Fossil fungus-related spheres with alga connection
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surreal pictureTwo preceding contributions on fungus-related spheres in one sample of Rhynie chert have provided surprising observations. (See Rhynie Chert News 116, 119.) In the Rhynie chert in general, spheres and globules of any size between about 10Ám and 1mm are 
usually related to fungi. They may be spores or fungus organs, called vesicles or chlamydospores in case of uncertain purpose, or sporangia, zoosporangia, oogonia, and the bigger ones may be alga cells, enormously bloated under the influence of some chytrid fungus [1]. In view of the different potential explanations, the interpretation of images like these ones becomes quite interesting.

Fig.1: Cut plane of a small sample of Rhynie chert with spheres up to 0.62mm, some of them arranged in a peculiar way apparently related to a charophyte alga. Picture width 4.3mm. All pictures: same scale, same sample as in Rhynie Chert News 116.

Let us, for short, refer to the arrangement on the right of Fig.1 as "aliens", one of them wielding a "sabre". The yellow sabre seems to be a hollow charophyte branch filled with reflecting quartz crystals. The lower part of the left "alien" resembles an old charophyte whorl, branches missing. For lack of another explanation, one may guess that head and belly of the alien are bloated branch parts. This might apply to the other alien, too: another charophyte branch transformed into a still more strange chain of globules.
This guess is doubtful in view of the fact that there are no upper branch parts emerging from the spheres, and that none of the other spheres in this picture show any connection to an alga. This applies also to the spheres, single or in pairs, in the preceding contributions.
The assumption of a causal connection between these spheres and a charophyte alga is supported by early work on the Rhynie Chert [2]: Spheres with sizes as seen in the present sample had been found among branches of an alga named there Palaeonitella. Moreover, some of the spheres were even found attached to the alga or to another sphere, thus forming a pair. However, the authors [2] were not right with their tentative interpretation of the spheres as "bulbils serving for vegetative reproduction".
PalaeonitellaThe origin of spheres and globules of this type has convincingly been explained by Taylor et al. [3] as alga cells bloated under the influence of either of two fungi, Milleromyces and Krispiromyces, with characteristic features shown in great detail [1,3,4,5]. A beautiful and instructive specimen of Palaeonitella with two bloated internodal cells is shown in every one of these publications, which might mislead to the assumption that this were the usual arrangement of the globules. Hence the numerous spheres shown in Rhynie Chert News 116, 119 may serve as a useful reminder of the fact that spheres or globules with sizes of about 0.3mm to 0.6mm are nearly always seen not connected to an alga or its parts. Nevertheless it is uncertain whether or not most spheres within this size range had originally been alga cells. An interpretation as fungus-affected alga cells raises the problem of how they got the decay-resistant wall distinctly seen here while the non-affected alga parts had mostly vanished before silicification.

Fig.2: Rare sight of several generations of Palaeonitella, silicified in different ways: clear or filled with quartz grains, white or stained with minerals; fungus-related spheres, 0.3mm and 0.6mm, apparently not attached to the alga; swamp gas bubble trapped among filamentous cyanobacteria, later filled with water and silica by diffusion, then turned into white chalcedony. Image width 3.2mm.

Plain evidence for the presence of Palaeonitella in this chert sample with abundant spheres is provided by Fig.2. It shows an uncommon combination of several successive generations of this alga and two spheres preserved by silicification. Perhaps the brown specimen at the bottom and the big three-pronged hollow fork with yellow fill in the background came next. Two narrow tubes in front of the fork are incidentally aligned along the cut plane and thus seen despite of poor contrast. The three-pronged fork in the background is probably hollow, with glittering quartz crystals at the inner wall.
Doubts concerning a close causal relation between spheres and algae are raised by Fig.3, with a sphere inside Rhynia. It is hard to imagine Palaeonitella penetrating into a dead plant and growing a big sphere there.
sphere in Rhynia
Fig.3 (right): Fungus-related spheres, 0.5mm and 0.4mm, inside and outside Rhynia, apparently not attached to alga parts. Image width 2.5mm.

Despite of apparently contradictory evidence it may be concluded that probably most of the big spheres or globules in the Rhynie chert are charophyte alga cells hypertrophied and made persistent under the influence of chytrid fungi in obscure ways.

H.-J. Weiss    2018

[1]  T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi. Elsevier 2015, p.64.
[2]  R. Kidston, W.H. Lang: On Old Red Sandstone plants … Part V,    Trans. Roy. Soc. Edinburgh 52 (1921), 855-902.
[3] T.N. Taylor, H. Hass, W. Remy: Devonian Fungi: Interactions with ... Palaeonitella.  Mycologia 84 (1992), 901-910.
[4]  T.N. Taylor, J.M. Osborn: The importance of Fungi in shaping the paleoecosystem. Rev. Pal. Pal. 90 (1996), 249-262.
[5]  T.N. Taylor, E.L. Taylor: The Rhynie chert ecosystem: a model for understanding fungal interactions,
       in: Microbial Endophytes, eds.:  Ch.W. Bacon, J.F. White Jr., Marcel Dekker Inc., New York 2000.
For correction of contradictory data in [1,3,5] see here or Google: errors palaeobotany.

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