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The ability of symbionts to recolonize their hosts after a period of dysbiosis is essential to maintain a resilient partnership. Many cnidarians rely on photosynthate provided from a large algal symbiont population. Under periods of thermal stress, symbiont densities in host cnidarians decline, and the recovery of hosts is dependent on the re-establishment of symbiosis. The cellular mechanisms that govern this process of colonization are not well-defined and require further exploration. To study this process in the symbiotic sea anemone model Exaiptasia diaphana , commonly called Aiptasia, we developed a non-invasive, efficient method of imaging that uses autofluorescence to measure the abundance of symbiont cells, which were spatially distributed into distinct cell clusters within the gastrodermis of host tentacles. We estimated cell cluster sizes to measure the occurrence of singlets, doublets, and so on up to much larger cell clusters, and characterized colonization patterns by native and non-native symbionts. Native symbiont Breviolum minutum rapidly recolonized hosts and rapidly exited under elevated temperature, with increased bleaching susceptibility for larger symbiont clusters. In contrast, populations of non-native symbionts Symbiodinium microadriaticum and Durusdinium trenchii persisted at low levels under elevated temperature. To identify mechanisms driving colonization patterns, we simulated symbiont population changes through time and determined that migration was necessary to create observed patterns (i.e., egression of symbionts from larger clusters to establish new clusters). Our results support a mechanism where symbionts repopulate hosts in a predictable cluster pattern, and provide novel evidence that colonization requires both localized proliferation and continuous migration.
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Comparative physiology reveals heat stress disrupts acid-base homeostasis independent of symbiotic state in the model cnidarian Exaiptasia diaphana
Climate change threatens symbiotic cnidarians’ survival by causing photosymbiosis breakdown in a process known as bleaching. Direct effects of temperature on cnidarian host physiology remain difficult to describe because heatwaves depress symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system to disentangle direct vs. indirect heat effects on the host, since it can survive indefinitely without symbionts. We tested the hypothesis that heat directly impairs cnidarian physiology by comparing symbiotic and aposymbiotic individuals of two laboratory subpopulations of a commonly used clonal strain of E. diaphana, CC7. We exposed anemones to a range of temperatures (ambient, +2°C, +4°C, +6°C) for 15–18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration, and host intracellular pH (pHi). Symbiotic anemones from the two subpopulations differed in size and symbiont density and exhibited distinct heat stress responses, highlighting the importance of acclimation to different laboratory conditions. Specifically, the cohort with higher initial symbiont densities experienced dose-dependent symbiont loss with increasing temperature and a corresponding decline in host photosynthate accumulation. In contrast, the cohort with lower initial symbiont densities did not lose symbionts or assimilate less photosynthate when heated, similar to the response of aposymbiotic anemones. However, anemone pHi decreased at higher temperatures regardless of cohort, symbiont presence, or photosynthate translocation, indicating that heat consistently disrupts cnidarian acid-base homeostasis independent of symbiotic status or mutualism breakdown. Thus, pH regulation may be a critical vulnerability for cnidarians in a changing climate.
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- Award ID(s):
- 1923743
- PAR ID:
- 10487624
- Publisher / Repository:
- The Company of Biologists
- Date Published:
- Journal Name:
- Journal of Experimental Biology
- ISSN:
- 0022-0949
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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