**5. Conclusions**

These results help to clarify the nature of changes in symbiotic relationships during discoloration syndrome development in freshwater sponges. We showed large fluctuations in microbial composition in diseased and healthy sponges based on our previously published data, which may be due to different collection points, depths and collection seasons. Given the limited number of samples, we analyzed experiments under the same conditions that are needed to identify the change in microbiome composition between healthy and diseased individuals and its causes. We also showed interspecific differences in discoloration syndrome development in Baikal sponges. Thus, more data are also needed from samples at different depths and seasons, and from different Lubomirskiidae species.

**Author Contributions:** Conceptualization, V.I. and J.S.; methodology, V.I., O.G. and O.K.; software, R.P., G.S.K. and J.S.; validation, V.I. and J.S.; formal analysis, R.P. and G.S.K.; investigation, V.I. and O.G.; data curation, V.I., R.P. and G.S.K.; writing—original draft preparation, V.I., O.K., R.P., G.S.K. and J.S.; writing—review and editing, V.I., O.K., R.P., G.S.K. and J.S.; visualization, R.P., G.S.K. and J.S.; supervision, V.I. and J.S.; project administration, V.I. and J.S.; funding acquisition, V.I. and J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The reported study was funded by RFBR and DST according to the research project № 19-54-45034, by RFBR according to the research project № 20-04-00868 and basic funding, project number 0279-2021-0011.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The raw sequencing reads were deposited in the NCBI-SRA portal (PRJNA665339).

**Acknowledgments:** We thank Igor Khanaev and Alexandr Kupchinsky for the help with samples collection. We thank Mallory Eckstut, from Edanz (https://www.edanz.com/ac, accessed on 1 November 2021) for editing a draft of this manuscript. We thank the three anonymous reviewers for their valuable comments. This study was carried out in the Large-Scale Research Facilities "Experimental Freshwater Aquarium Complex for Baikal Hydrobionts" (http://www.lin.irk.ru/aqua, accessed on 1 November 2021).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

