**4. Conclusions**

The active volcanic hot springs on the Kamchatka Peninsula are extreme environments for microbial ecosystems characterized by the persistent supply of nutrients and geothermal energy. The electron microscopic observations presented in this paper show diverse crystal habits and a wide range of pyrite crystal sizes in the hot springs studied. We propose that it is the continuous interplay between the geochemical environments of the volcanic hot springs and the microbial ecophysiological activities that sustain the continuous precipitation of pyrite and preserve the diverse crystal sizes and habits. We suggest that the complexity of crystal habits of pyrite in those hot springs represents a combined biological and geochemical contribution to the kinetics of pyrite mineralization and its preservation, and thus implies a biologically mediated process.

**Author Contributions:** Conceptualization, Y.-L.L. and M.T.; methodology, M.T.; validation, Y.-L.L. and M.T.; formal analysis, Y.-L.L. and M.T.; writing—original draft preparation, M.T.; writing—review and editing, Y.-L.L. and M.T.; supervision, Y.-L.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by RGC, grant number 17312016, NSFC, grant number 31970122 and Joint Foundation Project between Yunnan Science Technology Department and Yunnan University, grant number C176240210019.

**Acknowledgments:** We thank Juergen Wiegel of the University of Georgia for his generosity in sharing those precious samples.

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