Hyperepiphytes in the Shetland Islands

Gossa_Water_May19

I was lucky enough to spend a couple of days in the Shetland Islands during last week’s spell of warm weather and spent one of my mornings there hiking in shirtsleeves across moorland to a remote loch.   Good infrastructure is a legacy of the Shetland Islands’ association with the oil industry, and this includes a strong mobile network, meaning that I managed to find this particular loch using the Ordnance Survey maps on my smartphone. I would not normally rely upon a mobile signal to navigate across such remote terrain but in Shetland it is often possible.  I would, nonetheless, recommend keeping a paper map and a GPS in your kit just in case, as I did lose the signal on a few occasions during my stay.

Most of the lochs in the northern part of mainland Shetland are shallow, peaty water bodies, with soft water and relatively sparse assemblages of aquatic plants.   Parts of the littoral zone of this particular loch, however, had extensive growths of submerged mosses.  It is a long time since I was proficient at identifying aquatic mosses but these clumps look likeWarnstofia fluitans to me, though I am willing to be proved wrong.  I did try to remove some leaves and have a proper look but that task was complicated by tufts of attached filamentous algae.   In their submerged state, these formed distinct clusters at intervals along the straggly stems of the moss but, once removed, the filaments collapsed to smother the leaves and confound my attempts to run a scalpel blade along the stem.

Warnstofia_Gossa_Water_May19

Submerged colonies ofWarnstorfia fluitans(?) smothered byOedogoniumfilaments in Gossa Water, north Mainland, Shetland (HU 4354 6047). Gossa Water (one of five that share this name in the Shetland Islands!) is illustrated in the photograph at the top of this post.

The filamentous alga proved easier to unmask: the unbranched filaments, reticulate (net-like) chloroplasts and distinctive ‘cap cells’ all identifying it as the green alga Oedogonium.  As is often the case, however, the populations lacked any sexual organs so it was impossible to know which species (see “The perplexing case of the celibate alga“ and, for a rare case of a sexually-mature filament, “Love and sex in a tufa-forming stream”).   Abundant epiphytes can be another feature of Oedogonium: unlike several other filamentous green algae it produces little mucilage which makes it easier for diatoms, in particular, to colonise.  As well as colonies of needle-shaped cells of Fragilaria gracilis there were also several Achnanthidium cells and, entangled around the filaments and the moss, chains of Tabellaria flocculosa.   Given that the Oedogonium was, itself, an epiphyte, these diatoms are ‘hyperepiphytes’, a term that attracts remarkably few Google hits, almost all associated with lower plants.

The ‘cap cells’ are one of the most distinctive features of Oedogonium and results from a distinctive mode of cell division that leaves rings of scar tissue at the point where the two cells split.   That we see four or more of these scars on a few cells whilst the great majority have none suggests that we are looking at a primitive form of specialisation, with a few cells in a filament being responsible for all the cell division.  What is more, these cap cells are also often the ones that form oogonia (see “Love and sex in a tufa-forming stream” for an illustration of this) and asexual zoospores, so there must be something slightly different in the biochemistry within these cells that drives these processes.   However, at this point the formal scientific literature goes strangely silent apart from a single paper published in 1962.  Curiously, the evolution of multicellularity is one of those big questions that attract a lot of top academics (see the reference to a recent paper in Nature Scientific Reports below)  whilst a genus of algae that seem to show some faltering first steps towards specialisation of some cells are largely ignored.  Another case of the “trailing edge” of science?

Gossa_Oedogonium

Oedogonium filaments growing on Warnstofia fluitans in the littoral zone of Gossa Water, north Mainland, Shetland, May 2019.   The arrow on the top image shows the “cap cells”.   Note also the cluster of Fragilaria gracilis(plus a few cells of Achnanthidium) on the lowermost filament and, in the middle image, two of the many cells of Tabellaria flocculosa that were entangled with the Oedogonium filaments and moss stems.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 

Oedogonium_zoospores

A zoospore being released from a filament of Oedogonium.  This series of photographs was taken by me in about 1993 and I have no details of the location from which it came.  The filament is about 40 micrometres (= 1/25thof a millimetre) in diameter.

Reference

Herron, M.D., Borin, J.M., Boswell, J.C., Walker, J., Chen, I-C. K., Knows, C.A., Boyd, M., Rosenzweig, F. & Ratcliff, W.C. (2019).  De novo origins of multicellularity in response to predation.  Nature Scientific Reports 9, Article number: 2328

Rawitscher-Kunkel, E. & Machlis, L. (1962).  The hormonal integration of sexual reproduction in Oedogonium.   American Journal of Botany 49: 177-183.

St_Ninians_tombola_Shetland_May19

Sightseeing in Shetland: the tombolo (sandy isthmus) linking St Ninian’s Isle with Mainland in the Shetland Islands, May 2019.

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Nitzschia and a friend …

I drove from Bogle Bridge, my sample site on Stockerley Burn, up a low hill and down the other side into the next valley where another tributary stream of the River Browney flowed. I scrambled down to the stream and hunted amongst the stones for any algae. There were fewer green filaments here but I did find a small area of a stone covered with a dark brown mat that intrigued me.   My initial suspicion was that it was a mat of the filamentous blue-green alga Phormidium, but it turned out to be composed almost entirely of Nitzschia sigmoidea, a diatom that we last encountered in the brackish waters around Jarrow (see “All things bright and beautiful?”).

Nitzschia_sigmoidea

Nitzschia sigmoidea from Smallhope Burn, Knitsley Bridge, August 2014.   The upper image shows a single cell, the lower image two cells shortly after division.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

If you look back at the post on Nitzschia sigmoidea at Jarrow, you’ll see that it was smothered in epiphytes.   Some of the cells here at Smallhope Burn also had attached epiphytes, though this time it belonged to a different genus.

I have called this epiphyte “Synedrella subconstricta”, though it has had several different names during the course of my career.   When I started, it was Synedra parasitica var. subconstricta but it has also been moved to the genus Fragilaria and, from there, to Pseudostaurosira.   Some have regarded it as a variety, others as a distinct species.   What is generally agreed is that it is usually found attached to Nitzschia sigmoidea, and occasionally to other large motile diatoms.   Frank Round regarded this as a major reason for creating a new genus, Synedrella, for this and a related taxon, S. parasitica.   Don’t let this species name misguide you: there is no evidence that it is a parasite; it is more likely to be a commensal relationship of some kind.   I am less convinced by more recent efforts to move it to the genus Pseudostaurosira, which generally forms long chains rather than sharing the habit of single cells with a short but flexible stalk.   This move reflects the tendency of my fellow diatomists to spend too long staring at the empty cells of diatoms and not enough time watching them in their live state. I have hunted around to see if there is any molecular evidence to support any of these genus names but, so far, have not had any luck.   How, I wonder, would we get the pure culture that we need as our source of DNA when the diatom seems to be an obligate epiphyte?

And, one more question that puzzles me: what advantage does a sigmoid outline confer onto a diatom?   You’ll see from the second of my two images that there is also a cell of another sigmoid genus, Gyrosigma, in the same habit. This is a relative of Pleurosigma, which we at Whitburn last year (“Microscopic monsters in mud”) and there are also sigmoid species in the genus Stenopterobia.   The sigmoid habit must have evolved several times though it is interesting that all of these are motile genera and two (Nitzschia and Stenopterobia) also include straight species.  Any ideas?

N_sigmoidea_S_parasitica

Nitzschia sigmoidea with an epiphytic cell of Synedrella subconstricta and, just above them, a cell of Gyrosigma (probably G. acuminatum).   Scale bar: 10 micrometres (1/100th of a millimetre).

References

Medlin, L., Jung, I., Bahulikar, R., Mendgen, K., Kroth, P., Kooistra, W.H.C.F. (2008). Evolution of the diatoms. VI. Assessment of the new genera in the araphids using molecular data.   Nova Hedwigia Beiheft 133: 81-100.

Medlin, L., Yang, I. & Sato, S. (2012). Evolution of the diatoms. VII. Four gene phylogeny assesses the validity of selected araphid genera.   Nova Hedwigia Beiheft 141: 505-514.

Round, F.E. & Maidana, N.I. (2001). Two problematic freshwater araphid taxa re-classified in new genera. Diatom Research 17: 21-28.