Sunday, 10 December 2017

Little Stars

Continuing on from last night's blog post - it seemed that the tiny drop of water I'd collected from the edge of my local Sphagnum bog was absolutely hotching with microscopic plant life. Some of these don't exactly grab my attention by the jugular, but others do - especially the desmids! One family was very well represented in my bog water sample - Micrasterias. I've never even clapped eyes on one of these until midnight last night, but I'm really quite taken with them now. The mighty Freshwater Algal Flora of the British Isles introduces the family as follows -

Cells strongly compressed so that almost always observed in face view, approximately circular in outline and multilobed appearing like small, dissected leaves or little stars as the generic name indicates. 

If that wasn't cute enough in itself, the author gushes on with this rather unscientific prose - 

Micrasterias includes the most striking of all desmids, if not all plant and animal cells.

Wow, that's saying a lot! I can see why Alan Brook, the author of this section of the flora, is quite so taken with the genus. They're marvellous little things. Enough waffle, have some pics. I'll explain each species' diagnostic features as we go along.

Micrasterias rotata
So what you're looking at is one alga divided into two broadly symmetrical halves called subcells. Each subcell is itself divided into lobes which radiate outwards from the centre point, the middle (polar) lobe and two lateral lobes. Each of the lateral lobes is further divided halfway towards the outer edge, and again three quarters to the outer edge. Finally, these small divisions are also divided just before the outer margin. 

In order to determine which of the 18 British species you're looking at you need to run a set of characters through the key. Mostly this concerns the shape of the polar lobe's margin, how closely divided the lateral lobes are, extent of divergence of the lobes from each other, whether or not there are teeth present on the margins, spines or granules on the cell and the overall general outline shape.

The species in the image above is the commonest one in my sample. The key feature to look at is the polar lobe - see how it projects beyond the lateral lobes? Add to this the retuse shape of the polar lobe - it has a distinct notch on the margin - plus a few further details (such as the pronounced bidentate angles, deep constriction of the isthmus, linear sinus, narrow incisions between the lateral lobes), and that tells us we're looking at Micrasterias rotata. Sweet! 

Polar lobe extends beyond lateral lobes and has a retuse (notched) tip = Micrasterias rotata
The relevant text in the Flora says Subcosmopolitan, widespread in the British Isles in acid habitats, especially amongst Sphagnum and in the weedy margins of nutrient-poor lakes.

Next up, and the second commonest of the Micrasterias in my sample, is this chappie

Micrasterias denticulata
Note that, unlike M.rotata, the polar lobe does not project beyond the terminal lobes in this species. It also lacks the sharp angles to the lobe tip, hence exhibits a smoothly rounded rather than toothed appearance. The whole body of the cell is rounded rather than ovoid and the margins are smooth rather than toothed. This is Micrasterias denticulata and the flora has the following to say

Cosmopolitan, one of the most common and widespread of the British species. It can sometimes be obtained in almost pure gatherings especially amongst Sphagnum in small bogs, ditches or boggy springs. 

There is a very similar, closely related species which is also common and widespread and needs to be ruled out. The whitish circle in the centre of the alga is the nucleus. The undivided inner part of the alga surrounding this nucleus is known as the isthmus. Note it appears pretty much devoid of features. Also check the length of the polar lobe. It starts roughly halfway towards the outer margin. Agree with those two features? (I hope so!) Ok, so the lookalike species is M.thomasiana, which is also common and widespread in Britain. But...M.thomasiana shows three elaborate protuberances at the base of each semicell and isthmus (ie around the nucleus). There is a sneaky "variety" though which has reduced protuberances, so could easily be mistaken for M.denticulata. However, the polar lobe of M.thomasiana (both vars) is much longer than that of M.denticulata, starting well before halfway to the outer edge. 

At this juncture you may be thinking, "gee, this guy really knows his stuff!" Well guess what, looking at a couple more of my "denticulata" images, I think I may have thomasiana too. I've literally just seen it, if it is. I wasn't expecting that. I'm going to have to go back to the microscope and play around with the focus wheel, twisting it back and forth changes the depth of field. My 2-D images aren't good enough, I need a 3-D look at them. Here's one of the 'problem' specimens - check out the length of that polar lobe, well over halfway to the isthmus isn't it?
Could this be M.thomasiana var.notata with the reduced protuberances????

I'm wondering of the pair of darker green patches to either side of the nucleus are the protuberances. It would probably help if I had previous experience of these things. Or a grown-up to help me out. I need to pay attention to the minute detail of the polar lobe margins, there should be a couple of tiny, tiny teeth (or sometimes just one tooth) on the outside edge of the notch. Plus the lobe tips are meant to be different, though I suspect it's a sliding-scale interpretation difference, not a clear cut thing.

So, er...stay tuned folks. More madness and mayhem coming your way soon!

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