Thalassodendromyces purpureus Vohník & Réblová, 2025

Réblová, Martina, Nekvindová, Jana, Hynar, Ondřej & Vohník, Martin, 2025, From seagrass roots to saline soils: discovery of two new genera in Lulworthiales (Sordariomycetes) from osmotically stressed habitats, IMA Fungus 16, pp. e 157688-e 157688 : e157688-

Thalassodendromyces purpureus Vohník & Réblová sp. nov.

Figs 6 , 7

Etymology.

From Latin purpureus (purple, violet, dark red). Referring to the colour of the pigment produced in culture.

Type.

MAURITIUS • northeast coast of Mauritius near Plage des Vignots ; 20°06.98'S, 57°45.17'E; at a depth of 0.5–1.5 m; isolated from surface-sterilised roots of the seagrass Thalassodendron ciliatum ; 7 Dec 2023; leg. O. Hynar, isol. M. Vohník MAU- 7 (holotype PRA -22363 dried culture on PCASW, ex-type culture CBS 153577 = CCF 6837 , paratype PRA -22364 dried culture on MLA). GenBank: ITS = PV 441107 , LSU = PV 441121 , SSU = PV 441114 .

Culture characteristics.

On MLA colonies 11–15 mm diam. in 8 wk, circular, convex, margin ranging from entire to rhizoidal. The texture varied from mucoid to waxy, cobwebby at the centre and on the inoculation block, surface initially rugose, becoming cerebriform, and gradually developing deep radial furrows and cracks. Colonies were cream-coloured to yellow-beige, later exhibiting a deep purple pigment diffusing into the agar at the centre and margin, as well as forming irregular spots, often distinctly zonate. Over time, the colonies become purple to purple-brown, with submerged purple hyphae at the margins, reverse dark ochre with creamy or purple margin. On PCA colonies 17–20 mm diam. in 8 wk, circular to slightly irregular, flat, margin rhizoidal, cobwebby to mucoid, chestnut, ochre at the margin, with a conspicuous submerged growth, reverse of the same colour. On PCASW colonies 70–98 mm diam. in 2 wk. Colonies were initially composed of submerged mycelium, formed by sparse white vegetative hyphae visible only in translucent light (1 wk); later, hyphae became denser and subhyaline (2 wk) and gradually darkly pigmented (3 wk). In 8 wk, colonies were circular, flat, margin rhizoidal, dark brown, whitish grey at the centre, dark grey to almost black towards the margin; the texture ranged from cottony to sparsely floccose at the centre, cobwebby towards the margins, occasionally exhibiting zonation with alternating zones of sparse and dense growth, conspicuous submerged growth, with numerous colourless exudates on aerial hyphae, reverse none. Sporulation absent on MLA, PCA, and PCASW, except for the presence of monilioid hyphae (see description).

Description in culture.

Asexual morph. Conidiophores, conidiogenous cells, and conidia are absent. On PCASW, mycelium composed of hyphae 2–4 μm wide, cylindrical, septate, branched, hyaline at first, becoming subhyaline to brown; hyphae often slightly swollen, 5–7.5 μm wide, or monilioid. Monilioid hyphae develop either terminally or laterally, solitary or in clusters, which appear on the surface of colonies or immersed in agar. They are composed of thick-walled cells of different shapes: subglobose to globose 15–38 × 14–35 μm (mean ± SD = 23.6 ± 5.2 × 22 ± 4.6 μm), or ellipsoidal 15.5–38 (– 47) × 9–18 μm (mean ± SD = 26.2 ± 7.5 × 12.9 ± 3 μm), with walls 0.5–1 μm thick. The monilioid hyphae elongate on the terminal end; cells mature gradually, starting as hyaline and becoming dark brown to dark olivaceous brown with age, collapsing in old cultures (ca. 3 mo old). Sterile, globose structures developed at margin of the colonies; they are composed of dark brown, pseudoparenchymatous cells surrounded by sterile brown hyphae. On MLA, mycelium composed of hyphae 2–3 μm wide, cylindrical, septate, branched, hyaline, becoming dark brown in old cultures (ca. 4–6 mo old); hyphae often swollen, 3.5–5 μm wide, or monilioid. Monilioid hyphae occurring both on the surface of the colony and immersed in the agar. They are composed of subglobose to globose cells 11–17 × 9.5–15.5 μm (mean ± SD = 14.6 ± 2.0 × 12.2 ± 1.9 μm) and somewhat ellipsoidal cells 12.5–18 × 7.5–10 μm (mean ± SD = 15.2 ± 1.9 × 8.8 ± 0.9 μm), with walls 1–1.5 μm thick, hyaline. In old cultures (ca. 8 mo old), the cells in monilioid hyphae become dark brown. Sexual morph. Not observed.

Additional specimens examined.

MAURITIUS • northeast coast of Mauritius near Plage des Vignots ; 20°06.98'S, 57°45.17'E; at a depth of 0.5–1.5 m; isolated from surface-sterilised roots of the seagrass Thalassodendron ciliatum ; 7 Dec 2023; leg. O. Hynar, isol. M. Vohník MAU-4 (paratype PRA -22365 dried culture, culture CBS 153576 = CCF 6836 ) ; • Ibid.; MAU-5 (paratype CBS H-25755 , PRA -22509 dried culture, culture CBS 153646 ) ; • Ibid.; MAU-9 (paratype PRA -22367 dried culture, culture CBS 153578 = CCF 6838 ) ; • Ibid.; MAU-13 (paratype CBS H-25756 dried culture, culture CBS 153623 ) ; • Ibid.; MAU-16 (paratype CBS H-25757 , PRA -22508 dried culture, culture CBS 153624 ) ; • Ibid.; MAU-19 (paratype CBS H-25758 , PRA -22510 dried culture, culture CBS 153625 ) ; • Ibid.; MAU-17 (paratype PRA -22498 dried culture) ; • Ibid.; MAU-8 (paratype PRA -22366 dried culture) ; • Ibid.; MAU-1 (culture no longer viable) . All dried cultures are on PCASW.

Habitat and geographical distribution.

The species is known as a root mycobiont of the seagrass Thalassodendron ciliatum in Mauritius. No samples with identical sequences have been recorded in the GlobalFungi database.

Notes.

Clusters of dark monilioid hyphae composed of thick-walled, brown cells were only observed on PCASW (Fig. 7 A – G ). The dark, spherical aggregations of hyphae forming compact structures embedded within the agar medium remained sterile and may represent early stages of ascomatal formation (Fig. 7 I ). While monilioid hyphae were also present on MLA (Fig. 7 H ) and PCA, they mostly appeared as composed of irregularly swollen cells that remained hyaline, becoming pigmented in old cultures. Such cells never formed clusters and were reduced in both shape and size compared to those formed on PCASW.

On PCASW, the fungus developed extensive, dark, effuse colonies characterised by distinct submerged growth, sparse mycelium, and monilioid hyphae. Similar colonies were observed on PDA with NaCl. In contrast, on MLA and PCA lacking seawater, growth was considerably slower, and colonies appeared more compact with densely interwoven hyphae, displaying a mucoid-waxy texture. On MLA, the fungus produced a deep purple pigment. Growth on PDA and PDA supplemented with NaCl varied among the strains of T. purpureus ( CBS 153576 , CBS 153577 , CBS 153578 ), and in contrast to H. pluriseptata ( CCF 3788 ), was generally more vigorous on the salt-amended medium. The ex-type strain CBS 153577 exhibited the most robust growth, forming colonies 43–45 mm in diam. on PDA with NaCl within 2 wk, while growth on PDA alone was considerably slower, with colonies reaching only 6–7 mm in the same period. Strains CBS 153576 and CBS 153578 showed little to no growth on either medium, with mycelium developing only around the inoculation block.

Thalassodendromyces purpureus was the only fungus isolated from the roots of T. ciliatum that were heavily colonised by DS hyphae. This fact, together with similar hyphal morphology observed in the host and in culture (see the “ Microscopic observations ” in the Results), strongly suggests that the isolated fungus described here represents the mycobiont forming the newly described root-fungus symbiosis in T. ciliatum . However, a resynthesis experiment is necessary to confirm this in accordance with Koch’s postulates and to clarify the nature of the symbiosis, i. e. to determine where it lies along the mutualism – parasitism continuum.

Figures

Figure 6. Thalassodendromyces purpureus. A, B, E. Colonies showing aerial hyphae, radial growth, and a zonation pattern of sparse and dense growth. C, D, F, G. Colonies showing purple pigmentation and distinct zonation patterns. H, I. Colonies exhibiting variability in morphology and pigmentation. J – M. Diversity of colony morphology on MLA, MLA, PCA, and PCASW, respectively (from left to right) after 8 wk. Images: on PCASW (A, B, E); on MLA (C, D, F – I). Strains: MAU- 1 (A); CBS 153576 (I, J); CBS 153646 (C, H, K); CBS 153577 (D, G, L, M); CBS 153578 (E); PRA - 22498 (B); CBS 153625 (F). Scale bars: 500 µm (A); 5 mm (B, E, F – I); 1 mm (C, D); 1 cm (J – M).

Figure 7. Thalassodendromyces purpureus. A, B. Clusters of dark-pigmented monilioid hyphae. C. Monilioid hyphae comprising thick-walled, globose to subglobose cells. D – G. Different stages of monilioid hyphae formation. H. Septate, hyaline vegetative hyphae with irregular swellings. I. Dense, dark, spherical aggregation of hyphae forming a compact structure in culture. Images: on PCASW (A – G, I); on MLA (H). Strains: CBS 153577 (A, B, D – G); CBS 153578 (H); CBS 153624 (C, I). Scale bars: 200 µm (A, B); 50 µm (C); 10 µm (D – H); 100 µm (I).