Trizodia

Trizodia produces minute, whitish, sessile to short-stalked apothecia (tiny spore-producing fruit bodies) that are hemispherical to slightly top-shaped, smooth, and without a distinct rim. For the type species T. acrobia, the apothecia are translucent white to faintly pink when fresh, turning yellowish-brown on drying and up to about 0.24 mm across and 0.16 mm high (barely a quarter of a millimetre). The outer wall is made of long, parallel, colourless hyphae (an ectal excipulum of textura porrecta). The spore-bearing layer contains club-shaped asci (spore sacs) and slender paraphyses (sterile filaments) that do not overtop the asci. In T. acrobia, the asci stain blue in iodine after KOH pretreatment (a hemiamyloid reaction).^[2] In T. silvestris, by contrast, the asci are fissitunicate and apically thickened, with walls negative or only weakly blue in iodine even after KOH pretreatment, indicating a different staining behaviour.^[5] The spores are one-celled, colourless and smooth, and often contain one or more oil droplets (guttules). In T. acrobia they measure about 12.0–20.5 × 7.5–12.0 μm, while T. silvestris has ascospores roughly (9.7–)11–15(–17) × (6.0–)6.7–9.4(–14) μm.^[2][5]

The symbiotic organisation differs between the known species but centres on close association with Nostoc-type cyanobacteria.^[2][5] In T. acrobia (on living Sphagnum shoot apices), the fungal hyphae run over the moss surface and into the empty, water-holding hyaline cells via pores, where they envelop cyanobacterial colonies; the fungus was found wherever Sphagnum–Nostoc associations occurred and absent where cyanobacteria were lacking, consistent with obligate use of the cyanobiont. The association does not appear harmful; the authors suggested the hyphae may help position the colonies and aid passive water transport—functions also seen in lichen thalli.^[2]

Trizodia silvestris forms a true thallus—minute, granular cushions composed of globose to subglobose granules with a pseudoparenchymatous surface—within which the photobiont forms strands of Nostoc cells. Hyphae sometimes extend out of the thallus and may produce haustoria (feeding pegs) that intrude into the cyanobacterial cells. The apothecia of T. silvestris are abundant, white to whitish-orange when wet and yellowish-brown when dry, with the same basic wall construction (ectal excipulum of parallel hyphae); its paraphyses are apically swollen, and asci are clavate and fissitunicate. The cyanobiont of T. silvestris was identified by 16S rRNA data as Nostoc sensu stricto, supporting that the genus includes species with a lichen-like thallus built around this cyanobacterium.^[5]

Trizodia acrobia occurs only at the living tips of Sphagnum shoots; despite screening thousands of shoots it was absent from middle or dead parts. It occurs in boreal bogs and in coniferous forest sites where Sphagnum grows, and has been recorded on eight host species. On the moss, the fungal hyphae run over the leaf surface and into the water-holding hyaline cells to envelop Nostoc colonies. In 44 Sphagnum–Nostoc patches examined the fungus was always present, and it was not found where cyanobacteria were absent, implying a tight dependence on the cyanobiont. The holotype was collected in Finland (Kaijanneva bog, Haapavesi) on Sphagnum magellanicum. The protologue treated the distribution as Finnish and suggested it would also be found with Sphagnum in neighbouring regions.^[2] A later record from the Thuringian Forest (Germany) documents T. acrobia on the shoot tips of Sphagnum girgensohnii and S. russowii, representing the first report for Germany and extending the known range beyond Fennoscandia.^[7]

By contrast, T. silvestris was reported from humid temperate forests in Kyoto Prefecture, Japan, forming thalli and apothecia on moist peeled logs and on bryophyte mats (including the liverwort Dumortiera hirsuta). The species was typified from Sakyō-ku, Kyoto and named silvestris to reflect its forest setting, in contrast to the wetland-restricted habitat of T. acrobia. Sequencing showed that specimens from log and liverwort had virtually identical mtSSU sequences, indicating low substrate specificity within these forest microhabitats.^[5] As currently documented, the genus is known from peatland Sphagnum habitats in Finland and from humid temperate forests in Japan.^[2][5]