Date of Award

2026

Degree Type

Essay

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Dr. Jeffrey Johansen

Abstract

The genus Nostoc has long been known to be polyphyletic, and recent taxonomic work has segregated several Nostoc-like lineages into new genera, including Compactonostoc. Until now, Compactonostoc shennongjiaensis was the only described species in the genus. In a survey of biological soil crusts from Santa Catalina Island (California, USA), we isolated two Nostoc- like strains that phylogenetically affiliate with Compactonostoc. A polyphasic approach based on morphology, 16S rRNA gene phylogeny, 16S–23S ITS secondary structure, and cloned ribosomal operons revealed that each strain harbors two highly divergent ribosomal operon types. One operon (Type 2) is closely related to C. shennongjiaensis and contains the two tRNA genes in the ITS region, whereas the second (Type 1) lacks tRNA genes and is strongly allied to the horizontally acquired Type 1 operon previously reported in Kalymmatonema (as Scytonema hyalinum), indicating an ancient HGT event from an unsampled Nostocaceae lineage. In Compactonostoc chimaerum, we additionally detected two reciprocal chimeric operons (Type 2/1 and Type 1/2), with crossovers in a conserved region of the 16S rRNA gene, demonstrating ongoing recombination between operon types. ITS percent dissimilarity, ITS domain structures, and 16S percent similarity, together with consistent morphological differences, support recognition of the two strains as new species, Compactonostoc medioheterocytium and C. chimaerum, both clearly distinct from C. shennongjiaensis. Our results document rare ribosomal- operon HGT and reciprocal chimerism in Compactonostoc and highlight the need to examine multiple clones and operons when describing cyanobacterial taxa based on rRNA genes.

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