Eight spores good - a mossy mystery

Looking through moss and fragments of moist dead wood recently, I've found a range of small invertebrates such as mites, and spurred on to see what else dwells within, I've clocked up some more microscope time. Scanning a specimen of the creeping feather-moss Amblystegium serpens, a common species known from various habitats/substrates including both living and dead wood, I noticed some tiny red-brown structures underneath some of the leaves. Having a look through Atherton et al. (2010) and Watson (1981), no moss structures looked quite like it, so I started photographing and magnifying...

Structures growing from the stem of Amblystegium serpens

There's not a lot of detail here, but an idea of scale can be gained - the width of the stem (no more than about 0.2mm - the leaves are also tiny, about 0.5mm long) is clear where it reaches the left-hand side of the photo and the more-or-less rectangular (but not especially elongate) individual cells can be seen there. Just to the left of the pin, the stem is a little different however - the cells are orange-brown and a little larger. This might be of interest as many gall-causing Fungi induce changes in cell size and tissue colour. So, to see more detail in the brown masses, I made a simple 'squash' preparation for higher magnification.

Fibrous structure growing from the stem beneath a leaf of Amblystegium serpens

The fibrous structure is clearer here, including its point of attachment/outgrowth from just below the base of a leaf. The longest fibres are maybe 0.5mm long, maybe a little more, but it is still unclear exactly what they are. Mosses produce a number of structures worth considering here:

• Paraphyses - thin sterile hairs, sometimes club-shaped, usually multicellular. A possibility.
• Protonema - the young stage of a moss that develops when a spore germinates; generally appears as a system of green threads. Clearly not the case here.
• Gemma - a unit of vegetative propagation, may be single-celled, two-celled or multicellular. Another possibility.

To determine whether these were parts of the moss and/or a fungus growing on it, more detail was needed.

Various structures seen attached to Amblystegium serpens

A fungal spore found with a sample of Amblystegium serpens

In the first of this pair of photos, a number of structures are visible. Those indicated by blue arrows are unidentified - the larger fragment could be a paraphysis or similar (or an equivalent fungal paraphysis) while the 4-celled structure could be gemmal or a 4-celled fungal ascus (spore-bearing 'sac') - this is largely guesswork however, partly informed by checking fungal structure using Webster (1970). The structure indicated by a red arrow is rather clearer and appears to be the ascus (spore-bearing 'sac') of a discomycete fungus, complete with the typical eight spores seen when fully developed, though the left-most one is blurred out in the photo. Other similar structures could be seen, including the spore in the lower photo - it is almost spherical and measures around 13 x 15 um, and although again blurred out here, has a surface covered in tiny 'warts' (i.e. it is 'verruculose'). Consulting Ellis & Ellis (1998), there appears to be only one contender with this host and set of characteristics - the microfungus Octospora wrightii which is associated primarily with this moss, and is found from January to March.


As ever, comments, suggestions and corrections welcome - I am, as is so often the case, writing outside my comfort zone here; how else to learn though?


References

Atherton, I., Bosnaquet, S. & Lawley, M. (eds.) (2010). Mosses and Liverworts of Britain and Ireland: A Field Guide. British Bryological Society. [If you only buy one UK bryology book, I can recommend making it this one]
Ellis, M.B. & Ellis, J.P. (1998). Microfungi on Miscellaneous Substrates: An Identification Handbook (2nd ed.). Richmond, Slough.
Watson, E.V. (1981). British Mosses and Liverworts (3rd ed.). Cambridge University Press.
Webster, J. (1970). Introduction to Fungi. Cambridge University Press.