Survival of the fittest (1) …

Hadjipavlou_mine_March19

When I signed up to a trip to Cyprus in late March I was anticipating feeling some warm Mediterranean sun on my skin after the ravages of the British winter.  I did not expect snow and sleet.   However, as one of our destinations was the Troodos mountains, the location of Cyprus’ only ski resort, maybe it was a case of unrealistic expectations.   Fortunately, we realised our mistake just in time to pack some warm clothes, and the unseasonable weather did, at least, mean that the spring flowers at lower altitudes were, even by Cypriot standards, particularly impressive.

I was in Cyprus primarily as a camp follower on a reconnaissance trip for a geology and botany excursion next year.   Cyprus is, to put it in layman’s terms, the outcome of a collision between the African and European continental plates.   The Troodos mountains are a geologist’s paradise, having a wide range of features arising from this and from associated volcanic activity.   As the molten rocks cooled, minerals precipitate out and the resulting geological strata reflect differences in the melting points of these minerals.   Some of these minerals, such as chromite, are commercially valuable and have been mined for centuries.   Indeed, the name Cyprus itself is derived from cuprous, the Greek word for copper, which was mined here since 4000 BC.

The Hadjipavlou mine is set amidst pine forests close to the highest point of the Troodos.  It was an active chromite mine from 1950 to 1954 and from 1979 to 1982 but was abandoned when cheaper sources of chromite became available in South Africa.   Over a million tonnes of ore were extracted in the period when the chromite mines in the area were active, but now there are few obvious signs apart from this adit driven into the hillside.   A small stream bearing water that has percolated through the rocks and collected in the mine’s galleries emerges from the mine entrance and tumbles down the hillside to join the stream below.   This, on closer inspection, has some quite interesting microbial growths.

First of all, having been told that this is a chromite mine, you might expect the water to carry toxic concentrations of heavy metals.   So you might also be surprised to see abundant growths of bright green algae thriving in the stream immediately downstream of the mine entrance.   This is, in fact, a common phenomenon in mine waters and happens, we think, because the fast-growing algae evolve metal tolerance whilst the animals that feed on them are slower to adapt.   This is, literally, survival of the fittest and, with nothing to eat them, the algae grow prolifically.

These filaments belong to the genus Tribonemawhich, despite being bright green in colour, actually belongs to the yellow-green algae, the Xanthophyta, rather than to the green algae.  This group is actually more closely related to the diatoms than to the green algae, though it can be hard to understand why simply by peering through a microscope.  One useful test is to add a little iodine  solutionto the slide: this binds to the starch inside green algae cells, staining them a dark brown colour.   The Xanthophyta, by contrast, do not have starch as their storage product so the cells are not stained by iodine.   The only other member of this group that I have discussed in this blog is Vaucheria, a very different alga (see “Who do you think you are?”).

Tribonema_affine_Troodos

Tribonema cf affinein the channel draining the Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.   a. close-up of the alga in situ; b.  microscopic view of filaments; c. fragments of disintegrated filaments showing the H-shaped cell endings.  Scale bar: 10 micrometres (= 100thof a millimetre).   

Tribonemahas simple, unbranched filaments with two or more plate-like chloroplasts arranged around the cell periphery.   One other feature is the arrangement of the cell wall, which tends to consist of two overlapping halves.  When filaments disintegrate (as they often do) the fragments have an H-shape, with each end forming half the cell wall of a different cell.   The cells are, in fact, cylindrical but this is not apparent with the flattened perspective of a high magnification objective.   This feature is not universal in the Xanthophyta, nor is it unique to this group (a few green filamentous algae show the same characteristic) but it is a useful hint that you may be looking at Tribonema.

Whilst lush growths of algae is a common feature of streams draining mines, the species that form these growths can vary.   In the northern Pennines, I am used to seeing green algae in these habitats, but there are at least three different genera that I find.  Typically there is just one filamentous alga in this location, and they tend to be  constant over time: they are reliable sources for material for undergraduate practical classes as a result.  There is more to this story but I will have to come back to it at some point in the future.  .

There is also more to the algal flora of the Hadjipavlou chromite mine but, again, that will have to wait for another post.  I should also confess that, although I visited the mine briefly last year, these samples were collected by Heather, whilst I was sitting snugly below the snow line.

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The complexities of measuring mass…

Benthotorch_in_action

Once upon a time, measuring the quantity of algae growing on the beds of streams and rivers was a painstaking, slow process that invariably revealed large amounts of spatial and temporal variation that, very often, obscured the ecological signals you were looking for. That has changed in the last decade thanks to the availability of field fluorimeters such as the BenthoTorch.  This makes it much quicker and easier to measure chlorophyll concentrations, the usual proxy for algal quantity.  Thanks to devices such as this it is now much easier to discover that your ecological signal is masked by spatial and temporal variation.

We’ve generated a lot of data about the fluxes of algae in the River Ehen using a BenthoTorch over the past five years and are in a position where we can start to make some generalisations about how the quantity of algae vary over the course of a year.  In broad terms, the results I showed in “The River Ehen in January” back in 2014 have not varied greatly over subsequent years, with peak biomass in mid-winter and low biomass in the summer (due, we presume, to intense grazing by invertebrates).  Curiously, we see a much less distinctive seasonal pattern in the nearby Croasdale Beck, but that is a story for another day….

The BenthoTorch uses an algorithm to partition the fluorescence signal between three major algal groups and, though this is not without issues (see below), I thought it might be interesting to see how these groups varied with biomass trends, and consider how this links to ecological theory.  The first group I’m considering are the green algae which, in this river, are mainly filamentous forms.   The general pattern, seen in the graph below, is for a gradual increase in the proportion of green algae, which fits with the current understanding of thicker biofilms having greater structural complexity with filamentous algae out-competing attached single celled algae to create a “canopy” of algae that are more effective at capturing light and other resources.  The relationship is, however, strongly wedge-shaped so, whilst many of the thickest biofilms have a lot of green algae, there are also thick biofilms where green algae are scarce or even non-existent.  Croasdale Beck shows a similar, but less pronounced, trend.

green_algae_in_Ehen

Relationship between the proportion of green algae and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (a.) and Croasdale Beck (b.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.   The image at the top of the post shows Ben Surridge using a BenthoTorch to measure algal biomass beside Croasdale Beck in Cumbria.

The second graph shows that this pattern of a gradual increase in proportion is also the case for diatoms and, once again, there is a broad wedge of points with an upward trend.  But, once again, there are also samples where biomass is high but diatoms are present in very low numbers or are even absent.   What is going on?

The problem is clear I think, if one looks at the final image in “The only way is up …” where the very patchy nature of algal communities in the River Ehen (and, indeed, many other rivers).   There are plenty of algae on this boulder, but not organised in a homogeneous manner: some zones on the boulder are almost pure diatom whilst others are almost pure green algae (and there are also zones that are almost pure Lemanea– I’ll come to that in a future post).   We try to sample the stones as randomly as possible so you can see the potential for getting very different numbers depending on where, on a stone, we point the BenthoTorch’s sensor.

diatoms_in_the_Ehen

Relationship between the proportion of diatoms and the total quantity of benthic algae (expressed as chlorophyll concentration) in the River Ehen (c.) and Croasdale Beck (d.).   The blue lines show quantile regression fits at p = 0.8, 0.5 and 0.2.  

With experience, you can make an educated guess about the types of algae present in a biofilm.  I’ve tried to capture this with my watercolours, using washes of raw sienna for the diatoms and a grass-green for the green algae, which roughly matches the colour of their respective growths in the photo in my earlier post.   The two groups of algae a are relatively distinct on that particular boulder.   The top row roughly matches the upper “edge” of the graph showing variation in diatoms, whilst the bottom row emulates the upper “edge” of the graph showing variation in green algae.  These are the two extreme situations; however, we also often see darker brown growths in the field, which can be recreated by mixing the raw sienna and grass-green together.  When I peer through a microscope I often see green algae smothered in diatoms: genera such as Oedogoniumare particularly prone as they have less mucilage than some of the others we find in the Ehen. Their filaments often host clusters of Fragilariacells as well as Achnanthidium minutissimum, whilst stalked Gomphonemaand chains of Tabellaria flocculosaoften grow through the tangle of green filaments.   The dark brown colour is deepened yet further by the colour of the underlying rock, so my effort on white watercolour paper is a little misleading.

colour_patches

A colour chart showing how different proportions of green algae and diatoms influence the colour of biofilms.

The final graph shows how, as the average biomass increases in the River Ehen, so the variability in biomass also increases.   The River Ehen is one of the cleanest rivers I know but I suspect that this pattern in benthic algal quantity could be reproduced in just about any river in the country. What I would not expect to see in any but the purest and most natural ecosystems is quite so much variation in the types of algae present.   Once there is a little enrichment, so I would expect the algae to become more of a monoculture of a dominant filamentous alga plus associated epiphytes.  Like much that happens in the microscopic world of rivers, it is easier to describe than it is to measure.

That, however, is only part of the story but I’ll come back to explain the patterns in the other main groups of algae in the Ehen and Croasdale Beck in my next post.

mean_biomass_by_stdev

The relationship between mean chlorophyll density and the standard deviation (based on measurements from five separate stones) for samples from the River Ehen and Croasdale Beck. 

 

Secular icons?

paintings_at_botanic_gardens_Mar19

I’ve got two pictures on display as part of an exhibition at Durham University Botanic Gardens, both showing abstract or semi-abstract views of algae.  One is my sextych of the alga Apatococcus(see “Little round green things …”) and the other is a triptych based on Haematococcus, an alga which I wrote about in “An encounter with a green alga that is red” back in 2013.   Both pictures were painted for my final degree show back in 2008 and both addressed questions about the boundaries between abstract and representational art.

The point that I was trying to make with these images is that the boundary between abstract and representational art depends partly on what the viewer knows about the subject matter and, in the case of algae, this is usually not very much.  In cases such as these, the legend becomes very important as a means of bridging the gap between abstraction and reality by providing just enough information to help viewers make sense of the content (see “How to win the Hilda Canter-Lund Prize (2)”).   In the case of microscopic images, this should always include some indication of scale, written in terms that non-biologists can easily understand (I would always write “1/100thof a millimetre”, rather than “10 micrometres”, for example).

Haematococcus_MGK

Haematococcus. Triptych.   2008 50 x 130 cm Acrylic, resin and PVA on canvas.

This issue of viewers being able to “unlock” the meaning of images extends beyond the abstract/representational boundary that I encounter when displaying images of the microscopic world.  Exactly the same challenges occur when, for example, secular western Europeans look at eastern Orthodox icons, a subject that occasionally creeps into this blog (see, for example, “Unorthodox icons”.   My own curiosity about this art form led me to spend a week studying icon painting at the Quaker College in Woodbrooke, in the suburbs of Birmingham.  About ten minutes away from Woodbrooke there is the Serbian Orthodox church of St Lazar (built after the second world war by Yugoslav refugees with financial support from the Cadbury family, the Quaker philanthropists who also established Woodbrooke).

I talked a little about the practice of icon painting in “The art of icons …”.  Today, I am more interested in the symbolism.   A secular westerner can look at many of the icons I’ve depicted and broadly catagorise the contents: most would recognise that Fr Nenad, the priest of the Selly Oak Orthodox church, is holding an icon that depicts the crucifixion, for example, or that the icon just to the left of the centre of the doors in the iconostasis in the lower image depicts the Virgin and Child.  However, the symbolism goes much deeper.   I have a spotter’s guide to icons (sad, I know …) and it lists twenty eight different variants on the basic depiction virgin and child, differing in the physical relationship of Mary and Jesus, their facial expressions and the setting.  Each of these have a slightly different meaning for the Orthodox faithful.   The westerner sees “virgin” and “child”, the eastern Orthodox devotee sees so much more.

St_Lazar_Bournville

The Serbian Orthodox Church of the Holy Prince Lazar in Selly Oak, Birmingham with, right, Father Nenad displaying an icon of the crucifixion. 

What’s all this got to do with painting algae, you may ask.   Scientific illustratation and icon painting are two branches of applied art, where the subject matter serves a higher purpose.  Both, in their own way, try to help viewers understand their place in the world.  If you are not religious, you may not be comfortable with this comparison but, for most of Europe, east and west, until the Enlightenment, this would have been the case.   In both cases, however, the image cannot be viewed in isolation, the viewer needs the appropriate “keys” to unlock meaning.   Even then, the viewer is not a passive observer.   The icon requires a response from the viewer, it is the focus for contemplation and meditation and, I suggest, scientific images, when displayed as “art” should play a similar role, inviting viewers to reflect upon the mysteries of the natural world and demanding a response.

St_Lazar_iconostasis

The iconostasis at the Serbian Orthodox Church in Selly Oak.

A twist in the tale …

hamsterley_forest_jan19\

After my sojourn in East Durham, described in the previous post, I have travelled back to the Pennines for this one, crossing the River Wear at Wolsingham before driving up onto the fells and finally dropping down to the woodlands that are Hamsterley Forest.  This is a large man-made plantation, dating from the 1930s and popular for recreation. In January, however, the forest is quiet, and I only have a few mountain bikers and a lone dog walker for company as I peer into the peaty waters of Euden Beck.   This stream rises on the open fells of Hamsterley Common, between Weardale and Teesdale, before flowing through the forest and joining Spurl’s Wood Beck just downstream from where I am standing, to become Hamsterley Beck.  This then joins the Wear a few kilometres downstream from Wolsingham.

eudon_beck_jan19

Euden Beck, just above the forest drive in Hamsterley Forest, January 2019.  The photograph at the top of the post shows a view towards Hamsterley Forest. 

There is a mixture of diatoms growing on the stones here but I am most interested in the genus Fragilaria today.   One of the curiosities of this genus is that we often find several representatives growing at the same site at the same time, reminiscent of the old adage about London buses (“you wait ages, and then three come along at once”).   I’ve written about this before (see “Baffled by the benthos (2)” and “When is a diatom like a London bus?”) and Euden Beck is another good example of this conundrum in practice.

Today, I could see quite a few cells of Fragilaria teneraand smaller numbers ofF. gracilisplus a newly-described species that I will talk more about later in the post.  Fragilaria teneraforms long, needle-like cells, often clustering together to form sea urchin-like masses growing out from either a filamentous alga or particle to which they are attached (see “Food for thought in the River Ehen” for an illustration).  Most of the ones that I saw in my samples from Euden Beck were either single cells or pairs of cells, presumably following a recent division. Note how the second cell from the left in the figure below is not as straight as the others.   This is something that I often see with Fragilaria populations in streams in the northern Pennines, and indicates that there may be heavy metal pollution in the water.  There are a lot of abandoned lead mines in the northern Pennines and, sure enough, when I looked at a large scale map, I found one that I had not previously noticed in the upper part of Euden Beck’s catchment.

fragilaria_tenera.euden_jan19

Live cells of Fragilaria tenera(a. – d.) and F. heatherae from Euden Beck, January 2019.   a., b. and e. are valve views; c. and d. are girdle views.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The next image shows these valve abnormalities even more clearly, with almost all of the cells showing aberrations in their outline.   These images are from an older sample; the curiosity here is that whilst most of the Fragilaria tenera valves were twisted, fewer of the valves of Fragilaria gracilisare twisted, whilst few of the valves of the third Fragilaria species show any abnomality in their outline at all.   This species is very common in northern Pennine streams, and I have often seen distorted valves of this species in streams polluted by mine discharges.  This makes the discrepancy between the outlines of this and Fragilaria tenera in Euden Beck particularly intriguing.

fragilaria_tenera_rt#55

Fragilaria tenera from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

I say “Fragilaria gracilis” with a modicum of trepidation as a recent study in which I have been involved, suggests that there may well be at least two species.  These are, as far as we can tell, indistinguishable using characteristics that can be seen with the light microscope though we know that they are genetically quite distinct, and both are widespread, turning up not just in the UK but in other parts of Europe too.

The third species, to the best of our knowledge, does not match the description of any other Fragilaria species, and we are in the process of publishing it as a new species, Fragilaria heatherae.   We have found it a number of samples, not just from the UK but also from sites elsewhere in Europe.   These, by comparison with the other two species, show very little distortion at all.   Whilst several authors have noted this phenomenon in the past, the physiological cause is still not understood. My guess is that the metal ions are displacing a metal co-factor in an enzyme that is involved in the process of laying down the silica cell wall.   Fragilaria seems to be particularly susceptible, but this may be because their long needle-like cells show the distortions more clearly than in some genera but, based on the evidence from Euden Beck, there are clearly differences in susceptibility between species.

Once again, I seem to be ending a post having asked more questions than I have answered. That is always frustrating but another way of looking at this is to realise that the frontiers of ecology are only ever a short drive away from where you are now.  It is very nice to cross oceans to visit rain forests and coral reefs, but there are adventures to be had closer to your doorstep.

fragilaria_gracilis_rt#55

Fragilaria gracilis from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

fragilaria_heatherae_rt#55

Fragilaria heatherae” from a sample collected in Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King

References

Duong, T.T., Morin, S., Herlory, O. & Feurtet-Mazel, A. (2008). Seasonal effects of cadmium accumulation in periphytic diatom communities of freshwater biofilms.  Aquatic Toxicology90: 19-28.

Falasco, E., Bona, F., Ginepro, M., Hlúbiková, D., Hoffmann, L. & Ector, L. (2009). Morphological abnormalities of diatom silica walls in relation to heavy metal contamination and artificial growth conditions.  Water SA35: 595-606.

McFarland, B.H., Hill, B.H. & Willingham, W.T. (1996). Abnormal Fragilaria spp. (Bacillariophyceae) in Streams Impacted by Mine Drainage. Journal of Freshwater Ecology 12: 141-149.

Castle Eden Dene in January

castle_eden_burn_jan19

The story so far: in 2018 I made bi-monthly visits to the River Wear, my local river and tried to capture, in my posts, the changes in the algae that occurred over the course of 12 months (follow the links in “A year in the life of the River Wear” to learn more).  It was an interesting exercise, partly because last summer’s exceptional weather led to some intriguing changes over the course of the year.   Consequently, as 2019 dawned, I thought I should find a different type of stream within a short drive from my home and try again.  So, bearing in mind that Wolsingham is south and west from where I live, I turned in the opposite direction and drove due east instead, stopping on the edge of the brutal concrete housing estates of Peterlee, a most unprepossessing location for a National Nature Reserve.

My journey has brought me right across the Permian limestone that dominates the eastern Durham landscape. Its escarpment rises up close to my home, and I have written about the algae that live in the ponds at the foot of it (see “A hitchhiker’s guide to algae…”).  On the other side, however, the limestone ends in a series of cliffs overlooking the North Sea and small streams have cut into the limestone to create a series of wooded valleys, or “denes”.   I’ve come to Castle Eden Dene, the largest of these: if you want a cultural reference point, watch the film “Billy Elliott”, set just a few miles further north along the coast, or read Barry Unsworth’s The Quality of Mercy.

We made our way down the footpath into the dene on a crisp and very cold winter morning, past the old yew trees from which the name is derived, and myriad ferns.   A deer bounded across the path ahead and disappeared into some scrub, and then we turned a corner and looked into Castle Eden Burn, which runs along the bottom of the dene.   To my surprise, the stream was dry.   This is a valley that cuts through limestone, so it is common for the stream to be dry in the summer, but I had not expected it to be dry in the middle of winter.  Thinking back, however, I realised that there has not been much rain for some weeks, and this may have meant that the water table, still low, perhaps, after last summer’s dry weather, is too low for the stream to flow.

blunts_burn_jan19

Diatoms and cyanobacterial colonies in Blunt’s Burn, Castle Eden Dene, January 2019.   The top photograph shows diatom growths on bedrock; the lower image shows Phormidium retzii colonies, each about two millimetres across.   The photograph at the top of the post shows a yew tree overhanging Castle Eden Burn. 

A few hundred metres further down the dene, we finally heard the sound of running water where a small tributary stream, Blunt’s Burn, joined the main burn.  Judging from my OS map, it drains a good part of Peterlee so it might not have very high water quality.  It was, however, a stream and it did, as I could see with the naked eye, have some distinct diatom-rich growths.    These, I discovered later, were dominated by the diatoms such as Navicula tripunctataand N. lanceolata which are typical of cold weather conditions (see, for example, “The River Wear in January”).   A closer look showed that the orange-brown diatom growths were, in places, flecked with dark brown spots.  Somehow, I managed to get my cold fingers to manipulate a pair of forceps and pick up a few of these spots for closer examination.

blunts_burn_diatoms

Diatoms from Blunt’s Burn, January 2019: a. Navicula tripunctata; b. N. lanceolata; c.Gyrosigma cf. acuminatum; d. Nitzschiacf. linearis (girdle view); e. N. linearis(valve view).  Scale bar: 10 micrometres (= 1/100thof a millimetre).

I had a good idea, when I first saw these spots, that they were colonies of a filamentous cyanobacterium and, peering through my microscope a few hours later, once I had warmed myself up, I was relieved to see that I was right.  I picked out a dark patch and teased it apart before putting it onto a slide with a drop of water.  Once I had done this, I could see the tangle of filaments along with a mass of organic and inorganic particles and lots of diatoms.   The filaments themselves were simple chains of cells (a “trichome”) of Phormidium retzii, surrounded by a sheath.   There were also, however, a few cases, where I could see the sheath without the Phormidium trichome, and in some those I could also see diatom cells.

There are some diatoms that make their own mucilage tubes (see “An excuse for a crab sandwich, really …”) but Nitzschia is not one of those most often associated with tube-formation (there are a few exceptions).    On the other hand, there are some references to Nitzschiacells squatting in tubes made by other diatoms.   Some of those who have observed this refer to Nitzschia as a “symbiont” but whether there is any formal arrangement or is just a by-product of Nitzschia’s ability to glide and seek out favourable microhabitats, is not clear.  There are, as far as I can see, no references, to diatoms inhabiting the sheaths of Cyanobacteria, though Brian Whitton tells me he has occasionally seen this too.

We made our way back along the dry bed of Castle Eden Burn.  Many of the rocks here were quite slippery, suggesting that there had been some water flowing along it in the recent past.  That encouraged me to scrub at the top surface of one with my toothbrush and I managed to get a sample that certainly contains diatoms though these were mostly smaller than the ones that I found in Blunt’s Burn, and there was also a lot of mineral matter.   I’ll need to get that sample prepped and a permanent slide prepared before I can report back on just what diatoms thrive in this tough habitat.  Watch this space …

blunts_burn_phormidium

Cyanobacterial filaments from Blunt’s Burn, Co. Durham, January 2019: a. a single trichome of Phormidium retzii; b. and c. empty sheaths colonised by cells of Nitzschia; d. aPhormidiumfilament with a sheath and a trichome but also with epiphytes and adsorbed organic and inorganic matter.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   

References

Carr, J.M. & Hergenrader, G.L. (2004).  Occurrence of three Nitzschia(Bacillariophyceae) taxa within colonies of tube-forming diatoms. Journal of Phycology23: 62-70.

Houpt, P.M. (1994). Marine tube-dwelling diatoms and their occurrence in the Netherlands. Netherlands Journal of Aquatic Ecology28: 77-84.

Lobban, C.S. (1984). Marine tube-dwelling diatoms of the Pacific coast of North America. I. BerkeleyaHasleaNitzschia, and Navicula sect. Microstigmaticae.  Canadian Journal of Botany63: 1779-1784.

Lobban, C.S. & Mann, D.G. (1987).  The systematics of the tube-dwelling diatom Nitzschia martiana and Nitzschia section Spathulatae. Canadian Journal of Botany.  65: 2396-2402, 

 

A year in the life of the River Wear …

After six bimonthly visits to the River Wear at Wolsingham during 2018, I can now step back and have a look at the complete dataset to see what patterns emerge.   Over the course of the year, I have visited the site six times and recorded a total of 107 species: 5 Cyanobacteria, 32 green algae, 69 diatoms and one red alga.  The true figure is probably higher than this, as the green algae include a number of “LRGT” (see “Little round green things …”) and certainly did not receive the same level of attention as the diatoms.

This crude enumeration of species, however, disguises some interesting seasonal patterns with, as I described in “Summertime Blues” and “Talking about the weather …”, abundant growths of green algae during the heatwave and associated low flow periods.  This can be seen clearly in the bar chart showing the seasonal changes in the river: diatoms predominate in the early part of the year whilst green algae are very scarce.  The bloom of the green filamentous alga Ulothrix zonata that I expected to see in March was missing due, I suspected, to the hard weather we experienced in late Feburary (see “The mystery of the alga that wasn’t there …”) but, by the summer, the river had taken on a very different complexion and was dominated by small green algae.   The last sample of the year, collected in November, showed a return to diatom dominance with a late autumn showing of Ulothrix zonata(see “The River Wear in November …”).

wear_summary_2018

Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2018.  

Looking back at records of a similar exercise in 2009, I see that the beginning and end of the year were quite similar, with thick biofilms dominated by diatoms; however, the algae in the summer of 2009 were very different to those I found in 2018.  My 2009 exercise involved visits every month rather than every other month and I see that I recorded more Cyanobacteria in June and July 2009 than I found in Summer 2018.  These were mostly filaments of Phormidium retziiand tufts of Homoeothrix varians, which I assumed to be a consequence of intense grazing (there is evidence that invertebrates find Cyanobacteria to be less palatable than other algae).  By July, Cyanobacteria comprised over half the total biovolume of algae; however, there was a major spate soon after my visit.  I was surprised to find, when I visited in August, a noticeably thicker biofilm smothering the rocks and, when I looked closely, this was dominated by the small motile diatom Nitzschia archibaldii.   The Cyanobacteria had disappeared almost completely.   I attributed this change to the invertebrate grazers being washed away by the spate, allowing the algae to grow unhindered.  As the biofilm grew in thickness, so the algal cells start to shade each other, and a diatom that can glide through the biofilm has an advantage over any that are stuck to one place.  Diatoms remained dominant for the remainder of the year, although my November sample came just after another storm and the stones I sampled were completely bare.

wear_summary_2009

Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2009.   A sample was collected in November but no living algae were recorded from it.

Overall, however, the similarities between the years outweighed the differences in the summer assemblages, whilst the composition of communities between late autumn and late spring was remarkably similar across the two years.   The changes in summer 2018 extended beyond just a shift in the balance of algae in favour of greens: there were also changes in the composition of diatoms too.  In fact, the changes in diatoms proved to be quite powerful mirrors of the changes in the community as a whole.  I have demonstrated this in datasets spanning a number of sites in the past but it is reassuring to see that they are also reflecting patterns within one site.   On the other hand, if I only had examined the diatoms, I would have missed some of the most interesting changes in the river over the course of the year.

Another observation is that no single sample from 2018 contained more than a quarter of the total algal diversity that I recorded over the course of the year.  Every month saw some new arrivals and some departures (or, more likely in some cases, a few taxa that were present had dropped below my analytical detection limit).  Some of these were expected (the seasonal dynamics of Ulothirx zonata, for example); others not (e.g. dominance by Keratococcus bicaudatusin the summer).  I discussed this in “A brief history of time-wasting …” and, in honour of that post, am not going to repeat myself here. In an age when our environmental regulators are cutting back on the amount of data that they gather, I shall go into 2019 reflecting on Yuval Noah Harari’s comment that “the greatest scientific discovery was the discovery of ignorance”.

The big pictures …

If you read this blog regularly you will, I hope, have some sense of just how varied are the algae that live in our freshwaters.   It occurred to me, however, that, in cataloguing this diversity, I don’t often step back and give you some idea of how these many forms relate to one another. I drop terms such as “diatom” and “green algae” into my posts but have not, perhaps, discussed the meaning of these terms in very much detail for some time.

One of the problems is that the meaning of these terms can vary, as knowledge unfolds.  For the early part of my career, for example, I could define “green algae” quite easily, and point to several authoritative textbooks to support my case.   Depending on who wrote the book (and when), green algae were either a separate division (“Chlorophyta”) or a class (“Chlorophyceae”).  There was some dispute about whether Chara and relatives belonged in this group or formed a separate group (“Charophyta”) but that was pretty much the end of the story and taxonomists then got down to arguing about how the many genera and species of green algae should be arranged within this broad heading.

Opinion has, however, shifted over the last couple of decades, with the green algae now split between two separate phyla within the kingdom Plantae.   One of these phyla is the Chlorophyta and the other is the Charophyta, which includes not just Chara and relatives but also some quite important Classes of green algae.    We have met representatives from many of the Classes from both of these phyla in this blog over the years, with the exception of the Prasinophytes, which is an important group of marine plankton with only a few freshwater representatives, and the Trebouxiphyceae.

Viridiplantae_organisation

The organisation of the “green algae” subkingdom (“Viridiplantae”) showing division into two Phyla, and the major Classes found in freshwaters within each Phylum.   The organisation follows Algaebase and the Tree of Life website (see also Lewis & McCourt, 2004). 

Back in the summer I described a number of green algae that I found in the River Wear.   In “Summertime blues …” I wrote about algae that belong to the Chlorophyceae whilst, later in the summer, I explained how these had been joined by a number of desmids, which belong to the Conjugatophyceae (see “Talking about the weather …”).  The plate in that post includes a cell of Pediastrum boryanumbeside some of the desmids; if I was to put together a plate of animals sharing a similar level of kinship, I might include a human and a slug – representatives of two separate phyla within the same kingdom, Animalia (see “Who do you think you are?”).  That is a remarkable amount of diversity to pack into a group of microscopic cells.

The next figure shows the organisation within the Conjugatophyceae, one of the Classes of Charophyta.  The biggest group, in terms of number of species, is the Desmidales, which have featured in quite a few posts (see “Desmid diversity …”), but this class also includes Mougeotia and Zygnema, which we met in the previous post.  Again, just to give you some idea of the scale of the differences, Mougeotia and Zygnema are as closely related as we are to chimpanzees (different genera, same family), whilst their kinship to a desmid is on a par with ours to a warthog (different families, same order).

If you think that you are rather more different to a warthog than one microscopic green alga is to another, there are two things you need to remember: the first is that humans are, relatively speaking, rather good at knowing what features set different types of mammal apart, and that the absence of two short tusks protruding from the sides of the mouth, coupled with a bipedal gate, are highly relevant factors when struggling to decide whether or not the organism in front of you is a man or a warthog.  When trying to understand microscopic organisms such as algae, there are fewer obvious characters, and some of the most useful (such as the presence of flagellae during the reproductive stages) may be present only for a short period of the life cycle.   Straightforward observation, quite simply, is not so useful when trying to determine relationships between microscopic organisms.

Conjugatophyceae_orders

Organisation within the Conjugatophyceae, showing division into two Orders and Families.  After Algaebase and the Tree of Life website.

The other point to bear in mind is that algae having had far longer to evolve than mammals.   The two green algae lineages may have separated before the end of the Precambrian era, whilst the primates, the Order to which humans belong, split from other mammals only 65 million years ago.   That means that the green algae have had eight times as long to evolve subtle differences as humans have had to ensure no confusion with warthogs.   Just because these differences are not manifest in obvious features such as tusks does not mean that they are not there.

This brief overview of the green algae has had a side-benefit for me, as it has highlighted a couple of groups I have not previously written about.  One of these groups (the Prasinophytes) is uncommon in freshwaters but the other (Trebouxiphyceae) is quite common and I can even see a green patch formed by a member of this Class from my window as I write this post.   At least I know now what I should write about next …

References

Lewis, M.A. & McCourt, M.M. (2004). Green algae and the origin of land plants.  American Journal of Botany91: 1535-1556.

Leliaert F, Smith DR, Moreau H, Herron MD, Verbruggen H, Delwiche CF & De Clerck O (2012) Phylogeny and molecular evolution of the green algae. Critical Reviews in Plant Sciences 31: 1-46.

Appendix

Links to posts describing representatives of the major groups of green algae.  Only the most recent posts are included but these should have links to older posts.

Group Link
Chlorophyta  
Chlorophyceae Keeping the cogs turning …

Summertime blues …

Ulvophyceae Includes many important filamentous and thalloid genera from freshwaters:

Chaetophorales: Life in the colonies …

Cladophorales: Cladophora and friends

Oedogoniales: More about Oedogonium

Trentepoliales: Fake tans in the Yorkshire Dales

Ulothrichales: Spring in Ennerdale

Ulvales: Loving the low flows

Trebouxiphyceae Watch this space …
Prasinophyta Watch this space …
Charophyta  
Charophyceaee Life in the deep zone …
Conjugatophyceae Desmidiales: Desmid diversity

Zygnemetales: Fifty shades of green

Klebsormidiaceae The River Ehen in November