Casting the net wide …

A  month or so ago I wrote a couple of posts about the green algae that were thriving in the River Wear this summer (see “Keeping the cogs turning …” and “More green algae from the River Wear”).  In one of those, I promised to write a post about a related genus, Hydrodictyon.   I did try to find some recent populations but ran out of time so have fallen back on some old pictures along with more of Chris Carter’s spectacular photography.

Hydrodictyon reticulatum is commonly called the “water net” and can form extensive, and sometimes nuisance, growths, either floating in a lake or a slow-flowing river, or as a mat at the edge (see photos below).   The cylindrical cells are arranged in pentagons or hexagons which can be visible with the naked eye (hence the net-like appearance).   These have a mode of asexual reproduction that results in tiny zoospores being formed inside each cell.  Each of these develops into a small daughter cell whilst still inside the mother cell, and the ends of these daughters then join together to form mini-nets.  You can see this happening in Chris’ image at the top of the post: there are some young cells in the foreground with a mature “mother” cell full of “daughter” cells forming their own nets inside.  Eventually, the wall of the mother-cell disintegrates and the daughter net is released.

A mat of Hydrodictyon reticulatum from the lower River Tweed; b. macro, and, c. microscopic views of coenobia of H. reticulatum from Thrapston Lake, Northampton (b. and c. by Chris Carter).

This is an extremely effective way of enabling Hydrodictyon reticulatum to spread quickly when conditions favour its growth and the image below shows just how extensive these mats can be.   It is a species that is more common in the warmer parts of the world but it does occur in the UK as far north as Scotland and Brian Whitton has predicted that it is a species that is likely to be favoured in some climate warning scenarios.   Some authors have suggested that Hydrodictyon favours nutrient-rich water, but some of the locations where I have found it (the River Tweed in Scotland, Sunbiggin Tarn in Cumbria) do not meet this criterion.  Rather, I suggest that well-lit, relatively undisturbed summer conditions are the key factor and that this is more likely to be the case in lowland areas that are, in many cases, also rich in nutrients.  It is more likely to be a correlation than a cause, in other words.   Whatever the cause, there is a huge dichotomy between the beauty of the organism under the microscope and the nuisance that it can cause.

A huge growth of Hydrodictyon reticulatum at Manor Farm Weir on the Jubilee River (a flood alleviation channel of the River Thames near Maidenhead).  Photo: Environment Agency.

This mode of asexual reproduction – in which the zoospores aggregate inside the parent – is also a feature of Pediastrum.  Even though the shapes and dimensions of the organisms are very different, they share some fundamental properties.   Molecular phylogenetic studies have also shown that there is a close affinity between Hydrodictyon, Pediastrum and the other genera I mentioned in “Keeping the cogs turning …”.   However, their habits and ecology are very different and that raises some interesting questions about a different matter entirely … the subject of my next post.

Reference

John, D.M., Pentecost, A. & Whitton B.A. (2001).  Terrestrial and freshwater eukaryotic algae.   pp. 148-149.  In: The Changing Wildlife of Great Britain and Ireland (edited by D.L. Hawksworth).   Taylor & Francis, London.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

 

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Keeping the cogs turning …

A few algae lift my soul when I see them under the microscope through their beauty.   To see such intricate yet symmetrical organisation in something too small to be visible with the naked eye never ceases to amaze and delight me.   One of the genera that has that effect is the green alga Pediastrum, which forms cog-like colonies: flat plates of cells whose outer members bear horn-like projections.   One of its representatives, Pediastrum boryanum, was common in the River Wear when I visited recently (see previous post).   You can see, from the illustration above (the scale bar is 20 micrometres – 1/50th of a millimetre – long), the characteristic disc-like arrangement of cells, always in multiples of four (there are 16 in the colony above).   There are many species of Pediastrum, differing in the shape of both the inner and marginal cells, and the number and length of the horns.

I have found Pediastrum on many occasions in the Wear in the past, but never quite as abundant as it was in my most recent samples.  Pediastrum boryanum is the species I find most often, here and elsewhere, but other species occur too.  I have also found Pediastrum in some unusual places, including deep in lake sediments when I was searching for fossil pollen grains and there is evidence that the cell walls of Pediastrum contain both silica and a sporopollenin-like material (sporopollenin is the extremely tough material found in the outer walls of pollen grains (which probably explains why it had survived the fierce mix of chemicals that we used to prepare the lake sediments for pollen analysis).   I am guessing that the sporopollenin and silica both add some structural integrity to the cells.   There are references in the literature to Pediastrum being planktonic but I often find it in samples from submerged surfaces and associated with submerged macrophytes, so I suspect that it is one of those species that moves between different types of habitat.  It should not really be a surprise that a relatively large colonial alga with a payload of silica and sporopollenin in addition to the usual cellulose cell wall, is going to be common in benthic films in a river towards the end of a long, dry summer.

Pediastrum is another genus that has been shaken up in recent years as a result of molecular studies.  According to these, Pediastrum boryanum should now be called Pseudopediastrum boryanum although the Freshwater Algal Flora of the British Isles continues to use the old name.   Not everyone agrees with this split (see McManus and Lewis’ paper in the list below) but the divisions suggested by molecular data do also seem to match differences in morphological characteristics of the group (see Table below).

Pediastrum is part of the family Hydrodictyaceae and, as I was writing this, it occurred to me that I have never written about another interesting member of this family, Hydrodictyon reticulatum.  As I like to accompany my posts with my own photographs, I spent part of yesterday afternoon tramping around a location where I have found Hydrodictyon in the past.   All I got for my troubles, however, was two damp feet, so that post will have to wait for another day.

 

Differentiating characteristics of Pediastrum and similar genera (after Krienitz & Bock, 2011).

Genus Features
Pediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes
Lacunastrum Flat coenobia with large intercellular spaces, marginal cells with two lobes
Monactinus Flat coenobia with large intercellular spaces, marginal cells with one lobes
Parapediastrum Flat coenobia with intercellular spaces, marginal cells with two lobes, each divided into two projections
Pseudopediastrum Flat coenobia without intercellular spaces, marginal cells with two lobes, each with a single projection
Sorastrum Three-dimensional coenobia, each cell with two or four projections.
Stauridium Flat coenobia without intercellular spaces, marginal cells “trapezoid”* with deep incision to create two lobes, each with a concave surface, though the lobes are not really extended into “projections”

* not all of the illustrations show marginal cells that are strictly “trapezoid” (e.g. with at least one pair of parallel sides).

References

Buchheim, M., Buchheim, J., Carlson, T., Braband, A., Hepperle, D., Krienitz, L., Wolf, M. & Hegewald, E. (2005).  Phylogeny of the Hydrodictyaceae (Chlorophyceae): inferences from rDNA data.  Journal of Phycology 41: 1039-104.

Good, B.H. & Chapman, R.L. (1978).  The ultrastructure of Phycopeltis (Chroolepidaceae: Chlorophyta). I. Sporopollenin in the cell walls.  American Journal of Botany 65: 27-33.

Jena, M., Bock, C., Behera, C., Adhikary, S.P. & Krienitz, L. (2014).  Strain survey on three continents confirms the polyphyly of the genus Pediastrum (Hydrodictyaceae, Chlorophyceae).  Fottea, Olomouc 14: 63-76.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

McManus, H.A. & Lewis, L.A. (2011).  Molecular phylogenetic relationships in the freshwater family Hydrodictyaceae (Sphaaeropleales, Chlorophyceae), with an emphasis on Pediastrum duplex.   Journal of Phycology 47: 152-163.

Millington, W.F. & Gawlik, S.R. (1967).  Silica in the wall of PediastrumNature (London) 216: 68.

Getting started with microscopy

I talked about algae last week at an event organised by Durham Wildlife Trust (part of the publicity for The Natural History of Upper Teesdale) and I promised them a post about how to get started in microscopy and, more specifically, to start discovering more about algae.  I have illustrated the post with some of Chris Carter’s spectacular images of algae to whet your appetites for exploring the world of freshwater algae …

Broadly speaking, the natural historian wants a microscope for one of two tasks: to make small things bigger or to make invisible things visible.   There is not really a sharp dividing line between these two categories, as the illustrations of Cladophora filaments in “Summertime blues …” show.   You might start out looking at a handful of green slime pulled from your garden pond, but then you might see smaller algae growing on those filaments that you want to examine too.   The good news is that you should be able to get hold of a reasonable microscope with the capacity to magnify up to 400 times for about the same outlay as a digital SLR camera.   That should let you see all but the smallest algae.   If you are sure that your interests lie mainly in “making small things bigger” then you should consider a low power dissecting microscope (these are probably the best way of introducing children to microscopy, as there is a smooth transition between the tangible but small object that has piqued their interest and the larger, more detailed image that they see when they peer through the microscope’s eyepieces).

However, even though a basic microscope need not cost a fortune, good microscopes are expensive so my advice to a beginner is to search out a rerfurbished second hand microscope.  In north east England, I can recommend JB Microscopes but Google should help you find dealers elsewhere in the country.   A reasonably local supplier is necessary because you should really try out a microscope before you buy.   There are reputable mail order suppliers (e.g. Brunel Microscopes) but I would not want to spend a large sum on a piece of equipment that I had not had a chance to use first.

A colony of the diatom Meridion circulare.  The image at the top of the post shows desmids from the genus Micrasterias.   Both photographs by Chris Carter. 

If you are on a limited budget, I suggest you go for a good basic microscope with the option to fit a camera at a later stage.  It is possible to take a reasonable photograph by pointing a digital camera (or even a smartphone) down the microscope’s eyepiece and it is better to put up with the shortcomings of these images than to sacrifice the quality of the microscope itself.

Once you have your microscope, you will also need slides, coverslips, forceps, some plastic Pasteur pipettes, a couple of needles, a scalpel and some collecting tubes.  You can buy all of these from Brunel Microscopes and NHBS, both of whom cater for both the amateur and professional markets.   They also sell boxes of prepared slides, which are a good way to get some experience at using a microscope.

The microscopic world generally lacks the type of user-friendly well-illustrated identification guides that help us identify wild flowers, birds, butterflies and so on.   Most books are aimed at the academic market and are, consequently, expensive.   If you want to get started with freshwater algae, one useful resource is this guide to the larger algae found in rivers: RAPPER_manual_version1.7_May2016.  It was produced to accompany a method for rapid assessment of streams and rivers and, as the journey towards formal publication has stalled, I am happy to make it available here.

Hydrodictyon reticulatum, the water-net, photographed by Chris Carter.  500 mm (micrometres) is half a millimetre.

Useful websites include AlgaeVision and the Diatom Flora of Britain and Ireland.  As most freshwater algal genera are found throughout the world, Diatoms of North America is also a useful resource.

The Freshwater Biological Association have affordable booklets on the identification of desmids and diatoms and there is an AIDGAP key, too, for freshwater diatoms.   The latter is badly in need of updating but, people assure me, is still useful for beginners.

There are plenty of other online resources, but l would recommend visiting the website of the Quekett Microscopical Club, a long-established group of enthusiasts whose interests span the whole realm of natural history and optics.   www.microscopy-uk.org.uk is also worth a visit.   Both websites will help you as you start your explorations of the hidden worlds of nature.

Grazing on algae …

I comment on the role that grazers play in controlling algal biomass in rivers in these posts and this is the time of year when I, myself, take a more participatory role.   As it is spring, Lemanea fluviatilis is thriving in our rivers (the cleaner ones, at least) and I could not resist grabbing a couple of handfuls whilst out in the field recently for culinary purposes.

This time, I followed the routine I described in “More from the Lemanea cookbook … ” and washed, air-dried and then cut-up some Lemanea filaments into short lengths (they need to be about a centimetre long, otherwise they can form clumps).   My experience is that the fishy taste of Lemanea is a fine complement to freshwater fish, so decided to use it in a warm potato salad which I then served underneath a salmon fillet seasoned and sprinkled with dill and then wrapped in foil and baked with a couple of knobs of butter.

The warm potato salad needs a mayonnaise made from one egg yolk and about 150 ml of olive oil into which a couple of tablespoons of lemon juice are stirred, along with salt and pepper.   Add a generous handful of dried Lemanea to this and leave to soften for about 20 minutes, and also add a teaspoon of capers and a small handful of land (or water) cress.  Cook and drain enough new potatoes for two, then cut these into small chunks and stir the mayonnaise and algae mixture into these.   Divide between two warmed bowls and place half the salmon fillet on top of each.  Finally, add a few fresh pea shoots as a garnish, along with a wedge of lemon, and serve.

Definitely worth repeating.

Warm potato salad with lemon and Lemanea, served with salmon fillets.

Cypriot delights …

I could not return from my visit to Cyprus without an algal sample and a fine opportunity presented itself early last week when we visited the Avgás Gorge, on the Akámas peninsula at the west coast of Cyprus, just north of Pathos.  This is a spectacular limestone ravine whose steep sides offered welcome relief from the Mediterranean sun.  At points, as in the photograph above, the ravine narrowed to just a few metres wide, reminiscent of the siq which guards the entrance to Petra except that instead of exquisite carvings we stumbled across a Russian team conducting a glamour shoot.

A small stream made its way down the gorge.  The presence of woody debris at intervals suggested a considerable head of water during the winter months but, at this time of year the water has reduced in power, tumbling across a series of boulders into stagnant pools, interspersed with short runs shaded either by the high cliffs or the vegetation that flourished away from the harsh glare of the sun.

In the sections where water was still running there were clumps of Chara tangled up with filamentous algae, with plenty of bubbles of oxygen as evidence that both were busily photosynthesising away.  The filamentous algae was a coarse unbranched filament that was clearly a relative of Cladophora but which did not match any of the genera or species that I had encountered before (see “Fieldwork at Flatford” for similar situation).   There were pebbles and cobbles between these clumps, their surfaces criss-crossed by the galleries of caseless caddis larvae – probably Psychomyiidae, according to Richard Chadd.

Chara growing in the stream at Avgás Gorge in western Cyprus, April 2018.

Cladophora or a relative growing in the stream at Avgás Gorge in western Cyprus, April 2018.  The scale bar is 50 micrometres (= 1/20th of a millimetre). 

There were a number of diatoms present too, the most abundant of which was a chain-forming Ulnaria.  Unfortunately, despite having just co-authored a paper on Ulnaria from Cyprus, I cannot name the species, as I saw no cells in valve view.  I will have to return to this subject once I have prepared a permanent slide from the sample that I brought back.   The chloroplasts in the illustration below are not in a very healthy state because the sample lived in a fridge for almost a week before I was able to get it under a microscope.  Had I looked at this sample 20 years ago, I would have assumed that I was looking at a species of Fragilaria, as most keys then stated categorically that Synedra (the former generic name) were solitary rather than chain-forming.  However, we now know that there are several Ulnaria species that form chains although most that I see in my regular haunts in the UK do not.   Our paper states that the species we describe form short chains although, as we worked from cleaned samples collected by other people, I now wonder if that was an artefact of the preparation process and whether these, too, formed longer chains in their living state.

A chain of Ulnaria from the stream at Avgás Gorge in western Cyprus, April 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

Though it is rare for me to stray into describing the invertebrate life of streams, the Psychomyiidae are actually an important part of the story here, as the larvae graze algae from the surface of the stones.  They create a silk tube and this, in turn, becomes covered with fine sediment to create the galleries that are visible in the picture.   We know that invertebrates can change the composition of the attached algae by grazing but I sometimes wonder if the caseless caddis larvae also change the composition by creating myriad patches of very fine sediment across the rock surface.   If you look closely you can also see a couple of Simuliidae (blackfly) larvae.  These attach themselves to the rock by a circle of hooks at their last abdominal segment (I think that is “bum” in entomological language) and then use fan-like structures around their mouths to filter tiny particles from the water.  However, I have also seen Simuliidae larvae bent double on rock surfaces in order to hoover up particulate matter and algae that live there.

That, I thought, was just about enough natural history for one day.   I put my toothbrush and bottle back into my rucksack and made my way back down until, turning the corner, I stumbled across the Russian glamour models.   I’ve often written about the similarities between freshwater ecosystems in different parts of Europe but you don’t often see a bikini – or less – in April in my cold, damp corner.

Galleries of Psychomyiidae larvae on the top of a limestone cobble from the stream at Avgás Gorge in western Cyprus, April 2018.

Reference

Cantonati, M., Lange-Bertalot, H., Kelly, M.G. & Angeli, N. (2018).  Taxonomic and ecological characterization of two Ulnaria species (Bacillariophyta) from streams in Cyprus.   Phytotaxa 346: 78-92.

Wallace, I. (2003).   The Beginner’s Guide to Caddis (order Trichoptera).  Bulletin of the Amateur Entomologists’ Society 62: 15-26.

Letter from Cyprus

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Travel can be one of the most rewarding forms of introspection ….
Laurence Durrell, Bitter Lemons, 1957

When I stepped off flight EZY1973 from Manchester to Paphos on Saturday night I passed a personal milestone. Arriving in Cyprus means that I have now visited all 28 Member States of the European Union. Starting with (West) Germany in 1972 on an exchange visit before the UK was even a member of the European Economic Community, followed shortly after by a family holiday to southern Austria (where my father had been stationed just after the war) with a day trip to Slovenia (then part of Yugoslavia), the number started to increase in the late 1990s when I became involved in the work of CEN, the European Standards Agency and, from the mid-2000s onwards, with the intercalibration exercise associated with the Water Framework Directive. A few years ago I made a list and realised just how many I had visited, after which, I have to admit, my choice of conference and holiday destinations was driven by this rather childish whim. Latvia, Malta and Bulgaria, all subjects of posts on this blog, were ticked off, leaving just Cyprus. This year, a family holiday to celebrate my mother’s 80th birthday provided the opportunity and, after some shameless lobbying, we had booked a villa near Paphos via AirBnB and were on our way.

How Europe has changed in the 47 years since my first overseas trip. Twelve countries were behind the Iron Curtain, three of the remainder were right-wing dictatorships. Two have merged (East and West Germany) whilst seven have become disentangled from previous relationships (the Baltic States from the USSR, Slovenia and Croatia from the former Yugoslavia and the two former constituents of Czechoslovakia from each other). Cyprus, from where I am writing, was in political chaos in the early 1970s. A former British colony whose territory was argued over by Greece and Turkey, it was soon to be split into two, separated by a buffer zone. I used to browse my Collins World Atlas assuming national borders to be fixed and immutable; the older and wiser me wonders where (and when) the next changes will come from.

The intercalibration exercise, in particular, was an opportunity for an exchange of ideas and I counted co-authors from 23 of the 28 EU states on my publication list. Looking back, these papers show remarkable consistency in some aspects of ecology across Europe whilst, in other respects, I am much more cautious about assuming that knowledge gained in my damp corner of north-west Europe can be applied to warmer and more continental regions. This publication list includes, incidentally, two papers about Cyprus, despite never having either visited before or having a native Cypriot on my list of co-authors. In the first paper, we worked with an Austrian employed by the Ministry of the Environment but, for the second, the samples were collected and analysed by Italians and Germans whilst I helped out with data analysis. Scientific colonialism is not, perhaps, dead?

My favourite? I don’t think I should single one of the 28 out. The food and culture of the warm lands of the Mediterranean basin draw me but I think that the parched summer landscapes would lose their appeal if I was there for too long. I find the grey, damp climate of my own corner of Europe wearisome but the greenness of the Spring and Summer, and the Autumn colours almost compensate. My ideal, in other words, seems like it should be a semi-nomadic existence but that, too, would pale with time. The truth is that, for me, elsewhere, being wanted, is always more wondered at ….

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Natural lenses …

The photograph above is as about as far from Andreas Gursky’s careful constructions, described in the previous post, as it is possible to get.  It is a close-up of a green algal floc Heather noticed whilst on a walk around a local nature reserve.   I guess it fits the general description of “decisive moment” except that it takes a special sort of observer to find any interest at all in such an unprepossessing habitat.

Under the microscope, the floc turned out to be composed of filaments of Spirogyra, with a single helical chloroplast.  Members of this genus (and related genera such as Mougeotia) produce copious mucilage so are always slimy to the touch.  However, this mucilage makes it difficult for the waste gases produced by photosynthesis to diffuse away, leading to the production of bubbles within the mucilage mass.   The interest, today, however, was that these air bubbles are acting as tiny lenses through which it is possible to make out the individual filaments of Spirogyra.

The green floc beside a footpath in Crowtrees local nature reserve from which the other images in this post were derived. 

I should add the caveat here that the photograph was taken with the “super macro” facility of our Olympus TG2 camera but the end-product is, nonetheless, impressive.   It also offers us an insight into the world of the very earliest microscopists.  Anton van Leuwenhoek’s microscopes consisted of a metal plate which held a tiny sphere of glass which acted as a convex-convex lens capable of up to 266x magnification to a resolution of little  more than a micron (1/1000th of a millimetre) (follow this link for more details).  To give an idea of what he might have seen with this, the right hand image below used 400x magnification.

That, however, only tells us part of the story of Anton van Leuwenhoek’s genius.   Whilst we should not underestimate the skill required to make the lenses and their mounts, the other essential element is curiosity.   Curiosity is, itself, multifaceted: in a few weeks we will probably make a trip out to an old quarry where we know we will find several species of orchids, and maybe some excursions to locations new to us but where others have reported interesting assemblages.  That’s one type of curiosity.  However, simply looking harder at the habitats all around us involves a different type of curiosity: a recognition that there is more to know even about things we think we already know about.   The former broadens our experiences, the latter deepens them …

The algal floc at Crowtrees local nature reserve in close-up: left: an extreme macro view of a single bubble from the image at the top of the post and, right: filaments of Spirogyra photographed under the microscope.  Scale bar: 20 micrometres (= 1/50th of a millimetre).