Across diverse taxa, including those of human hunter–gatherers, gut microbial
communities have been shown to fluctuate with seasonality and latitude or altitude
either directly through changes in environmental conditions (temperature, water
availability) or indirectly due to changes in physiological state (e.g., hibernation)
or food supply that occur in association with these changes (Smits et al. 2017;
Sepulveda and Moeller 2020). But since gut microbial community composition
varies among animal taxa and trophic level, effects of climate-associated variables
are also likely to vary among host taxonomy and niche. For example, members of the
bacterial phylum Firmicutes often dominate in vertebrate guts and elevated temperatures often reduce, by unknown mechanisms, their abundance across a range of
endo- and ectothermic taxa (Sepulveda and Moeller 2020; Fontaine et al. 2018;
Bestion et al. 2017). In contrast, the guts of invertebrate animals are typically
enriched with members of the Proteobacteria, but these tend to positively correlate
with temperature (Sepulveda and Moeller 2020; Moghadam et al. 2018). Thus, while
gut associates are certain to be impacted by a warming world, the observed variability renders it difficult to make general predictions about these associations.
Highly specialized symbioses, on the other hand, often share characteristics
across taxa that allow for general predictions regarding responses to GCC. Many
specialized symbioses are widely distributed across the planet with symbionts
performing key roles in the exploitation of particular niches. In marine systems,
many animals have symbioses with photosynthetic bacteria or eukaryotes (informally algae) while those that live beyond the reach of sunlight may partner with
chemosynthetic bacteria (Venn et al. 2008; Dubilier et al. 2008). In terrestrial
systems, most plants are associated with mycorrhizal fungi or nitrogen-fixing bacteria important in nutrient acquisition (Smith and Read 2008), while animals specialized on nitrogen-poor diets have N-fixing, N-recycling, and N-provisioning
symbionts (Hansen et al. 2020), those restricted to vertebrate blood have
B-vitamin provisioning symbionts (Vogel and Coon 2020), and those consuming
difficult to digest plant polymers harbor microbes that help degrade these substances
(Wertz and Béchade 2020). There are also symbionts with varying degrees of
specialization that confer conditional benefits, including defense against natural
enemies, often by the production of toxins (Oliver and Perlman 2020; Florez et al.
2015).
Key climate challenges that specialized symbiotic organisms encounter include
increasing temperatures, water variability, extreme weather events, elevated CO 2
levels, ocean acidification, and hypoxia. These are often exacerbated by other
anthropogenic stresses, especially habitat loss and pollution. These factors may
impact the abundance and distribution of specific symbioses, by, for example,
converting mutualistic associations into parasitic ones. Symbiotic organisms may
respond to climate change by reorganizing relationships: expelling symbionts,
switching symbiotic partners, or acquiring completely novel symbionts. Host and
symbiont genotypes may also acclimate or adapt to changing conditions, but the
shorter generation times of the latter suggest they may be first responders. Any
emerging symbiotic solutions have the potential to be shared widely among macroscopic organisms via horizontal transfer, but since stresses will come in many forms,
16 Symbiosis in a Rapidly Changing World
269
communities have been shown to fluctuate with seasonality and latitude or altitude
either directly through changes in environmental conditions (temperature, water
availability) or indirectly due to changes in physiological state (e.g., hibernation)
or food supply that occur in association with these changes (Smits et al. 2017;
Sepulveda and Moeller 2020). But since gut microbial community composition
varies among animal taxa and trophic level, effects of climate-associated variables
are also likely to vary among host taxonomy and niche. For example, members of the
bacterial phylum Firmicutes often dominate in vertebrate guts and elevated temperatures often reduce, by unknown mechanisms, their abundance across a range of
endo- and ectothermic taxa (Sepulveda and Moeller 2020; Fontaine et al. 2018;
Bestion et al. 2017). In contrast, the guts of invertebrate animals are typically
enriched with members of the Proteobacteria, but these tend to positively correlate
with temperature (Sepulveda and Moeller 2020; Moghadam et al. 2018). Thus, while
gut associates are certain to be impacted by a warming world, the observed variability renders it difficult to make general predictions about these associations.
Highly specialized symbioses, on the other hand, often share characteristics
across taxa that allow for general predictions regarding responses to GCC. Many
specialized symbioses are widely distributed across the planet with symbionts
performing key roles in the exploitation of particular niches. In marine systems,
many animals have symbioses with photosynthetic bacteria or eukaryotes (informally algae) while those that live beyond the reach of sunlight may partner with
chemosynthetic bacteria (Venn et al. 2008; Dubilier et al. 2008). In terrestrial
systems, most plants are associated with mycorrhizal fungi or nitrogen-fixing bacteria important in nutrient acquisition (Smith and Read 2008), while animals specialized on nitrogen-poor diets have N-fixing, N-recycling, and N-provisioning
symbionts (Hansen et al. 2020), those restricted to vertebrate blood have
B-vitamin provisioning symbionts (Vogel and Coon 2020), and those consuming
difficult to digest plant polymers harbor microbes that help degrade these substances
(Wertz and Béchade 2020). There are also symbionts with varying degrees of
specialization that confer conditional benefits, including defense against natural
enemies, often by the production of toxins (Oliver and Perlman 2020; Florez et al.
2015).
Key climate challenges that specialized symbiotic organisms encounter include
increasing temperatures, water variability, extreme weather events, elevated CO 2
levels, ocean acidification, and hypoxia. These are often exacerbated by other
anthropogenic stresses, especially habitat loss and pollution. These factors may
impact the abundance and distribution of specific symbioses, by, for example,
converting mutualistic associations into parasitic ones. Symbiotic organisms may
respond to climate change by reorganizing relationships: expelling symbionts,
switching symbiotic partners, or acquiring completely novel symbionts. Host and
symbiont genotypes may also acclimate or adapt to changing conditions, but the
shorter generation times of the latter suggest they may be first responders. Any
emerging symbiotic solutions have the potential to be shared widely among macroscopic organisms via horizontal transfer, but since stresses will come in many forms,
16 Symbiosis in a Rapidly Changing World
269