Little round green things …

Apatococcus_on_Fence

Nature does not get much more prosaic than this: my garden fence covered with a fine, powdery green coating. For most of the year this is hidden behind the foliage of our apple and willow trees but as autumn gives way to winter, so the bare green slats became visible again.  Last week, staring out of the window whilst completing my previous post, it occurred to me that, in the six years that I have been writing this blog, I have never made the short journey across the grass to look at one of the most common algae in the country.

I scraped a single-sided razor blade across the surface of one of the slats to harvest a small quantity of the damp, powdery film, and put a few specks under my microscope in order to take a closer look.   What this revealed was lots of clumps of small near-spherical green cells.   That, along with an ability to live in terrestrial habitats are about all the natural historian has to go on when trying to name this organism.  My old copy of West and Fritsch suggests Pleurococcus naegelii, adding that “there is probably no other alga about which there has been so much confusion” whilst the latest guide to British algae would call it either Apatococcus lobatus or Desmococcus olivaceum– their descriptions are very similar.   Desmococcus olivaceum has been described as “the commonest green alga in the world”, which is a bold claim.  Certainly, green powdery coatings such as these are found in shaded locations in a great many places but is the singular “alga” really appropriate?  Cells such as these offer so few visual clues that the microscopist is apt to latch onto a phrase such as “one of the commonest terrestrial algae” alongside a description that roughly matches the material, and considers it to be job done.  These groups have also been referred to as “LRGT” (“little round green things”) – the phycological equivalent of the ornithologist’s “little brown jobs”.   Recent molecular studies suggest that there is a lot of diversity within these powdery films, and this is almost certainly going to be very difficult to resolve with traditional methods.  It looks as if we going to struggle with these “LRGT” for the foreseeable future.

The Class Trebouxiphyceae seems to have a particularly large number of LRGT.   Some (such as the one I am describing in this post) are free-living and capable surviving desiccation, but this group also includes many of the algae that unite with fungi to form lichens, whilst others prefer to live in truly aquatic situations.   But it is the fence-dwelling forms that are of interest to me today, and even if I cannot put the exact name onto my powdery film, I can perhaps offer some thoughts on why it thrives where it does.

Apatococcus_lobatum_181218

Cells of Apatococcus lobatum(?) from a garden fence in County Durham.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   The photograph at the top of the post shows the fence in my back garden from which it was collected.

Some of the other terrestrial (or semi-terrestrial) green algae that I’ve described in this blog are endowed with brightly coloured pigments that protect them from the damaging ultra violet rays in sunlight (see “Fake tans in the Yorkshire Dales” and “An encounter with a green alga that is red”).  Apatococcusand Desmococcus, by contrast, do not come with preloaded sunscreens.  They thrive, by contrast, in relatively shaded locations where the gradual accumulation of cells on the fence surface means that the outer cells take the primary ultra violet hit and, in the process, protect those cells underneath.   There is also evidence of Apatococcus producing lots of stress compounds.  These belong to a class of compounds call “polyols” – complicated alcohols.  A lifestyle that involves single cells sitting on a damp fence indefinitely might seem like an evolutionary dead end.   However,  they have the last laugh as, unlike us, they are genetically adapted to produce their own booze when the going gets tough.

A further adaptation that has been observed is that the cells can switch between producing their own simple sugars via photosynthesis, and absorbing sugars and other organic compounds directly, a strategy known as “mixotrophy”.   Walls and fences are challenging habitats for any organism so having the ability to mop up any spare fuel (leaking down from one of those outermost cells that took one for the team, perhaps?) might give the organism a slight competitive advantage over time.

Back when I was doing my Fine Art degree, I was using algae to explore the boundaries between abstraction and representational art.  My thesis was that an image of an alga could be either representational or abstract depending on how much prior knowledge the viewer brought to the image.   I used the Apatococcus (or is it Desmococcus) from my garden fence as subject matter for this exploration, creating a sextych (honestly, that’s the word for a painting on six panels) that juxtaposed the minimal outline of fence panels with microscopic views of the alga.  The three fence panels offer the unprepossessing view that most people will walk past for their entire life without a thought, whilst the microscopic views give an insight into the hidden world even though the arrangement of shapes and colours will not match any of the schemata lodged in the memories and experiences of most of the viewers (see “Abstracting from reality …”).

Apatococcus

Apatococcus. 2008 50 x 130 cm.  Acrylic and photomicrograph on canvas.

References

Gustavs, L., Schumann, R., Karstens, U. & Lorenz, M. (2016).  Mixotrophy in the terrestrial green alga Apatococcus lobatus(Trebouxiphyceae, Chlorophyta).  Journal of Phycology52: 311-314.

Laundon, J.R. (1985). Desmococcus olivaceus– the name of the common subaerial green alga.   Taxon 34: 671-672.

Lemieux, C., Otis, C. & Turmel, M. (2014).  Chloroplast phylogenomic analysis resolves deep-level relationships within the green algal class Trebouxiphyceae.  BMC Evolutionary Biology14: 211.

 

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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

 

The River Ehen in November

There is a gravel bank on the inside of the meander at one of our sites on the River Ehen and we get a pretty good idea of how high or low the river is as soon as we see how much of this bank is exposed when we arrive.   When the river is low, there is always a zone just above the water line where many of the stones are covered with a bright green film of algae.

Under the microscope, this turns out to be composed of narrow unbranched filaments of an alga called Klebsormidium flaccidum, a member of a small genus of green algae that are often found in habitats that are not fully submerged or only periodically damp.   A single chloroplast encircles half or three-quarters of the circumference of each cell.

Ehen_Klebsormidium_flaccidu

A stone (about 20 cm across) covered with Klebsormidium flaccidum on a gravel bank beside the River Ehen, November 2013

As is often the case with filamentous green algae, naming the genus to which it belongs is relatively straightforward but deciding the name of the species is much more difficult.   The problem is that there are so few diagnostic characters available to differentiate one species from another and that the algae are naturally variable (think about how different a houseplant looks when grown in the shade to on a well-lit windowsill).   Fabio Rindi, who wrote the key in The Freshwater Algal Flora of the British Isles, suggested that his key was provisional and that we should really grow the species in culture and extract its DNA if we want to be sure of it’s identity.

This all creates a problem for ecologists who want to use algae to infer properties from the environment as much of the literature about these species is scattered liberally with errors made in good faith by earlier generations  of biologists.   We have to pick our way through their conclusions with great care and, regrettably, often find that generalisations based on the properties of the genus are the best we can do.

Klebsormidium_flaccidum_sta

A filament of Klebsormidium flaccidum from the River Ehen, November 2013.  The scale bar is 10 micrometres (= 1/100th of a millimetre).

Reference

Rindi, F., Guiry, M.D. & López-Rautista, J.M. (2008).  Distribution, morphology and phylogeny of Klebsormidium (Klebsormidiales, Charophyceae) in urban environments in Europe.  European Journal of Phycology 44: 1529-1540.