The acid-base relevant molecules carbon dioxide (CO2), protons (H + ), and bicarbonate (HCO3 -) are substrates and end products of some of the most essential physiological functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acidbase homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H + and HCO3have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments 2 presents organisms with distinct acid-base challenges that are not found in terrestrial environments. These include a relatively high CO2 relative to O2 solubility that prevents internal CO2/HCO3accumulation to buffer pH, a lower O2 content that may favor anaerobic metabolism, and variable environmental CO2, pH and O2 levels that require dynamic adjustments in acid-base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example, acidifying mechanisms can enhance O2 delivery by red blood cells, lead to ammonia trapping for excretion or buoyancy purposes, or lead to CO2 accumulation to promote photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms can serve to promote calcium carbonate skeletal formation. This non-exhaustive review summarizes some of the distinct acid-base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.

Summary Diatom cell walls, called frustules, are main sources of biogenic silica in the ocean and their intricate morphology is an inspiration for nanoengineering. Here we show dynamic aspects of frustule biosynthesis involving acidification of the silica deposition vesicle (SDV) by V‐type H+ ATPase (VHA). Transgenic Thalassiosira pseudonana expressing the VHA B subunit tagged with enhanced green fluorescent protein (VHAB‐eGFP) enabled subcellular protein localization in live cells. In exponentially growing cultures, VHAB‐eGFP was present in various subcellular localizations including the cytoplasm, SDVs and vacuoles. We studied the role of VHA during frustule biosynthesis in synchronized cell cultures of T. pseudonana. During the making of new biosilica components, VHAB‐eGFP first localized in the girdle band SDVs, and subsequently in valve SDVs. In single cell time‐lapse imaging experiments, VHAB‐eGFP localization in SDVs precluded accumulation of the acidotropic silica biomineralization marker PDMPO. Furthermore, pharmacological VHA inhibition prevented PDMPO accumulation in the SDV, frustule biosynthesis and cell division, as well as insertion of the silicalemma‐associated protein SAP1 into the SDVs. Finally, partial inhibition of VHA activity affected the nanoscale morphology of the valve. Altogether, these results indicate that VHA is essential for frustule biosynthesis by acidifying the SDVs and regulating the insertion of other structural proteins into the SDV.

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