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Summary Symbiosis between eukaryotic microalgae and heterotrophic hosts is a widespread, phylogenetically convergent, and ecologically important phenomenon in aquatic ecosystems. Partners include taxonomically diverse microalgae interacting with multicellular or unicellular hosts in marine or freshwater environments. While progress has been made recently, there are still major knowledge gaps on the microenvironmental conditions of microalgaein hospite(e.g. nutrient and CO₂availability), the algal carbon metabolism (production and storage), and the cellular mechanisms of carbohydrate export to the host. This review aims to provide current knowledge on the physiology and metabolism of symbiotic microalgae, to highlight whether there are commonalities across different photosymbioses, and to identify new approaches and technologies for disentangling photosymbiotic interactions at relevant temporal and spatial scales. 

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 acid–base homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H+, and HCO3− have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments 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/HCO3 − accumulation 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 nonexhaustive review summarizes some of the distinct acid–base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.