No evidence that lulworthioid fungi are dark septate endophytes in the roots of the dominant Mediterranean seagrassPosidonia oceanica

Abstract

A previous study from Sicily, Italy indicated that the dominant Mediterranean seagrass Posidonia oceanica forms a dark septate endophytic (DSE) association with a lulworthioid fungus (Lulwoana sp.), which is in conflict with several other studies from the NW Mediterranean Sea that point at the recently described pleosporalean fungus Posidoniomyces atricolor. I collected P. oceanica roots at eight sites around Sicily and checked them for fungal colonization using light microscopy. At three sites, root fungal symbionts (=mycobionts) were isolated into pure cultures and identified using sequencing of the ITS rDNA gene. Posidoniomyces atricolor represented the most frequent mycobiont (56 isolates), closely followed by lulworthioid fungi (51). The obtained mycobiont spectrum also comprised Cladosporium (2), Alternaria (1), Corollospora (1), Fusarium (1), Penicillium (1) and Vishniacozyma (1) isolates. The characteristic DSE root colonization similar to those occurring in terrestrial plants but not known from any other seagrass was found in all investigated P. oceanica individuals. The microscopic screening suggests that P. atricolor is indeed responsible for the observed DSE colonization. This study extends the known range of P. atricolor and the DSE association characteristic for P. oceanica for southern Tyrrhenian Sea/Sicily. While lulworthioid fungi regularly occur in P. oceanica tissues, including terminal fine roots, their significance and functioning are unknown and beg further investigation. However, there are currently no proofs that they belong among dark septate endophytes of this seagrass.