The Microfungal Communities in Deep-Sea Sediments from the Equatorial Atlantic

Microfungi of deep-sea sediments, and especially those several meters below the water–sediment interface, are poorly studied. In this work, for the first time, microfungal communities isolated by cultivation from deep-sea sediments of the eastern part of the Equatorial Atlantic (the Romanche and Chain Fracture Zones) were investigated. Fungi were isolated from sediments sampled at each of 12 stations from horizons 1.0–4.7 m below the sediment–water interface. To study microscopic fungi, one sediment horizon was isolated from each core. The fungal abundances were within the range of 0.0–3300.0 CFU g-1 sediment dry weight. A total of 19 fungal taxa from the phyla Ascomycota (18) and Basidiomycota (1) were identified, and Mycelia sterilia 1 strain was also isolated. Seven fungal species were encountered only once. In this case, the maximum similarity of species composition, in terms of the Bray – Curtis coefficient, was 57.14% (horizons 1.0 and 3.6 m, four common species). A comparison of the taxonomic structures of fungal communities from the study area was made with those from sediments of the Indian and Pacific Oceans and other areas of the Atlantic. The fungal communities from sediments in the study area were compared with those from the Indian and Pacific Oceans and other areas of the Atlantic. From the literature data and present study results, a list of fungal species with 180 names was compiled. The fungi belonged to 97 genera, 57 families, 32 orders and 13 classes of the phyla Ascomycota, Basidiomycota, and Mucoromycota. The diversity of fungal communities was assessed using indicators of taxonomic richness (number of taxa from different ranks), proportions (genera/families, species/families, species/genera), Average Taxonomic Distinctness index (AvTD, Δ+) and Variation in Taxonomic Distinctness index (VarTD, Λ+). Four and twelve fungal classes were identified in sediments in the Eastern Equatorial Atlantic and the Indian Ocean, respectively. The species/genera proportions in the communities varied from 1.33 (Indian Ocean) to 3.8 (other areas of the Atlantic Ocean). For the fungal communities of the Eastern Equatorial Atlantic, the AvTD index value was minimal (Δ+ = 50.19), the VarTD index was maximal (Λ+ = 945.38), and they were beyond the 95% confidence interval. This was due to the small number of the fungal classes and vertical and horizontal unevenness of species distribution along taxonomic branches, which was manifested in the dominance of species of the family Aspergillaceae (78.9% of the species in the class Saccharomycetes and Eurotiomycetes), only two species belonging to the classes Sordariomycetes and one species belonging to the class Microbotryomycetes (phylum Basidiomycota). Consequently, statistically significant differences were found between the taxonomic structures of the fungal communities of the Eastern Equatorial Atlantic and the other regions of the World Ocean, which are due to the insufficient amount of data obtained on the species composition of fungi in the sediments of this area. The study did not reveal any pattern in the change in the number of fungal species and their abundance in relation to the water characteristics (temperature, pH, and salinity), horizon depth in the sediment core, sediment type, or sampling station location in the Romanche and Chain Fracture Zones.