Table 2.

Benefits based on services in microbial symbioses

Benefit type	Description	Examples
Protection
Habitat (protection against abiotic factors)	Providing safe environment by removing, neutralizing, or buffering environmental risks and fluctuations including harmful products and agents [114, 188]	Providing extracellular adhesive polymer for biofilm structure [67, 189] Detoxification or sinking of a reverse-inhibiting product Dictyostelium carrying and dispersing food bacteria to reintroduce them at a new site to provide new food crop, also helping the symbiont to survive and spread [190] Parasite inducing resistance in the host, e.g., Holospora-infected Paramecium hosts can tolerate more heat than the uninfected due to symbiont-induced expression of heat‐shock resistant proteins [191]
Resistance (protection against biotic factors)	Restricting or inhibiting access of biological agents, by providing structural or antibiotic resistance	Extracellular matrix of the biofilm [192] Species providing resistance for the whole biofilm consortium [67], against, e.g., bacteriophages [193] Endosymbionts inducing antibiotic production (mostly in Metazoa, e.g., fungus-growing ants [194])
Nutrition benefits
Trophic interaction	Feeding on the partner, actively reducing its population size. Direct benefit of providing nutrients for the predator; direct control over its population size	Ciliate-alga symbioses that lead to internal, living alga-population [195] (also see under farming)
Metabolite exchange	The metabolic product of one party serves as a resource for another. Localization, spatial patterns, and correlated presence of partners are crucial in rendering metabolite exchange efficient [196, 197]	Free diffusion within the medium: nitrogen-cycling network [42, 67, 198] Directed transfer via transport structures (i.e., reducing loss due to leakiness)
Transportation	Providing transportation for the other party to find resources	Chlorochromatium aggregatum: the motile central Betaproteobacterium transports its nonmotile photosynthetic epibionts towards light [177], while the epibionts provide energy for the host [155] The slime mould Dictyostelium can set aside and distribute with its spores phagocytized Burkholderia cells to colonize new habitats and provide food for the new colony [103]
Indirect benefits and costs	Secondary effects that stem from the primary relationship, but manifest only occasionally or are comparably smaller in effect	Metabolic exchange: the metabolite is the primary benefit, while the host also has access to an alternative metabolic pathway within the symbiont for energy harnessing. Metabolically diverse symbioses (e.g., biofilms), provide the benefit of robustness against harsh or fluctuating conditions [199]. The symbiont or the nutrients coerced from it could provide a more stable and balanced diet for the host [121] Aggregating with cooperative partners is indirectly beneficial as it reduces interactions with non-cooperative individuals [200] Predatory or parasitic interactions can provide metabolic complementation or acquired resistance against pathogens, that are only beneficial in particular environments
Cross-feeding	Mutualistic metabolism in which partners depend on metabolic products of each other	Removing the product of the partner can be synergistic as it could turn otherwise endergonic reactions to exergonic for the partner [42]; hence, cross-feeding can be seen as a combination of nutrition and protection (detoxification) benefits The majority of biofilm-forming microorganisms are coupled metabolically due to complementarity and dependency (auxotrophy) [42, 67, 201]
Farming	A combination of nutrition and protection benefits: the host can feed on the stored stack; the symbiont is protected against environmental fluctuations and predators	Typical examples are eukaryotic ciliate-alga symbioses [195] Theory has demonstrated the selective advantage of storing living prey internally for poor times, especially if resource-poor times are sufficiently long and/or frequent [53] Extant alga-protist examples: the metabolite of the photosynthetic symbiont is only provided during daylight; otherwise, the partnership is costly for the host [58]
Enabling new niches	The host–symbiont association can extend or even enter a new ecological niche where they enjoy reduced competition	Aerobic mitochondria could have provided the possibly anaerobic (microaerophilic) host an opportunity to venture into aerobic habitats, while its direct competitors could not. This has happened to a purple protist [202] According to a hypothesis, it was heat generation by mitochondria that allowed the hyperthermophile archaeal host to find cold yet unoccupied niches [203]