Date of Award

2026

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Dr. Jeffrey Johansen

Abstract

Biological soil crusts (BSCs) in arid and semiarid environments support diverse microbial communities that contribute to soil stabilization, nutrient cycling, and ecosystem resilience. Among these organisms, terrestrial green algae are taxonomically challenging due to their simple morphology, phenotypic plasticity, and lack of diagnostic life history traits. This study investigated the diversity and taxonomy of green algae from BSCs collected on San Nicolas Island, San Clemente Island, and Santa Barbara Island using the polyphasic approach integrating morphology, phylogenetic analyses, molecular analyses, and ecological data. A total of 198 strains were isolated, of which 79 green algal strains underwent molecular analysis using 18S rRNA, rbcL rRNA, and 5.8S-28S ITS2 rRNA sequence data. Twenty-two species representing 18 genera were identified across the Chlorophyceae, Trebouxiophyceae, Charophyta, and Eustigmatophyceae clades. Several taxa previously reported from the islands were confirmed, while numerous additional taxa were newly recorded, including Ava limnothalassea, Pseudostichococcus monallantoides, Elliptochloris subsphaerica, and Leptosira terrestris. Comparative analyses with a previous study of San Nicolas Island (Flechtner et al., 2008) demonstrated shifts in algal diversity over time and among islands. Polyphasic evidence further suggested the presence of at least three putative new species, including strains affiliated with Diplosphaera and Phyllosiphon. The results of this study emphasize the importance of combining morphological, phylogenetic, molecular, and ecological evidence for resolving cryptic terrestrial algae and improving species delimitation in desert ecosystems.

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