Under stress conditions, anemones that host different species of algae could be choosing which symbiont they expel

Background/Question/Methods

Coral reefs are experiencing significant impacts due to the bleaching events related to higher sea water temperatures, changes in irradiance and loss of their algal symbionts (Symbiodinium spp.). Recent studies have revealed high genetic diversity within the genus Symbiodinium, raising the possibility that flexibility in the symbiosis may make corals more resilient to climate stress than has been previously thought. Corals may become more stress tolerant through selection for more thermo-tolerant symbionts. The underlying physiological and cellular processes that lead to these shifts, however, are largely unknown.

Under heat and light stress, sea anemones in the genus Anthopleura can shift their symbiont populations from the more temperature sensitive  green chlorophyte (Elliptochloris marina), to the more thermo-tolerant dinoflagellate Symbiodinium muscatinei. These 2 symbionts are readily identifiable by microscopic examination, producing an excellent opportunity to study changes in symbiont complement.

To gain a better insight on the underlying physiological factors that affect symbiont drift and determine if A. elegantissima can selectively expel their heat and light sensitive green algal symbionts (E. marina) in favor of  the more tolerant brown ones (S. muscatinei),anemones with mixed populations of the two symbionts were exposed to a high light/high temperature (20°C) treatment or to control conditions (low light/8°C) for 5 days. The water from the container holding each anemone was collected each day, centrifuged to concentrate the expelled symbionts, and frozen for later symbiont counts. On day 5, the anemones were homogenized and symbiont complements were counted.  By comparing the ratios of green/brown symbionts expelled to those in the anemone homogenates, we were able to determine whether the anemones were selectively expelling the green symbionts.

Results/Conclusions

We did not find great statistical significance for most of our data. However, our data suggets that the less resilient green symbionts (zoochlorellae) are being expelled from the anemone at a higher rate and proportion than the more tolerant brown zooxanthellae. Many factors such as initial anemone symbiont density (brown:green cells inside the host) and width of anemones tissues could have affected the expulsion response of the anemones to the experimental conditions. Increasing plot size and selecting anemones with similar symbiont populations and similar tissue width could help measure less variant data.