title: Influence of temperature on the physiological performance of zoochlorellae in two intertidal hosts (Anthopleura elegantissima and A. xanthogrammica)
Creators: name: Woodhouse, Gemma Smith
Publisher: name: Western Washington University
Year: 2014

description: <p>The ability of a symbiotic organism to tolerate and respond to stress is dependent on a complex integration of the physiological processes of both host and symbiont. In the intertidal zone, where organisms are exposed to numerous environmental stressors, physiological tolerance limits of algae and animals are often within 1°C - 3°C of the body temperatures they experience there. To understand the association between intertidal sea anemones and their photosymbionts, and how these associations may change with increasing climatic stress, I examined two spatially dominant species in the genus Anthopleura (A. elegantissima and A. xanthogrammica) in symbiotic associations with their relatively sensitive chlorophyte photosymbiont, Elliptochloris marina. Anemones hosting E. marina were exposed to an increasing thermal regime from 10 - 28°C, under two light treatments, over the course of 10 weeks to establish the upper thermal tolerance limit of E. marina in each host, while examining the response of the anemones themselves to the thermal stress. Of the two hosts, A. xanthogrammica was less tolerant of high temperatures. A contraction response was triggered for A. xanthogrammica at temperatures above 18°C, but A. elegantissima showed no contraction until temperatures reached 24°C. To determine how the E. marina were responding to the temperature increases, I examined the photochemical efficiency of PS II by measuring photosynthetic efficiency (Fv/Fm) and photosynthetic capacity (rETRmax) of symbionts within each anemone host at each temperature interval. Photochemical efficiency was strongly affected by temperature; however, there were no apparent host-specific differences. From 10 - 22°C, Fv/Fm remained stable, averaging 0.6 ± 0.1 (SD) for both species. At temperatures above 22°C, photochemical efficiency steadily declined, indicating photoinhibition and the upper thermal tolerance limit of E. marina. This relatively low thermal tolerance may influence the competitive balance of symbionts under conditions of increasing global temperatures. Increasing temperatures may cause anemones to adaptively expel symbionts to switch to a more tolerant species. In this study, both A. elegantissima and A. xanthogrammica expelled symbionts as the temperatures rose. By 28°C, both Anthopleura species had expelled the majority of their symbionts; however, A. xanthogrammica was able to retain a higher percentage (A. elegantissima: 96.7 ± 4.6 % loss; A. xanthogrammica: 84.0 ± 18.1 % loss), indicating that they may have an increased ability to buffer temperature changes and maintain algal symbioses during prolonged periods of high temperatures. Field measurements of the internal body temperatures of A. xanthogrammica indicated that the anemone has a moderate ability to buffer its symbionts from thermal stress, as the internal body temperatures of lower intertidal anemones remained 6.2 ± 1.1°C cooler than ambient temperatures. This ability to moderate the internal temperature is likely due to host-specific morphological traits, such as a large body size and thick host tissues, which may ultimately provide a more favorable environment for their symbionts under periods of high stress. The relative abilities of A. elegantissima and A. xanthogrammica to buffer their symbionts, as well as the physiological tolerances of E. marina, may have important ecological implications, controlling the range of zoochlorellae at both latitudinal and microhabitat scales.</p>