Diatoms in low pH environments: diversity, adaptations, mechanisms, ecological roles, and applications.

Diatoms inhabit a broad pH spectrum, from neutral lakes to highly acidic waters shaped by natural organic acids and anthropogenic inputs such as acid mine drainage (AMD). This review outlines the key chemical drivers of low-pH environments, including natural and industrial acidification. We then synthesize diatom community responses to acid stress-declining taxonomic richness, dominance of acidophilic taxa, and frustule deformities-highlighting how proton toxicity can be a dominant structuring force in highly acidic environments, although in many AMD systems it interacts synergistically with metal stress to shape assemblage composition. At the cellular level, diatoms exhibit adaptive traits including proton-pumping ATPases, dynamic carbon concentrating mechanisms, pH-modifying frustule surface chemistry, and flexible silicon uptake. Genomic studies reveal DNA regulatory elements linked to energy conservation, metabolic rewiring, and enhanced proton/metal homeostasis. Ecologically, acid-tolerant diatoms contribute to carbon, silica, and trace metal cycling, support unique trophic webs, and serve as reliable indicators in biomonitoring and palaeoecological reconstructions. Despite limited experimental data, acidophilic diatoms offer promise for biotechnological applications, including biomimetic nanomaterials, lipid-rich biofuels in acidic photobioreactors, and biofilm-based remediation. We identify key research gaps in genomic resources, taxonomic resolution, and cultivation methods, and propose a multidisciplinary agenda integrating omics, eDNA, and engineered bioreactors. This synthesis underscores the ecological and biotechnological value of acid-tolerant diatoms in understanding and addressing aquatic acidification.