Bollihope Bhavacakra*


My explorations of the biology of Ulothrix zonata have taken me from Bollihope Burn in Weardale (see “Bollihope Burn in close-up”) to upper Teesdale (see “The intricate ecology of green slime”) and one of the outcomes is this representation based on the diatom-smothered filaments that I observed in Bollihope Burn, close to the sink hole.   The picture illustrates the suggestion that I made in the post about Bollihope Burn – that the switch from “vegetative” to “reproductive” mode leads to less energy being available for the alga to manufacture the slime that it needs to stop epiphytes gaining a foothold.   By intercepting the limited light that penetrates into the water, these algae can shade the host plant to such an extent that it cannot gain the energy it needs to grow.   The mucilage is the equivalent of the “anti-fouling paint” that mariners use to stop barnacles encrusting their hulls.

My image shows a single healthy filament of Ulothrix zonata at the bottom right of the image and, on the left, two filaments of cells that are producing flagellated gametes that will eventually be released.  I write “gametes” with mild trepidation, as they may also be nascent zoospores associated with the asexual phase (see below).   A third filament, in the background, is composed mostly of empty cells that have already released their gametes.   There are no “male” or “female” gametes; any two can combine to form a zygote, so long as they come from different filaments.   This zygote then attaches to the substratum and does little more over the summer.

In my image, the Ulothrix filaments have been colonised by needle-like cells of Fragilaria gracilis, Achnanthidium minutissimum cells on short stalks, and a couple of cells of Gomphonema pumilum.   There are also a couple of cells of Ulnaria ulna and some zig-zag colonies of Diatoma tenuis.   The effect of these cells on the appearance of the Ulothrix zonata is marked, smothering the filaments entirely so that, with the naked eye, the assemblage appears brown rather than green.

The diagram below summarises the life cycle of Ulothrix zonata and emphasises the point that the green filaments that most people associate with this alga are only a small part of the story.  The cell contents divide in one of two ways.  The first produces zoospores, each with four flagellae, which are released, settle and grow directly into a new vegetative filament.  The second, however, produces a number of cells which are smaller but otherwise look similar to the zoospores except that each has two rather than four flagellae.  These gametes then fuse with gametes produced by another filament to produce a “zygote” which, in turn, germinates to produce several (typically eight) zoospores, each with four flagellae, from which new vegetative filaments grow (see illustration of putative “germlings” in “The intricate ecology of green slime”).


The life-cycle of Ulothrix zonata, following Lokhorst and Vroman (1974).   “2n” refers to diploid stages; “n” refers to haploid phases (note that the vegetative filament is also haploid).

The asexual phase can be produced at any time, but is stimulated by high temperatures; the sexual phase, however, is more strictly regulated.   The formation of gametes only occurs under “long day” conditions, which means that it will happen as daylight hours extend in the spring.   By contrast, the division of the zygote requires short day conditions and low temperature, meaning that the zygote is relatively inactive over the summer months, only dividing to produce zoospores, and ultimately, new filaments, in late autumn and winter.   This creates a useful niche for the organism during a period of the year when nutrients are relatively plentiful in upland rivers (as they are washed off the land following rainfall) and grazers are relatively inactive.   It also means that this apparently simple green filament actually has some sophisticated controls that regulates how and when it divides.

I’ve talked about algal life cycles in the past, commenting that the concepts behind these are not always easy to grasp (see “Reflections from the trailing edge of science …”).   The problem is that undergraduates of my generation were taught this as part of a broader overview of plant evolution and the variations between patterns in different groups tended to befuddle rather than enlighten students already struggling to grasp the big picture.   The interesting twist to my explorations of Ulothrix zonata is that it has shown how the idiosyncrasies of an organism’s life-cycle can have a practical significance that helps the organism survive in a particular habitat.   Knowing about the life cycle can, in turn, inform our understanding of processes occurring within a stream or river.  The problem is that these topics have largely fallen off the agenda both for teaching and research, so we are generally limited to interpreting descriptions from old journals, and often forget completely the role that these factors may play in creating the mosaic of algae in a stream.


Lokhorst, G.M. & Vroman, M. (1974).  Taxonomic studies on the genus Ulothrix (Ulotrichales, Chlorophyceae) III.  Acta Botanica Neerlandica 23: 561-602.

* “Bhavacakra” is a symbolic representation of the cyclical nature of existence used in Tibetan Buddhism.  The title of this post is also an affectionate tribute to Brian Moss, who died a few days ago.

Bollihope Burn in close-up

Bollihope Burn does not disappear dramatically down a single swallow hole in the way that Gaping Gill swallows up Fell Beck on the slopes of Ingleborough.  Rather, there is a gradual diminishment of flow, as the river percolates through the joints in the limestone, before the remnants of the stream swirl down a final sinkhole (see “Co. Durham’s secret Karst landscape”).   I was intrigued to see how the organisms that inhabited Bollihope Burn reacted to these stresses so got down on my knees close to this final sinkhole to get a closer look.

My waterproof Olympus TG2 (see “Getting close to pearl mussels with my underwater camera”) set to super-macro mode is equivalent to putting my head under the surface of the water and then peering at the rock through a magnifying glass … but gets fewer odd looks from passers-by.   Fortunately, this is an isolated corner of Weardale and passers-by were limited to a few rabbits, because sticking a camera into a stream to take a photograph of a stone is, itself, odd enough to attract stares from most people.

These close-up views of freshwater algae in their natural habitat continue to surprise me.  It is only in the last few years that waterproof digital cameras with macro facilities have fallen to an affordable price.  Before this, underwater photography required special kit that few freshwater biologists could afford.  Yet, removing a stone to photograph the algal growths meant that the algae were never photographed in their natural habitat, and were deprived of the buoyancy that the water afforded them.   I have plenty of photographs of green or brown gunk composed of different algae but, with the algae removed from their context, these photographs offer few insights into the biology of the stream bed.  The photograph below, however, shows a community with a distinct structure – a “turf” of near-vertical filaments waving in the gentle eddies of the stream as it swirls around before disappearing down the swallow hole.


A cobble in Bollihope Burn, close to the swallow hole, covered by a short “turf” of algae, April 2016.   Scale bar: approximately two centimetres.

Under the microscope, the structure of this “turf” starts to reveal itself.   The filaments appear to be aggregations of diatoms around dying filaments of the green alga Ulothrix zonata.   This is an alga that is common in Pennine streams in the winter and early Spring but which disappears as the weather starts to warm up. It often forms very conspicuous green patches on the river bed for a short period of time, as in the following picture, which I took a few kilometres away from my current location, in the River Wear at Wolsingham.   The difference in appearance between the alga in the two photographs is mostly due to the Bollihope population being smothered with diatoms whilst the Wolsingham population was virtually a pure growth of Ulothrix.   This may be partly due to the Bollihope picture being taken taken two months later than the Wolsingham image.   Ulothrix zonata produces copious quantities of mucilage and the Wolsingham population was slimy to the touch.  I rarely see epiphytes on this or any other slime-producing algae in their healthy state.   However, Ulothrix is a species that thrives in cold water.   Indeed, a study has shown that when the water starts to warm up and the day length increases, the Ulothrix filaments switch into their dispersal and reproductive modes and that is what may be happening here.   As the rate of photosynthesis declines, so there is less carbohydrate from which the slime molecules can be made and, as a result, less of a deterrence to any diatom looking for a perch.   From now until next winter, Ulothrix zonata will not be very obvious in the streams that I visit.  This is because the zygotes which are produced by sexual reproduction lie dormant until day length decreases and temperature drops.   At this point, they germinate and divide to produce zoospores which, in turn, grow into new Ulothrix zonata filaments.


Growths of Ulothrix zonata on cobbles in the River Wear at Wolsingham, February 2009. 

The photographs taken under the microscope illustrate this well.  On the left hand side there is one of the few healthy looking Ulothrix filaments that I found, with a chloroplast wrapped around the inside of the cell wall   On the right hand side you can see that the chloroplasts have gone, replaced by dark green blobs which are (I think) bundles of gametes awaiting release.   More significantly, you can also see several diatoms around the Ulothrix filament, taking advantage of it to lift themselves up above the rock surface.

The paradox is that these algae are entering their senescent phase just as most of the plant life in Weardale is flourishing.   This is probably not a coincidence: life in cold water means fewer grazing invertebrates and less shade to intercept the precious winter sunlight.   I suspect that algae, once masters of the planet, have gradually adapted and evolved to live a subordinate life, flourishing in those periods of the year when most of us are content to stay indoors.


Ulothrix zonata from Bollihope Burn, April 2016.  The left hand image shows a healthy vegetative filament; the right hand image shows zoospore production and colonisation by diatom epiphytes. 


Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1985).  Light, temperature and photoperiod as factors controlling reproduction in Ulothrix zonata (Ulvophyceae).  Journal of Phycology 21: 235-239.

van den Hoek, C., Mann, D.G. & Jahns, H.M. (1995).  Algae: an Introduction to Phycology.  Cambridge University Press, Cambridge.

Co. Durham’s secret Karst landscape

When I was at school in London, a field trip to see the classic limestone scenery that is a standard part of the geography curriculum involved a major expedition: a week-long journey to the Craven district of Yorkshire where we had our first opportunity to see features described in our textbooks and on the blackboard for ourselves.  Now, an inhabitant of northern England for over half my life, I can get to a miniature “Karst” landscape with a drive of under an hour drive from my home.

Bollihope Burn rises on the moorland between Weardale and Teesdale, and looks like many other small tributaries of the Wear as the water tumbles through a series of riffles towards its confluence with the River Wear near Frosterley.   As is the case for many streams in this area, there are spoil heaps and other relics of the lead mining industry close by the water and, were I to look closely, I am sure that I would see evidence for their malign effect on the ecology of the stream.   A couple of kilometres above the confluence with the Wear there is, however, a rather more serious problem for the fish and other aquatic life: a complete absence of water.   Scrabbling down the bank into the dry stream bed and walking upstream for a short distance, I can start to hear the sounds of a babbling brook again and, on turning a corner, see the stream as it disappears through a series of swallow holes.


Bollihope Burn at approximately NZ 034 362, as it disappears down a swallow hole, with the dry river bed beyond.

Walk a short distance downstream and more “Karst” landscape features are on display.   The dry river channel continues over a waterfall and then in a steep gorge, which once was an underground cavern created as the water eroded away the limestone.  Just at the point where the stream should have tumbled over the waterfall, the limestone is darker and close examination reveals a patterning of lighter-coloured fossils.   This is the famous but misnamed “Frosterley Marble”; it is, in fact, a dark-coloured variant of the Great Limestone that is common in the Pennines.   The most prominent fossils in the rock are corals and crinoids, reminding us of the origin of these rocks in shallow, warm tropical seas of the Carboniferous period over 300 million years ago.


Bollihope Burn showing the dry waterfall in the foreground and the collapsed cavern in the background.


A polished block of Frosterley marble beside Bollihope Burn, April 2016.

The fine grain of this particular means that it can be cut and polished to give the appearance of marble, although a geologist would reserve this word for particular metamorphic variants of limestone.   There is no true marble in northern England, and the Frosterley marble is a fine substitute, cropping up in several places including, most famously, Durham Cathedral, where columns (probably quarried from the bed of Bollihope Burn) support the roof of the Chapel of Nine Altars and the rood screen.   Much great architecture (and certainly that which blends most sympathetically with the landscape) is really just a rearrangement of local geology.

However, I did not make this journey to muse about landscapes and geomorphology.   The time has come to unpack my sampling equipment and take a closer look at the life in Bollihope Burn.   That will be the subject of my next post …


Pillars of Frosterley marble in Durham Cathedral.  Left: in the Chapel of Nine Altars; right: part of the rood screen between the nave and choir.