Photosymbiosis

Photosymbiosis is a symbiotic relationship in which one partner is photosynthetic and provides energy-rich organic carbon to the other. The association is usually mutualistic, though it can be facultative or context-dependent. The photosynthetic partner, typically an alga or cyanobacterium, or a dinoflagellate in some marine hosts, carries out photosynthesis and transfers a portion of the fixed carbon to the host. In return, the host offers a protected habitat and access to inorganic nutrients, carbon dioxide from respiration, and often minerals or nitrogen.

Marine examples are among the best studied. In corals, endosymbiotic dinoflagellates (commonly Symbiodinium and related taxa) reside within coral tissues and supply substantial portions of the coral’s energy needs, facilitating rapid growth and reef formation. This partnership is sensitive to temperature and light, and stress can cause loss of the symbionts, a phenomenon known as bleaching. Terrestrial and freshwater photosymbioses include lichens, partnerships between a fungal mycobiont and a photosynthetic partner (green algae or cyanobacteria) that together tolerate harsh, nutrient-poor environments. Cyanobacteria in some lichens can also fix nitrogen, adding another ecological function.

Other instances include transient or specialized associations such as kleptoplasty, where some sea slugs sequester chloroplasts from algal prey to perform photosynthesis within their own cells for a period. Transmission of symbionts can be horizontal (acquired from the environment) or vertical (passed from parent to offspring), influencing the stability and coevolution of the relationship.

Ecologically, photosymbioses contribute to primary production, nutrient cycling, and energy flow in both marine and terrestrial ecosystems, while environmental stressors like heat and pollution can disrupt these intimate associations.