Abstract
Collodaria (Retaria) are important contributors to planktonic communities and biogeochemical processes (e.g. the biologic pump) in oligotrophic oceans. Similarly to corals, Collodaria live in symbiosis with dinoflagellate algae, a relationship that is thought to explain partly their ecological success. In the context of global change, the robustness of the symbiotic interaction and potential subsequent bleaching events are of primary interest for oceanic ecosystems functioning. In the present study, we compared the ultrastructure, morphology, symbiont density, photosynthetic capacities and respiration rates of colonial Collodaria exposed to a range of temperatures corresponding to natural conditions (21°C), moderate (25°C) and high (28°C) thermal stress. We showed that symbiont density immediately decreased when temperature rose to 25°C, while the overall Collodaria holobiont metabolic activity increased. When temperature reached 28°C, the holobiont respiration nearly stopped and the host morphological structure was largely damaged, as if the host tolerance threshold has been crossed. Over the course of the experiment, the photosynthetic capacities of remaining algal symbionts were stable, chloroplasts being the last degraded organelles in the microalgae. These results contribute to a better characterization and understanding of temperature-induced bleaching processes in planktonic photosymbioses.
Highlights
IntroductionWithin Radiozoa, Collodaria have been recently unraveled as key players in oceanic ecosystems (Biard et al, 2016; Guidi et al, 2016)
Retaria encompass Foraminifera and Radiolaria sensu stricto or Radiozoa
Bleaching is commonly described in the phylum Cnidaria as a loss of color originating in the exclusion of the symbiotic dinoflagellates from the animal, and/or the degradation of photosynthetic pigments in the chloroplasts of the symbionts (Douglas, 2003)
Summary
Within Radiozoa, Collodaria have been recently unraveled as key players in oceanic ecosystems (Biard et al, 2016; Guidi et al, 2016). Overlooked because of their fragility, the use of non-destructive, in situ, imaging tools showed that Collodaria represents nearly 30% of zooplankton biomass at subsurface in oligotrophic oceans (Biard et al, 2016). Collodaria were reported as central actors in plankton community networks related to carbon export to the deep ocean via both primary. Haeckel toward the end of the 19th century (Haeckel, 1887), Collodaria have been little studied so far, with only a few landmark studies conducted on radiolarian morphology and physiology (Anderson, 1983)
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