Aposymbiosis

Aposymbiosis refers to the physiological state in which an organism that normally maintains one or more symbiotic relationships lacks its symbionts. It can be natural, temporary, or induced experimentally. Aposymbiotic individuals may persist but typically show altered metabolism, reduced growth, or increased susceptibility to stress, depending on the reliance on the symbionts.

A well-known example is reef-building corals, which host endosymbiotic dinoflagellate algae (Symbiodiniaceae). Under thermal stress or pollution, corals can lose their zooxanthellae in a process called bleaching, becoming aposymbiotic. While alive, aposymbiotic corals can supplement energy through heterotrophy but often experience reduced calcification and higher mortality if stress persists. Some corals can reacquire symbionts from the environment, restoring their symbiotic state (re-symbiosis).

Aposymbiosis is also observed in other systems, such as insects that rely on intracellular bacterial endosymbionts (for example Buchnera in aphids) or microbial communities in some plants. In these cases, the loss of symbionts can lead to reduced nutrient provisioning and compromised fitness, though temporary aposymbiosis may occur during experiments or environmental perturbations.

Research on aposymbiosis helps clarify the ecological and evolutionary dynamics of symbioses, including the costs and benefits of hosting symbionts, the stability of these partnerships, and the mechanisms by which hosts manage or compensate for loss of symbionts. Detection commonly involves microscopy and molecular markers to confirm absence of symbionts.