acclimation

Acclimation refers to the physiological, biochemical, or developmental adjustments an organism makes in response to changes in its immediate environment, enabling it to maintain function and homeostasis. These changes are usually reversible and occur within an individual’s lifetime, in contrast to genetic adaptation that arises through evolution over generations. The term is often used to distinguish short-term, plastic responses to environmental variation from longer-term evolutionary change. Acclimation can be studied under controlled conditions (acclimation) or observed in natural settings (acclimatization).

Common stimuli include temperature, oxygen availability (altitude), salinity, light, and nutrient status. Humans ascending to high altitude experience ventilatory acclimatization and increased erythropoiesis, leading to higher hemoglobin concentration and capillary density. Plants adjust leaf anatomy, stomatal conductance, and osmotic balance, sometimes accumulating compatible solutes such as proline and soluble sugars. Ectotherms, including many fish and reptiles, exhibit temperature acclimation that can shift enzyme kinetics and membrane fluidity, affecting metabolic rate.

Mechanisms underlying acclimation involve changes in gene expression, protein synthesis, enzyme activity, membrane composition, and hormonal regulation. Some adjustments occur within minutes to days, while others develop over weeks; many are reversible when conditions revert. The extent of acclimation varies among species and traits, and there are costs and trade-offs that may reduce performance in other contexts.

Acclimation is distinct from, yet related to, acclimatization and broader phenotypic plasticity. In ecology and physiology, it is a key concept for understanding how organisms cope with environmental variability, including implications for health, agriculture, and conservation.