multiple symbiont solutions would be required. Also, eukaryotic organisms are
already relocating in response to GCC, but for the majority of organisms with
environmentally acquired microbes, less is known about the movement of their
symbionts. Presumably, organisms that associate with less specific microbes may
experience fewer constraints on migration, while mismatches in dispersal ability
between plants and more specific symbionts may curtail dispersal ability.
16.3 The Early Results Are in, and Symbiotic Animals Are
Not Faring Well
16.3.1 Widespread Losses of Corals
Photosynthetic symbionts are widely distributed across the planet and eukaryote taxa, including animals and fungi, as well as plants and microbial eukaryotes
where plastids derive from ancient symbioses (Venn et al. 2008; Palmer 2003;
Falkowski et al. 2004). In shallow tropical oceans, where sunlight is abundant,
benthic animals have become abundant through photosymbiosis. However, warming
oceans pose a general threat to photosymbiotic animals, as many, including corals,
sponges, and anemones, expel their photosynthetic symbionts during thermal stress.
This phenomenon, called bleaching, is mediated by oxidative stress in one or both
partners and often results in the loss of host color (Oakley and Davy 2018).
Now unfortunate icons of the threats posed by GCC, corals are a symbiosis
between anthozoan animals (Phylum Cnidaria) and photosynthetic dinoflagellates
in the Symbiodiniaceae, such as Symbiodinium. Coral reefs, engineered by
scleractinian stony corals, are hotspots for production and biodiversity despite
covering just 0.1% of the ocean surface (Roberts et al. 2002). Their associated
ecosystem services (e.g., coastal protection, food products, and tourism) are valued
in the trillions of USD, and sustain roughly 10% of the world’s human population
(Costanza et al. 2014). However, stress from a variety of factors, including warming
and pollution, can convert microbial mutualists into parasites (Vega Thurber et al.
2014; Morris et al. 2019; Hughes et al. 2017a; Fitt et al. 2001; Baker et al. 2018).
Since most of the animal’s energy is obtained from the photosymbionts, extended
periods of loss result in partial to complete coral mortality. Effects of bleaching
radiate outward, for example, homogenizing coral-supported communities
(Richardson et al. 2018).
In addition to gradual increases in surface temperature, heat waves are causing
ever more widespread, severe, and frequent mass bleaching events, limiting corals’
capacity to recover (Hughes et al. 2018). Three pan-tropical mass bleaching events
occurred between 1997 and 2016 with just 1
C warming (Hughes et al. 2017b). And
the Great Barrier Reef suffered its most widespread bleaching event in 2020; the
third event in just 5 years. The latter is especially troubling given that it occurred in
the absence of a normally facilitating El Niño event (Stone 2020). Intensifying
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K. M. Oliver and C. H. V. Higashi
already relocating in response to GCC, but for the majority of organisms with
environmentally acquired microbes, less is known about the movement of their
symbionts. Presumably, organisms that associate with less specific microbes may
experience fewer constraints on migration, while mismatches in dispersal ability
between plants and more specific symbionts may curtail dispersal ability.
16.3 The Early Results Are in, and Symbiotic Animals Are
Not Faring Well
16.3.1 Widespread Losses of Corals
Photosynthetic symbionts are widely distributed across the planet and eukaryote taxa, including animals and fungi, as well as plants and microbial eukaryotes
where plastids derive from ancient symbioses (Venn et al. 2008; Palmer 2003;
Falkowski et al. 2004). In shallow tropical oceans, where sunlight is abundant,
benthic animals have become abundant through photosymbiosis. However, warming
oceans pose a general threat to photosymbiotic animals, as many, including corals,
sponges, and anemones, expel their photosynthetic symbionts during thermal stress.
This phenomenon, called bleaching, is mediated by oxidative stress in one or both
partners and often results in the loss of host color (Oakley and Davy 2018).
Now unfortunate icons of the threats posed by GCC, corals are a symbiosis
between anthozoan animals (Phylum Cnidaria) and photosynthetic dinoflagellates
in the Symbiodiniaceae, such as Symbiodinium. Coral reefs, engineered by
scleractinian stony corals, are hotspots for production and biodiversity despite
covering just 0.1% of the ocean surface (Roberts et al. 2002). Their associated
ecosystem services (e.g., coastal protection, food products, and tourism) are valued
in the trillions of USD, and sustain roughly 10% of the world’s human population
(Costanza et al. 2014). However, stress from a variety of factors, including warming
and pollution, can convert microbial mutualists into parasites (Vega Thurber et al.
2014; Morris et al. 2019; Hughes et al. 2017a; Fitt et al. 2001; Baker et al. 2018).
Since most of the animal’s energy is obtained from the photosymbionts, extended
periods of loss result in partial to complete coral mortality. Effects of bleaching
radiate outward, for example, homogenizing coral-supported communities
(Richardson et al. 2018).
In addition to gradual increases in surface temperature, heat waves are causing
ever more widespread, severe, and frequent mass bleaching events, limiting corals’
capacity to recover (Hughes et al. 2018). Three pan-tropical mass bleaching events
occurred between 1997 and 2016 with just 1
C warming (Hughes et al. 2017b). And
the Great Barrier Reef suffered its most widespread bleaching event in 2020; the
third event in just 5 years. The latter is especially troubling given that it occurred in
the absence of a normally facilitating El Niño event (Stone 2020). Intensifying
270
K. M. Oliver and C. H. V. Higashi