Genetic variation in Breviolum antillogorgium, a coral reef symbiont, in response to temperature and nutrients

Abstract

Symbionts within the family Symbiodiniaceae are important on coral reefs because they provide significant amounts of carbon to many different reef species. The breakdown of this mutualism that occurs as a result of increasingly warmer ocean temperatures is a major threat to coral reef ecosystems globally. Recombination during sexual reproduction and high rates of somatic mutation can lead to increased genetic variation within symbiont species, which may provide the fuel for natural selection and adaptation. However, few studies have asked whether such variation in functional traits exists within these symbionts. We used several genotypes of two closely related species, Breviolum antillogorgium and B. minutum , to examine variation of traits related to symbiosis in response to increases in temperature or nitrogen availability in laboratory cultures. We found significant genetic variation within and among symbiont species in chlorophyll content, photosynthetic efficiency, and growth rate. Two genotypes showed decreases in traits in response to increased temperatures predicted by climate change, but one genotype responded positively. Similarly, some genotypes within a species responded positively to high‐nitrogen environments, such as those expected within hosts or eutrophication associated with global change, while other genotypes in the same species responded negatively, suggesting context‐dependency in the strength of mutualism. Such variation in traits implies that there is potential for natural selection on symbionts in response to temperature and nutrients, which could confer an adaptive advantage to the holobiont.

Microbiome dynamics in the tissue and mucus of acroporid corals differ in relation to host and environmental parameters

“…is distinct between species, dynamic and flexible (corals associate with different Symbiodiniaceae types at different life stages, for example), and contributes significantly to physiological attributes of the coral holobiont. For example, different algal genotypes can affect the nutrient availability (e.g., carbon and nitrogen) in the coral holobiont ( Pernice et al, 2015 ; Bayliss et al, 2019 ). Environmental factors such as seawater temperature can also lead to temporal changes in the symbiont community ( Cooper et al, 2011 ; Howells et al, 2012 ; Rocker, Willis & Bay, 2012 ).…”

Corals in the hottest reefs in the world exhibit symbiont fidelity not flexibility

“…Recent studies show that Symbiodiniaceae populations do exhibit functional variation and thus have the potential to adapt. For example, populations of two Breviolum B1 species (B. antillogorgium and B. minutum) from the same host species and reef site encompass standing genetic variation in heat and nutrient tolerance, as well as photosynthesis traits (Bayliss, Scott, Coffroth, & terHorst, 2019). Additionally, a monoclonal culture of Cladocopium C1 became adapted to elevated temperature within 2.5 years (~80 asexual generations) and tended to improve the outcomes for inoculated coral hosts under heat stress (Chakravarti, Beltran, & van Oppen, 2017; see also Chakravarti & van Oppen, 2018).…”

Divergent symbiont communities determine the physiology and nutrition of a reef coral across a light-availability gradient

“…Differences in symbiont densities can also be influenced by Symbiodiniaceae growth rates and/or responses to nutrient availability [86]. For example, symbiont-derived photosynthates stimulate the recycling of ammonium waste by the host and nitrogen incorporation into amino acids, serving as a negative-feedback loop regulating symbiont densities by limiting nitrogen to the symbiont [87].…”