*Article* **Microbiome Changes of Endemic Lake Baikal Sponges during Bleaching Syndrome Development**

**Valeria Itskovich 1,\*, Oxana Kaluzhnaya 1, Olga Glyzina 1, Ragothaman Prathiviraj 2, George Seghal Kiran <sup>3</sup> and Joseph Selvin <sup>2</sup>**


**Abstract:** The sponge (Porifera) microbiome is an indicator of both natural and anthropogenic stressors. Studying Baikal sponge microbial communities could help reveal if there is a connection between bacterial symbionts and a mass sponge bleaching event that was recently detected; 16S rRNA sequencing was performed among healthy and diseased freshwater sponges of *Lubomirskia baikalensis* and *Baikalospongia intermedia*, which were collected from Lake Baikal, Russia. A phylum-based taxonomic classification showed that Chlorophyta, Acidobacteria, Bacteroidetes, Actinobacteria and Cyanobacteria were most abundant across samples. When comparing healthy and diseased *L. baikalensis* samples, large variations in microbial composition were found at the phylum level. Comparative analyses, which were performed for the first time for *B. intermedia*, showed a decrease in Chlorophyta (unicellular green algae) and an increase in Bacteroidetes and Cyanobacteria in diseased specimens. At the genus level, the *Opitutus* (Verrucomicrobia), *Planctomyces*, and *Nitrospira* content increased in all diseased sponges, which reflected a general tendency toward an increase in Cyanobacteria in diseased sponges. Comparative analysis of the diseased and healthy sponge metagenomes showed that diseased sponges underwent various nonspecific changes in bacterial composition. The bacterial community composition is probably influenced by sponge type and degree of disease affection.

**Keywords:** Porifera; microbiome; sponge disease; 16S rRNA sequencing; Baikal
