Symbiodiniaceae in and ex hospite have differential physiological responses under different heat stress scenarios

ABSTRACT

Extreme seawater temperatures are linked to increasingly frequent cnidarian bleaching, a breakdown of the symbiosis between host and endosymbiotic dinoflagellates. This can be driven by several anthropogenic stressors, of which seawater warming is a major and frequent one. It remains unclear whether this is caused by intrinsic properties of the host or exclusively by the symbiont. This study investigates the response to elevated temperatures of dinoflagellate symbionts in and ex hospite, particularly to find out whether thermal tolerance differs between endosymbionts and free-living micro-algae. Maximum quantum yield of photosystem II (F_v/F_m) and symbiont cell density were measured in seven different cnidarian species and five cultures of isolated endosymbionts subjected to temperatures of 26°C (control), 30°C and 34°C for 21 days. Isolated dinoflagellate cells showed a susceptibility to elevated temperatures of 30°C, evidenced by a decrease in photochemical efficiency and cell density of the cultures, as well as a progressive disintegration of cellular structures and loss of pigmentation of all but two cultures during the first week of exposure. Bleaching of coral holobionts at 30°C could be explained by reduced density of algae cells in host tissue. This effect was particularly evident in soft corals. Exposure to 34°C resulted in drastic bleaching of stony coral species, anemones and jellyfish, and even the death of soft corals. Data from this study show different responses to elevated thermal conditions across several species and different genera of symbionts and emphasize the important role of dinoflagellate symbionts in the holobiont response to elevated seawater temperatures.

KEYWORDS:

• Bleaching
• corals
• PAM fluorometry
• photochemical efficiency
• stress
• holobiont

Acknowledgements

The authors wish to thank Jörg Christian Frommlet from the University of Aveiro, Portugal for providing cell cultures. Nico Steinel, José Garciá Barranca and Beatrice Brix da Costa for assistance with experimental setup and Epiphane Yéyi and Sebastian Flotow for their technical assistance. Many thanks also to Nora Diehl for excellent support with the statistical analysis.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Availability of data (data transparency)

The data that support the findings of this study are available at the Centre for Open Science database at 10.17605/OSF.IO/Q3ZJV.

Additional information

Funding

This research was supported by inhouse funding of ZMT (CC 6073).