Novel Pathway-Based Classification of Interactions Between Microalgae and Their Symbionts Based on Currencies, Contact and Context

Microalgae rarely grow alone. Bacteria, fungi, yeasts, other algae and animal hosts surround, attach to or enclose algal cells, and these associations control primary production, nutrient recycling, bloom dynamics and the performance of engineered cultivation systems. The interactions are still classified mostly by their outcome, as mutualism, commensalism, parasitism, amensalism, competition or neutralism. However, the same co-culture can move from mutualism to pathogenicity when growth phase, nutrient supply, light, temperature or pH change. This review proposes a classification based on the pathway that connects the organisms rather than on the outcome. Seven pathway classes are defined by what crosses the interface: metabolic exchange, growth factor provision, chemical signalling, oxidative stress modulation, algicidal and antagonistic activity, attachment and physical association, and gene transfer. Each interaction is further described by its mode of association, the obligate or facultative dependence of each organism and its context dependence. The review compares how these classes differ in their physiological effects on the alga and its symbionts in natural and engineered environments, the genes and physical structures that encode them and the magnitude of environmental effects acting on them. It then shows that pathways succeed and substitute for one another, and it discusses how mutation, gene loss, horizontal gene transfer and environmental change will redistribute pathways across algal symbioses.