Diversity metrics for the evaluation of cultivable Actinobacterial variety from the limestone soils of the Cuddapah Basin

Abstract

Aim: Limestone ecosystems represent geochemically unique and underexplored habitats that can harbor specialized Actinobacteria with vital applications. The present study investigates the culturable actinobacterial diversity from limestone-embedded soil of the Cuddapah Basin, India. Methodology: A total of 36 isolates were obtained on selective media and identified by 16S rRNA gene sequencing. Results: Quarry A yielded five genera, dominated by Streptomyces (41.7% of isolates), whereas Quarry B harbored eight genera, with a strong dominance of Streptomyces (58.3%). Despite the differences in richness, both quarries exhibited comparable Shannon diversity values of 1.468 and 1.465, respectively. Only two genera (Streptomyces and Rhodococcus) were shared between the two quarries, resulting in a low Jaccard similarity index of 0.182, indicative of limited overlap in genus composition. The Bray-Curtis dissimilarity value of 0.611 further supports noticeable differences in genus-level abundance structure between the two-quarry sites, proving dissimilarities in genus typed between two samples. Phylogenetic analysis resolved all isolates into well-supported lineage that correspond to their respective genera, reflecting broad representation across the orders Streptomycetales, Micrococcales and Mycobacteriales. Interpretation: This study presents the first report of actinomycetes isolated from limestone quarry and provides the first baseline of combining culturedependent isolation with quantitative diversity metrics to evaluate actinobacterial diversity from the Cuddapah Basin limestone ecosystems of Southern India. Key words: Actinobacteria, Cuddapah Basin, Culturable diversity, Limestone soil, Streptomyces