated many times until, eventually, cell size diminishes to a threshold whereupon sexual reproduction is initiated.
Cleaned cells of Hannaea arcus collected from the same site on the River Ehen as the live cells photographed above, but a year earlier, in spring 2013. Scale bar: 10 micrometres (100th of a millimetre).
We do not know much about the specifics of the life-cycle of Hannaea arcus but a colleague, David Jewson, suspects that many freshwater benthic diatoms have a two-year life cycle. The reduction in size between 2013 and 2014 would support this assertion. The big question, then, is what size will the cells be when we return to the river in spring 2015?
One aside on Hannaea arcus: a couple of months ago, I wrote about the late John Carter (“remembering John Carter”) and I recalled seeing a short paper that he wrote in 1946. In those days, Hannaea arcus was known as Ceratoneis arcus and he wrote about a sample he had collected in 1927 which was an almost pure growth of this species. That, in itself, is uncommon but it was the location that surprised me: he had found it in a water-filled hole, nearly twenty feet (6.5 metres) up an oak tree. Having only known him as a staid old man, the image of the younger John Carter clambering through the branches of an oak tree in search of algae brought a wry smile to my face.
Carter, J.R. (1946). Diatom notes: the importance of records. The Microscope 6: 70-73.
Friday morning arrived with blue skies, scattered cloud and sunshine over the Northumberland hills. More importantly, the river levels were much lower than on Thursday and my plans for fieldwork in the upper Coquet were back on course.
The River Coquet, just above the confluence with Rowhope Burn, May 2014.
I’m here in search of a remarkable diatom which grows luxuriantly on the rocks in this river and nearby rivers in Northumberland. The growths are visible with the naked eye in May; later in the year, they will be even larger. Each has the texture of damp cotton wool and, when you look at them through a low power microscope, you can see why: they are composed of dense masses of branched stalks, each topped by the distinctive, sarcophagus-shaped cells of Didymosphenia geminata. The left-hand image below shows the side (girdle) views of a group of cells which have recently divided; the right-hand image shows the front (valve) view. Each of the stalks was, in turn, smothered with many other diatoms.
A stone from the bed of the River Coquet, covered with colonies of Didymosphenia geminata, May 2014.
Didymosphenia has been the subject of a lot of interest over the last decade as mass growths (much larger than those I saw in the Coquet) have appeared in rivers in several parts of the world, notably New Zealand and Canada. It does not seem to have changed its distribution in Britain or Ireland markedly over this time. The paradox is that these huge biomasses seem to occur in rivers that are naturally low in nutrients.
Looking at Didymospenia down the microscope suggests a partial solution: the growths you can see smothering the stones are largely composed of the stalks which are made from carbohydrates which are just composed of carbon, hydrogen and oxygen and not phosphorus and nitrogen, the nutrients that normally limit growth in freshwaters. However, the Didymosphenia cells still need nutrients to survive and grow and some recent research has suggested an intriguing explanation for how low nutrients might actually be responsible for the high biomass that is often associated with Didymosphenia.
Didymosphenia geminata from the upper River Coquet, May 2014 (left hand image) and May 2006 (right-hand image). Scale bar: 20 micrometres (1/50th of a millimetre).
The first part of the story comes from a paper published by Brian Whitton and Neil Ellwood in 2007 which suggested that the stalk actually plays a role in helping the Didymosphenia cells scavenge phosphorus. Extracellular enzyme activity located in the upper part of the stalk helps the cells to liberate phosphorus that is bound into organic particles (from peat, for example). Even though routine measurements often indicate concentrations of phosphorus dissolved in the water are very low, there are occasional pulses of peaty water, associated with rainfall, that the Didymosphenia (and other algae) are ready to tap into.
The second part of the story follows on from this: Max Bothwell and Cathy Kilroy showed that low phosphorus actually stimulates growth of the stalk, presumably (my speculation here) to increase the potential to trap these organic phosphorus sources. They also lift the Didymosphenia colonies out of the narrow boundary layer close to the rock surfaces where it is exposed to any nutrients that might drift downstream.
The irony, as Bothwell and colleagues point out, is that most aquatic biologists associate high biomass of algae with high nutrients, whereas Didymosphenia actually seems to be associated with the opposite situation. Another irony, as I point out on my web pages is that, , when detached from the stream bed, these brownish masses floating downstream are often mistaken for raw sewage. So we have the rather unusual situation of an unsightly natural phenomenon (in the case of Northumberland, at least) being driven by the absence of pollution. So much for cleaner rivers!
Bothwell, M.L., Taylor, B.W. & Kilroy, C. (2014). The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms. Diatom Research doi: 10.1080/0269249X.2014.889041.
Ellwood, N.T.W. & Whitton, B.A. (2007). Importance of organic phosphate hydrolysed in stalks of the lotic diatom Didymosphenia geminata and the possible impact of climate change. Hydrobiologia 592: 121-133.
My fieldwork in Northumberland on Thursday was not particularly successful. After several days of dry weather, it started to rain on Wednesday night and, by Thursday morning, the rivers I wanted to sample were running very high. First rule of sampling: the ecologist is a benthic organism. Never planktonic. Second rule of sampling: discretion is the better part of valour. However, as I drove towards the raging torrent that was Wooler Water, salvation appeared in the guise of an Environment Agency van. The two guys whose van it was knew the Northumberland rivers well and assured me that, with no more rain forecast, the rivers would be low again by the next morning. Fortunately, I had decided to stay overnight in Rothbury, so I was able to reorganise my itinerary and return to Wooler Water and the nearby River Coquet in the morning.
This is a good example of the “streamcraft” I mentioned in my recent post (“Slow science and streamcraft”): I have a good idea of how my local rivers react to rainfall but, here in Northumberland, I needed to draw upon the experience of people who visited these rivers regularly. Once settled in my hotel, I was able to test their advice in real time, using the Environment Agency’s excellent system of hydrographs (see “The River Ehen in January”). The closest hydrograph whose results are available online was at Rothbury, a few hundred metres from my hotel and, assuming Wooler Water was behaving in a similar manner, my visit earlier in the day had coincided with the peak flow for this particular rainfall event. Over the next 12 hours, however, the flow gradually decreased and, by Friday morning, though the river was still higher than normal, it was possible to wade in and collect my samples. I should, in retrospect, have checked the hydrograph before I went out on Thursday, but it is useful to have some local knowledge in order to “calibrate” the readings in terms of the activities you want to perform.
A screenshot of the Environment Agency’s hydrograph at Rothbury between 30 April and 2 May (see http://apps.environment-agency.gov.uk/river-and-sea-levels/120694.aspx?stationId=8173). My first visit was at 14:00 on 1 May; my return visit at about 10:00 on 2 May.