Concentrating on carbon …

On the other side of Ennerdale Water I could see plenty more submerged stones, all covered with green filaments but these belonged to different genera to those that I wrote about in my previous post.   Both are genera that we have met previously – Mougeotia, which has flat, plate-like chloroplasts which rotate around a central axis in order to control its rate of photosynthesis – and Spirogyra.  When light levels are low, Mougeotia’s flat chloroplast is perpendicular to the light in order to capture as much energy as possible, but in bright light it rotates so that the plate is parallel to the direction of the light, in order to slow the photosynthesis mechanism down and prevent internal damage (see “Good vibrations under the Suffolk sun” for another approach to this problem).

However, too much sunlight is the least of an alga’s problems in the Lake District.   This post looks at a different challenge facing freshwater algae and our starting point is the spherical nodules, “pyrenoids”, that you should be able to see on the chloroplasts of both Mougeotia and Spirogyra in the images below.   Photosynthesis involves a reaction between water and carbon dioxide to make simple sugars (turning fizzy mineral water into “pop”, in other words).   A submerged alga does not have a problem obtaining the water it needs, but what about carbon dioxide?   Gases are not very soluble in water, so this presents a much bigger problem to the algae.   Explaining why also presents a big problem to a blogger who conscientiously avoided physics and chemistry from age 16 onwards.  Here goes …

Mougeotia from the littoral zone of Ennerdale Water, April 2017.  Scale bar: 20 micrometres (= 50th of a millimetre).

The concentration of a gas in a liquid depends upon the concentration of that gas in the surrounding atmosphere.   As far as we know (and this is still an area of contention amongst geologists), concentrations of carbon dioxide in the deep past were much higher than they are today, in part because there were no land plants to suck it out of the atmosphere for their own photosynthesis.  So the earliest photosynthetic bacteria and, subsequently, algae, lived in water that also had higher concentrations of carbon dioxide.   As land plants spread, so the carbon dioxide concentration in the atmosphere dropped as they used it to fuel their own growth.  As a result, carbon dioxide concentrations in the water also dropped, thus depriving the algae of an essential raw material for photosynthesis.

However, carbon dioxide is not the only source of carbon available to aquatic organisms.   There is also carbon in many rocks, limestone in particular, and this can mineralise to carbonate and bicarbonate ions dissolved in the water.  Aquatic plants can get hold of this alternative carbon supply via an enzyme called carbonic anhydrase.   By concentrating the carbonic anhydrase activity in a small area of the chloroplast, the algal cell can boost the activity of the Rubisco enzyme (which evolved to function at a higher concentration of carbon dioxide).   This whole process is one of a number of forms of “carbon concentrating mechanism” that plants use to turbocharge their photosynthetic engines (see “CAM, CAM, CAM …” on my wife’s blog for more about a terrestrial version of this).

A two-chloroplast form of Spirogyra from the littoral zone of Ennerdale Water, April 2017.  Scale bar: 20 micrometres (= 50th of a millimetre).

Pyrenoids are widespread amongst algae, though a few groups (notably red algae and most chrysophytes) lack them.   Cyanobacteria (blue-green algae) use an organelle called a “carboxysome” for a similar purpose.   The only group of land plants with pyrenoids are the hornworts, relatives of mosses and liverworts.   About half of all hornworts have pyrenoids and a recent study has suggested that the ability to form pyrenoids has evolved up to five times in this group during their evolution.   The appearance of pyrenoids in distinct evolutionary lineages of algae also suggests that there may have been several evolutionary events that precipitated their formation.  And, it is important to stress, some algae which lack pyrenoids have alternative methods of concentrating carbon to enhance Rubisco activity.

So let us end where we started: in the littoral zone of Ennerdale Water on an April morning, gazing at a fine “fur” of filamentous algae clinging to the submerged rocks.   Back in October last year, I talked about how Ennerdale fitted into a pattern of increasing productivity of Cumbrian lakes first noticed by Pearsall in the early part of the 20th century (see “The power of rock …”).   Now we can start to understand that pattern in terms of basic biochemical processes: getting enough carbon from a combination of atmospheric carbon dioxide and the surrounding rocks for Rubisco and the other photosynthetic enzymes to convert to sugars.   In Ennerdale Water, one of the least productive of the Cumbrian lakes, we can see these algae during the winter and spring because the amount of biomass that those biochemical reactions produces is still just ahead of the amount that grazing invertebrates such as midge larvae can remove.  In a month or so, the grazers will have caught up and the rock surfaces will be, to the naked eye at least, bare.

Rubisco is the enzyme whose gene, rbcL, we use for molecular barcoding, subject of many recent posts (see “When a picture is worth a thousand base pairs …”).  My early desire to avoid physics and chemistry at school translated into as little biochemistry as possible whilst an undergraduate and, over the past few-years, I’ve developed a frantic urge to catch-up on all that I missed.   Just wish that those lectures explaining the Calvin cycle had been a little less … tedious …

References

Giordano, M., Beardall, J. & Raven, J.A. (2005).  CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution.   Annual Review of Plant Biology 56: 99-131.

Villareal, J.C. & Renner, S.S. (2012).  Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years.  Proceedings of the National Academy  of Science of the USA 109: 18873-18878.

 

Spring in Ennerdale …

My latest trip to Ennerdale Water, in the Lake District, has yielded its usual crop of spectacular views and intriguing questions (see “Reflections from Ennerdale’s far side”).   This time, my curiosity was piqued by lush growths of green algae at several locations around the lake shore.  The knee-jerk reaction to such growths is that they indicate nutrient enrichment but I am always sceptical of this explanation, as lush green growth are a common sight in spring (see “The intricate ecology of green slime …”) and these often disappear within a month or two of appearing.

Two points of interest: first, the lake seems to be lagging behind the River Ehen, which flows out of Ennerdale Water.   We often see these lush growths of algae on the river bed in winter but by this time of year the mass of algae there is lower than we saw in the lake littoral.   Second, the lake bed looks far worse (see photograph below, from the north-west corner of the lake) than the actual biomass suggests.

Filamentous algae (Ulothrix aequalis) smothering cobble-sized stones in the littoral zone of Ennerdale Water, April 2017.

Under the microscope, this revealed itself to be unbranched filaments of a green algae, whose cells each contained a single band-shaped chloroplast lapping around most of the perimeter.   This is Ulothrix aequalis, a relative of Ulothrix zonata, which I wrote about a few times last year (see link above).   Like U. zonata, this species is very slimy to the touch and, I suspect, the payload of mucilage adds to the buoyancy of the organism and means that we look down on a fine mesh of filaments which trap light and add to the unsightly appearance of the lake bed at this point.   That this part of the lake shore is close to a tributary stream draining some improved pasture triggers some suspicions of agricultural run-off fuelling the algal growths but, looking back at my notebook, I see that the lake bed was almost clear of green algae when we visited this location in July last year.  I suspect that a return visit this summer would also show a clean river bed.  Appearances can often be misleading (see “The camera never lies?”).

Ulothrix aequalis from the littoral zone of Ennerdale Water, April 2017.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

This was not the only site that we visited that had conspicuous growths of green algae, though the mass of algae was greatest here.   All of the sites at the western end had these growths (see “A lake of two halves” for an explanation of geological differences within the lake) but, curiously, the genus of alga that we found differed from site to site.   In addition to Ulothrix aequalis in this corner of the lake, we found Mougeotia on the south side and Spirogyra close to the outfall.  This diversity of forms is, itself, intriguing, and I have never read a convincing explanation of what environmental conditions favours each of these genera.   I see both spatial and temporal patterns of green algae in the River Ehen too and, again, there is no satisfactory explanation for why the species I find can differ along short distances of the river and between monthly visits.

The Mougeotia and Spirogyra both have another story to tell, but that will have to wait for the next post …

A new diatom record from West Sussex

Some part-time sleuthing on a sample that I was sent a couple of weeks ago have resulted in a new addition to the UK freshwater diatom flora.   The slide came to me from an Environment Agency laboratory with a question mark over a small diatom that was quite abundant but which did not match any of the species with which they were familiar.  It was a small diatom, only about 10 micrometres (1/100th of a millimetre) long, with very fine features, but there were enough features visible for me to realise that it was not something that I had seen before either.   I sent images off to a couple of colleagues and we decided that it was a species of Nupela, probably N. neglecta.

Nupela was only established in 1991. Before that, species that we now place in this genus were spread between Achnanthes and Navicula, as people struggled to understand its characteristics.   If you look at diatom keys written before Nupela was established (and several written subsequently – including my own), the presence of a raphe on either or both valves is seen as an important distinguishing characteristic, and the small number of genera that have a raphe on just one valve were generally assumed to be related.   Nupela, however, has some representatives that have a raphe on one valve (formerly placed in Achnanthes) and representatives that have a raphe on both valves (formerly placed in Navicula).  Nupela neglecta has a raphe on both valves, but one of the valves has raphe slits that only extend for about half the total length.   Stir in the small size and morphological details that are barely visible with the light microscope and there is ample scope for confusion.

nupela_neglecta_raphe_cfc

Valves of Nupela cf neglecta with a full raphe.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  The double lines indicate a single valve at different focal points.  Photographs: Chris Carter.

nupela_neglecta_partial_rap

Valves of Nupela cf neglecta with a short raphe.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  The double lines indicate a single valve at different focal points.  Photographs: Chris Carter.

The sample was collected from the River Stor, a tributary of the River Arun, in West Sussex, downstream of Storrington sewage works (NGR: TQ 0681 1641).  This is a small hard water stream (average pH: 7.9; average alkalinity: 170 mg L-1 CaCO3) with very high concentrations of nutrients (average dissolved reactive phosphorus: 0.99 mg L-1; average total oxidised nitrogen: 5.0 mg L-1).   These observations are similar to those made by Marina Potapova and her colleagues for habitatas where they found N. neglecta in the USA.  And this raises an interesting paradox: normally, the presence of a rare and exotic organism is considered to be a reason for conserving a habitat.  In this case, however, the rare species seems to be associated with a polluted habitat and, as a result, the Environment Agency will be doing their best to drive any organism that thrives here (however rare) to extinction.   Discuss.

References

Krammer, K. & Lange-Bertalot, H. (1991). Süswasserflora von Mitteleuropa2: Bacillariophyceae; 4. Teil: Achnanthaceae, Kritische Ergänzungen zu. Achnanthes s.l., Navicula s. str., Gomphonema, Gesamtliteraturverzeichnis Teil 1-4.  Spektrum Akademischer Verlag, Heidelberg.  (see p. 440 for account of Nupela)

Potapova, M.G., Ponader, K.C., Lowe, R.L., Clason, T.A. & Bahls, L.L. (2003).  Small-celled Nupela species from North America.   Diatom Research 18: 293-306.

Vyverman, W. & Compère, P. (1991).  Nupela giluwensis gen. & spec. nov.  A new genus of naviculoid diatoms.  Diatom Research 6: 175-179.

When is a sample not a sample?

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If you have followed this blog over the years, you will probably have worked out that the only inevitable outcome of a close study of diatoms is that you are older at the end than you were at the start. Whether you are also wiser is, alas, not guaranteed.   The older : wiser ratio can vary quite a lot, depending on what, exactly, you are studying and a further factor to stir into the mix is that a freelance ecologist such as myself needs to be prepared to forego the pursuit of wisdom if the price is right.

And so it is that I have spent a fair part of my time since Christmas staring down my microscope at a batch of samples that I have been sent whilst, at the same time, cursing my pecuniary instincts.   These samples are one part of a large survey and, I know, are not collected by people with any experience of freshwater algae.  Judging by the muddy sludge that I get in some of the sample tubes, I am not wholly sure that all can be trusted to distinguish a stream from a field, let alone find stones likely to yield a representative crop of diatoms.   But when, I wondered, after an hour hunting around a slide for fragments of diatoms to identify, do I throw up my hands and say “enough”?

The two photographs in this post are from one of these irksome slides.  In both cases, there is a single diatom but, also, quite a lot of mineral matter.   I would expect maybe five to ten diatoms in a field of view on a well-prepared slide from a good sample,.  In this one, there were more fields of view without diatoms than there were with (typically, I had to scroll past two empty fields between each identifiable diatom, but there could be as many as four or five empty fields between diatoms).   In theory, my first action when confronted with a slide such as this is to make another, more concentrated slide but this will also concentrate all that mineral matter.

117024_2

A field of view with a single valve of Achnanthidium minutissimum (top right) from a sample from an unnamed stream.   The image at the top of the post shows a different field view, this time with a single valve of Cocconeis euglypta.  Note the large quantity of inorganic matter in the sample.

Here then, are a few questions to ask when you encounter a very sparse slide.

  • Who collected the sample? Do you trust them or not?   A lot of samples these days are collected by people who have little understanding of the ecology of benthic algae and who will not know when a sample is unlikely to yield enough diatoms for analysis;
  • Is there a lot of particulate matter that has resisted oxidation during the preparation stages? These might be telling you something about the habitat itself: mineral particles suggest a depositional, rather than an erosional, habitat.  Some organic materials, particularly from peaty habitats, are also resistant, and can obscure diatoms, unless a dilute preparation is made;
  • What is the state of the diatoms that are present on the slide? If a large proportion are broken, this may suggest that there was not a viable community of algae at the time the sample was collected and you are, in fact, counting diatoms that have been washed in from elsewhere in the catchment;
  • Do the diatoms that you do find in a sample tell a consistent story? Sometimes the diatoms I find in a sparse sample have ecological profiles which, when combined, suggests a particular interpretation but, on other occasions, I see samples that are both very sparse and very diverse, with species representative of several different environments.  When the ecological profiles are not broadly consistent then, again, it is a warning that you may be dealing with washed-in diatoms and fragments, and not an assemblage that is telling you much about the site in question.

I believe that you should be able to count at least 100 valves and have answered the second, third and fourth questions after no more than an hour’s analysis.  This is a good point at which to decide whether it is worth pushing on to complete the analysis or abandon the count.   I try to make this clear in my terms and conditions, emphasising that it takes about as long to decide that a sample cannot be analysed as it does to perform an analysis on a “normal” sample.   I should also emphasise that these suggestions apply to samples from rivers and lake littoral zones and different criteria may need to be applied when dealing with other types of samples (e.g. for palaeoecological or forensic work).

The judgements that you need to make are easier if you have direct knowledge of the site from which the sample was collected; however, this is often not possible.   As “streamcraft” is undervalued by managers (see “Primed for the unexpected” for my most recent moan on this topic), the natural habitat of the diatom analyst is the laboratory not the field and sample collection is often delegated to less-highly trained individuals.   The determination of my fellow analysts to wring every last mote of knowledge from empty silica frustules has also contributed to a greater focus on the laboratory, rather than the field.   Most of the time, to be fair, sample quality is not a factor.  We produced some PowerPoint presentations a few years ago to help people collect diatom samples (see “A cautionary tale…” for the whole story) and, let’s be honest, collecting a decent diatom sample should not be rocket science.   The question underlying all of these is whether or not the diatoms you have on a slide are an accurate representation of the assemblage of living diatoms present at a particular point in space and time.   If you cannot say “yes” with confidence, then you will certainly be older, but no-one will be any wiser.

Reflections from Ennerdale’s Far Side …

ennerdale_far_end_170105

Ennerdale Water is, as I have described in earlier posts, is a lake of two halves, with a south eastern end influenced by granite and the north western end by softer mudstones and sandstones.  That has a big effect on the algae that we find in the littoral zone, with Cyanobacteria (blue-green algae) abundant in the south-east end and Chlorophyta (green algae) more conspicuous at the other end.   Diatoms are conspicuous in the littoral zone all around the lake, although there are some differences in the types of species encountered.  That is a story for another day, but I did find one species in some of the samples I collected from the south-eastern end that point to one other influence on the ecology of Ennerdale’s littoral zone.

Look at the photograph at the start of this post.  It was taken as I walked up to the south-eastern end (circa NY 127 140) and shows the view up the lake, with Angler’s Crag visible on the left hand shore in the distance.   The River Liza enters the lake on the right hand side (just out of the frame) and the low lying area between the River Liza and the raised ground where I was standing is an area of wet heath with a range of Sphagnum species and several boggy pools.   The shoreline of the lake itself is formed by a shingle spit which acts as a barrier between the wet heath and the lake itself.

ennerdale_gravel_bar_170105

The shingle spit separating the wet heath at the south-east end of Ennerdale from the lake itself.   Photographed in January 2017.

Several of the diatoms that I found at this end of the lake were species that I associate with acid conditions although, curiously, the limited chemical data that we have does not show a lower pH here than elsewhere in the lake.   I suspect that the proximity to the acid Sphagnum heath may lead to occasional pulses of acid water entering this area and exerting a subtle effect on the attached algae before being diluted by the water of the lake as a whole.   Of the species that I found, the most intriguing was Stenopterobia sigmatella, a long, sigmoid diatom with a single plate-like chloroplast.

The genus Stenopterobia fulfils most of my criteria for a genuinely rare diatom (see “A “red list” of endangered British diatoms”).   I only have 11 records in my dataset of 6500 samples, and in only one case did Stenopterobia constitute more than one percent of the diatoms in the sample.   These samples are all from acid habitats (mean pH: 6.1), with low nutrient concentrations (never more than 2 mg L-1 reactive phosphorus).  Those for which we have location information are plotted below.   The record in East Anglia needs further investigation (meaning: “I don’t believe it … but I haven’t had a chance to track down the slide for a closer look”). If we ignore this, the distribution is confined to mountainous regions of western Britain, and these Ennerdale samples also fit this trend, although the lake has soft water and is circumneutral rather than acid.

Stenopterobia sigmatella is another diatom with a sigmoid outline, and this brings me back to a question that I have posed before (see “Nitzschia and a friend …”): what advantages does a sigmoid outline confer on a diatom?  I cannot think of any other genera of algae that has species with a sigmoid outline, which only adds to the mystery. All of the diatoms that are sigmoid are motile, so I guess that the explanation may be linked to movement, but I don’t know for sure what the reason may be.   For all of the rich diversity that we see in diatoms, there is still, to pick up on a phrase from my biography of Humboldt, a “poverty of meaning” …

stenopterobia_sigmatella_en

Stenopterobia cf sigmatella from Ennerdale Water, October 2016 and January 2017.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

stenopterobia_distribution_

A distribution map of records of Stenopterobia in Great Britain.   S. curvula is a synonym for S. sigmatella (see taxonomic note below).  Map prepared by Susannah Collings (see “Why do you look for the living among the dead?” for more details of how this was done)

stenopterobia_densestriata_

A valve of Stenopterobia densestriata.  Photograph from the ADIAC database (photographer: Micha Bayer).  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Taxonomic note

I have used the name “Stenopterobia sigmatella” in this post, but this still needs confirmation as there is a closely-related species, S. densestriata (Hustedt) Krammer 1987 (see image above).  S. sigmatella has < 24 striae in 10 micrometres whilst S. densestriata has > 26 striae in 10 micrometres.  S. densestriata also has slightly smaller overall dimensions.

David Mann made the following comment about Stenopterobia sigmatella on the website Common Freshwater Diatoms of Britain and Ireland (predecessor to the new Diatom Flora of Britain and Ireland: “A nomenclatural mess. For most of the 20th century, this species was referred to (wrongly) as S. intermedia. Ross (in Hartley, 1986) stated that there is an earlier name, sigmatella, that could be applied to this species and made a new combination S. sigmatella. Unfortunately, this was wholly ignored by Krammer (in Lange-Bertalot & Krammer, 1987; and see Krammer & Lange-Bertalot, 1988) who made the new combination S. curvula. However, Nitzschia curvula of W. Smith is preceded by N. sigmatella of Gregory (1856, 1854, respectively).”   The references can all be found on the Common Freshwater Diatoms website.

 

Not so Bleak Midwinter?

ennerdale_170104

Occasionally – just occasionally – the gods smile on us when we least expect it.  And Wednesday was one of those days: fieldwork on a glorious winter day in the Lake District without a cloud in the sky and barely a breath of wind.  The pleasure of being outside on such a day was offset slightly by the necessity of plunging my arm into freezing cold water at intervals, but the views of the mountains beyond Ennerdale Water more than compensated for these temporary discomforts.

The coldness of the water, today, offers me a link to a book I am reading, about the 19th century German scientist Alexander von Humboldt, a polymath who was ahead of his time in many ways, and whose writing pre-empted ecological thinking of the twentieth century.   One of his strongly held beliefs was that scientists could not really understand nature from a laboratory: they had to be outside, experiencing nature first hand.   That seems to be a fine New Year message in a world where ecologists seem to spend more and more time staring at screens, and their managers are increasingly reluctant to let them spend time in the field.

The ecology of lakes and rivers in this area in winter continues to fascinate me.   Look at the picture below: a stream bed at the coldest time of year that is covered with lush growths of algae in a range of hues, most strikingly the pink-red of the Rhodophyta Audouinella, complemented by the green and blue-green algae around it.  The first young olive-green filaments shoots of Lemanea, another Rhodophyta, were also apparent at a couple of the sites that I visited, and there were thick brown diatom blooms smothering many of the stones too.   These are all thriving at a time of year when either most nature has shut down for the winter or most natural historians have plonked themselves onto the sofa to watch Living World II rather than challenging the first clause in this sentence.  You decide.

ehen_ob_170104

A riot of colour on the stony substrata of the River Ehen, a few kilometres downstream of Ennerdale Water, Cumbria, January 2017. 

One of Humboldt’s big concerns was that scientists saw the big picture (“naturgemälde”) rather than getting bogged down with details.   He was someone whose mind had been formed by the Enlightenment, when the necessity of cataloguing and classifying the diversity of nature was a primary concern.  However, he saw that this was not enough, and that one had to understand the connections between these different life forms, and between each of these and their environment.  He saw the natural world as a web of interdependencies, and humans as potential disruptors of the delicate balances that existed.

The problem we have in the modern age is balancing the need to see the big picture in focus without losing site of important details.  Or, as Ed Tipping said during a meeting at CEH last year: “we stick to the principle of simplifying to just short of the point of naivety”.   He had his tongue in his cheek but there is an important point here: the complexity of the natural world means that its secrets will only be yielded to those scientists who can keep their natural proclivity to get lost in detail in check.   At the same time, if we forget that those details are out there we may reach erroneous conclusions.  And, I fear, microscopic benthic algae may be ecology’s Sirens, sitting on submerged rocks and luring the unsuspecting into a world of taxonomic detail that is too rarely accompanied by profound ecological insight.

William Wordsworth, born in Cockermouth, just a few miles away from Ennerdale, was one of Humboldt’s readers.  He recognised the need to be outside experiencing nature applied as much to a poet as to a scientist and reacting against the dry, dissected knowledge that the Enlightenment encouraged.  His words offer a succinct conclusion for this first post of 2017, and encapsulate my resolution to be as holistic as possible in my thinking during the year ahead:

For was it meant
That we should pore, and dwindle as we pore,
For every dimly pore on things minute,
On solitary objects, still beheld
In disconnection dead and spiritless,
And still dividing and dividing still
Break down all grandeur …

William Wordsworth, The Excursion, 1814

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Diatoms from the Valley of Flowers

valley_of_flowers_5_aug16

My visit to the Valley of Flowers in India (see “Into the Valley of Flowers …”) is a fast fading memory but I have finally managed to get the diatom samples that I smuggled into my suitcase properly mounted and spent some time last weekend peering down my microscope and trying to match what I could see with the habitat that I remembered.

The sample I collected came from a first order stream which appeared from the mouth of a glacier a couple of hundred metres above us on the [north] side of the valley.  About 500 metres downstream it joined the Pushpanati River, a tributary of the Aleknanda, itself a tributary of the mighty Ganges.   It was just over a metre wide and a few centimetres deep and had a mixture of pebbles and gravel as its substratum.  Some of the larger stones were encrusted with what looked like growths of Chamaesiphon  (see “A bigger splash …“).  There were also a few flocs of green algae which turned out to be Zygnema, a relative of Mougeotia and Spirogyra ( see “Fifty shades of green …”) with two distinctive star-shaped chloroplasts.  They are not at their best in the photograph below because they made the journey from India to the UK soused in local vodka (a cheap and effective preservative for algae: your liver will not begrudge you this particular experience …).

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Vinood, our guide, looking at the stream that I sampled in the Valley of Flowers, August 2016.

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Vodka-soused Zygnema sp from a glacier-fed tributary stream of the Pushpanati River (Valley of Flowers),  August 2016.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The diatoms were a surprise as almost all belonged to a single species, Diatoma mesodon, a species familiar to me from very high quality streams in Europe.  Ninety-five percent of all the diatoms I looked at belonged to this species, an unusually high proportion compared with other samples that I have examined, especially as there are no human pressures in the area that might influence diversity.   The Diatoma cells formed zig-zag chains, though these fell apart during the preparation process and the images just show individual valves.   Other diatoms present in small numbers included Meridion circulare var. constrictum (syn: Meridion constrictum) and two species of Eunotia, all of which suggest relatively soft water.

The low diversity intrigued me.   I have seen very low diversity with headwater streams, possibly because there is low potential for organisms from upstream to “seed” the location.   As small tributaries merge, so incocula from the sparse assemblages of these headwater streams will combine to create more diverse communities downstream.  Curiously, a recent paper argues the opposite: that headwaters are, in fact, hotspots of microbial diversity, and that this declines with increasing distance downstream.   However, this study takes a much broader view than just algae.  The authors suggest that it the close connection between headwater streams and the surrounding catchment leads to soil bacteria being washed into the stream.   So this result does not really contradict my observation; rather it highlights the limited insight that one may glean through looking at a single group of organisms.

Had I had more time (and more samples containers), it would have been interesting to follow the valley as far upstream as possible, to see if the other streams flowing down from the hillside had similar assemblages of algae, or if their algae were different.   My guess is that the patchwork of habitats would mean that the total diversity for the valley (“beta diversity”) would be considerably greater than the diversity at any particular site (“alpha diversity”).   If anyone wants to test this hypothesis, then all they need to do is make a three day journey from Delhi, with an extra day set aside for acclimatisation, followed by a two day hike up to a height of 3500 m.  And don’t forget to pack that bottle of vodka …

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Diatoma mesodon from a glacier-fed tributary stream of the Pushpanati River (Valley of Flowers), August 2016.   a. – e.: valve views; f. – g.: girdle views.  Scale bar: 10 micrometres (= 1/100th of a millimetre.

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Other diatoms from a glacier-fed tributary stream of the Pushpanati River (Valley of Flowers),  August 2016,  a. – b.: valve and girdle views of Meridion circulare var. constrictum; c. Eunotia islandica; d. E. paratridentula; e. Cymbella cf naviculiformis.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Reference

Besemer, K., Singer, G., Quince, C., Bertuzzo, E., Sloan, W. & Battin, T.J. (2016).  Headwaters are critical reservoirs of microbial diversity for fluvial networks.  Proceedings of the Royal Society of London Series B 280: DOI: 10.1098/rspb.2013.1760