SYMBIONT THERMAL HISTORY AND SENSITIVITY INFORM PHOTOCHEMICAL PERFORMANCE IN THE SEA ANEMONE EXAIPTASIA DIAPHANA DURING ACUTE HEATING
Date
2023-05
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Increasing seawater temperature due to global climate change is one of the
largest threats to coral reefs. Excess heating often results in the host expelling the
symbiotic algae, leading to a phenomenon commonly called coral bleaching.
However, in some symbioses, these algae have the capacity to adjust to heating and
can modulate this stress response. I examined how two genotypes of the symbiotic
dinoflagellate Breviolum minutum responded to acute heat stress in the sea anemone
Exaiptasia diaphana. Two phenotypes were tested for each genotype, one of which
was thermally tolerant and previously exposed to gradual heating in free-living
culture, and the other of which was thermally sensitive (i.e., wild-type) and naïve to
heating. Anemones were acclimated to two different light levels to determine if
photoacclimation differed by symbiont type, and if photoacclimation and symbiont
type interacted to change the heat stress response. Anemones were then exposed to six
hours of acute heating (from 24°C to 28, 32, 34, and 36°C), followed by a 16-hour
recovery period at 28°C. At the end of acute heating, the maximum quantum yield of
photosystem II (Fv/Fm) decreased with increasing temperature for all groups, but
thermally selected algae exhibited less decline at low light levels. By the end of the
recovery period, anemones exposed to 34°C showed some recovery in
photochemistry, while Fv/Fm remained low in anemones exposed to 36°C. While Fv/Fm
was sensitive to short-term heating, symbiont density and chlorophyll concentration
remained similar to control values, and animal-based reactive oxygen species
production did not increase with temperature. Overall, thermally selected symbionts
did confer some tolerance to the host, and acute heat stress may predict the overall
thermal tolerance of host-symbiont pairings. However, both the particular response
variables and the time at which they are measured are critical for accurately
determining short-term thermal responses in this symbiosis.