First Report on Soil Algae From Ultramafic Substrates in Bulgaria

Abstract

Ultramafic substrates, characterized by high nickel concentrations and a low calcium-to-magnesium ratio, are unique natural phenomena. Their global biodiversity is relatively well documented for vascular plants, while data on cryptogamic inhabitants are scarce or lacking. This is particularly true for these substrates in the Eastern Rhodope Mountains, Bulgaria, where soil algae have never been studied. This paper presents the first data on algae from Rhodopean serpentine soils, which are intrinsically associated with ultramafic rock. Surface samples were collected from five sites in 2007 (2), 2010 (1), and 2024 (2) and were analysed by light microscopy in 2025 following 2 months of simultaneous cultivation on two different media. Despite the limitations of these conventional methods and differences in sample age, a relatively rich algal flora (19 species) was detected. However, it cannot be considered unique or endemic, as it contains typical soil representatives possessing specific traits already well known as morphological adaptations for an edaphic mode of life, thus showing that these classical adaptations also function in such chemically unusual and adverse environments. The distinct species composition at each studied site was structured by multiple interacting environmental gradients, among which the main drivers were Mg, altitude, Zn, Fe, and Ca, with some support from Ni, as demonstrated by the values of the Jaccard and Sørensen similarity indices and the results of canonical correspondence analysis. Our findings highlight the need for future studies expanding on the biodiversity, ecological characteristics, and biotechnological potential of algae inhabiting ultramafic substrates.