Historically, microbes represent the dominant organisms on the planet; life began
approximately 4 billion years (Ga) ago and was entirely microbial for the first 2 Ga
(Knoll and Nowak 2017). The earliest microbes were likely chemoautotrophs
deriving energy from the oxidation of inorganic compounds, and the first ecosystems
were anaerobic, with patchy distributions restricted to sources of geochemical
energy (Judson 2017). Sunlight was abundant and the evolution of oxygenic photosynthesis 2.7 Ga, preceded by anoxygenic photosynthesis a billion years before, was
certainly among the most important evolutionary innovations. Photosynthesis
greatly improved productivity and allowed ecosystems to exist independent of
geochemical sources. Although it took time, oxygenic photosynthesis led to the
oxygenation of the atmosphere (2.3 Ga) and later the oceans. This Great Oxidation
Event created the protective ozone layer, and created a plethora of new niches
ranging from anoxic to oxygen rich (Judson 2017). Even more impactful, the
availability of oxygen as an energy source opened the door to the rise of large,
multicellular organisms; a transition enabled by the endosymbiotic acquisition of an
alpha-proteobacterium that became mitochondria in eukaryotes (Lane and Martin
2010). These large-bodied organisms themselves became niches for microorganisms, which in turn, provided novel capabilities to their hosts, enhancing existing
functions or creating novel ones. For instance, oxygenic photosynthesis likely
evolved just once in the ancestors of cyanobacteria, but was shared broadly among
eukaryotic lineages via symbiosis, cumulatively representing the bulk of primary
productivity across the planet (Venn et al. 2008). In another example, fungi and
plants co-invaded the land, with each partner using the other for successful terrestrial
establishment, with coevolutionary interactions driving diversity and major evolutionary events in each group (Lutzoni et al. 2018). Increased energy and mobility
further created opportunities for predation and parasitism, leading to arms races
around the evolution of defenses and counter-defenses that likely drove the diversification of multicellular eukaryotes into “endless forms most beautiful” (Judson
2017; Darwin 2009).
Today, most multicellular eukaryotes interact intimately with microorganisms.
For example, tissues exposed to the environment, including animal guts, are typically colonized by microbes, although at highly variable degrees of specificity and
specialization (Engel and Moran 2013; Douglas 2014). While gut microbes may not
be critical for every species, they often increase the digestive efficiency of hosts by
breaking down recalcitrant polymers or toxins, facilitating nutrient and energy
uptake, and modulating immune function, often by providing colonization resistance
against ingested pathogens (McFall-Ngai et al. 2013; Ley et al. 2008a, b; Hammer
et al. 2017; Gould et al. 2018). Climate-induced changes in the abundance, composition, or function of microbiomes have the potential to impact these common
phenotypes and hence host fitness. There will be opportunities for microbe-mediated
acclimation to GCC, but also the conversion of beneficial or commensal microbes
into pathogens (Alberdi et al. 2016). Studies are limited, but heat-stressed individuals have been associated with decreased digestive and immune performance,
although it is difficult to show causation because of feedback between host and
microbiome (Sepulveda and Moeller 2020).
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approximately 4 billion years (Ga) ago and was entirely microbial for the first 2 Ga
(Knoll and Nowak 2017). The earliest microbes were likely chemoautotrophs
deriving energy from the oxidation of inorganic compounds, and the first ecosystems
were anaerobic, with patchy distributions restricted to sources of geochemical
energy (Judson 2017). Sunlight was abundant and the evolution of oxygenic photosynthesis 2.7 Ga, preceded by anoxygenic photosynthesis a billion years before, was
certainly among the most important evolutionary innovations. Photosynthesis
greatly improved productivity and allowed ecosystems to exist independent of
geochemical sources. Although it took time, oxygenic photosynthesis led to the
oxygenation of the atmosphere (2.3 Ga) and later the oceans. This Great Oxidation
Event created the protective ozone layer, and created a plethora of new niches
ranging from anoxic to oxygen rich (Judson 2017). Even more impactful, the
availability of oxygen as an energy source opened the door to the rise of large,
multicellular organisms; a transition enabled by the endosymbiotic acquisition of an
alpha-proteobacterium that became mitochondria in eukaryotes (Lane and Martin
2010). These large-bodied organisms themselves became niches for microorganisms, which in turn, provided novel capabilities to their hosts, enhancing existing
functions or creating novel ones. For instance, oxygenic photosynthesis likely
evolved just once in the ancestors of cyanobacteria, but was shared broadly among
eukaryotic lineages via symbiosis, cumulatively representing the bulk of primary
productivity across the planet (Venn et al. 2008). In another example, fungi and
plants co-invaded the land, with each partner using the other for successful terrestrial
establishment, with coevolutionary interactions driving diversity and major evolutionary events in each group (Lutzoni et al. 2018). Increased energy and mobility
further created opportunities for predation and parasitism, leading to arms races
around the evolution of defenses and counter-defenses that likely drove the diversification of multicellular eukaryotes into “endless forms most beautiful” (Judson
2017; Darwin 2009).
Today, most multicellular eukaryotes interact intimately with microorganisms.
For example, tissues exposed to the environment, including animal guts, are typically colonized by microbes, although at highly variable degrees of specificity and
specialization (Engel and Moran 2013; Douglas 2014). While gut microbes may not
be critical for every species, they often increase the digestive efficiency of hosts by
breaking down recalcitrant polymers or toxins, facilitating nutrient and energy
uptake, and modulating immune function, often by providing colonization resistance
against ingested pathogens (McFall-Ngai et al. 2013; Ley et al. 2008a, b; Hammer
et al. 2017; Gould et al. 2018). Climate-induced changes in the abundance, composition, or function of microbiomes have the potential to impact these common
phenotypes and hence host fitness. There will be opportunities for microbe-mediated
acclimation to GCC, but also the conversion of beneficial or commensal microbes
into pathogens (Alberdi et al. 2016). Studies are limited, but heat-stressed individuals have been associated with decreased digestive and immune performance,
although it is difficult to show causation because of feedback between host and
microbiome (Sepulveda and Moeller 2020).
268
K. M. Oliver and C. H. V. Higashi