The free-living Symbiodinium reservoir and scleractinian coral symbiont acquisition

Abstract

The genus Symbiodinium (Dinophyceae, Suessiales), a group of geographically widespread marine dinoflagellates, comprises numerous ecologically and genetically distinct taxa. Symbiodinium spp.establish intracellular symbioses with cnidarians, infecting hosts such as jellyfish, sea anemones, octocorals and reef-building corals. The energetic demands of hosts can be met at varying levels by Symbiodinium through translocation of photosynthetically-fixed carbon, assisting in the formation of coral reefs. The cnidarian-dinoflagellate symbiosis is relatively well studied. In contrast, empirical data for population dynamics, distribution, and physiology of free-living Symbiodinium are limited.Therefore this thesis aims to characterise the dynamics between free-living Symbiodinium cells, hosts, and their habitat.Free-living Symbiodinium cells have been identified in reef sediment, and this study investigated whether adult coral colonies are a significant source of benthic Symbiodinium cells. Acropora millepora colonies were translocated to bare patches of sediment (Heron Reef, Great Barrier Reef, Australia), and the surrounding sediment was sampled for Symbiodinium cells over a period of 18 months. An 8-fold increase in visually-identified cells relative to the background population was recorded from the sediment at the immediate base of coral colonies. Symbiodinium abundance returned to background levels after the removal of the coral. These fine-scale changes in the distribution of cells suggests that hard-coral may be an important 'passive' source of Symbiodinium.The 'active' supply of Symbiodinium cells to the benthos was investigated through direct contact of coral tissue with sediment. Symbiont loss was induced in the coral Pocillopora damicornis through burial of coral branches. Peak cell abundance in the sediment occurred after four days, but the photophysiology (F v /F m ) of the Symbiodinium cells was significantly impaired. By day 12, Symbiodinium cells were present only in low concentrations in sediment samples, and the majority of cells were substantially degraded. In this study, Symbiodinium appeared to survive only transiently following expulsion, with an approximate window of viability of seven days. Such a period may be sufficient for coral recruits to make contact with potential symbionts in situ.Elevated temperatures induce a range of serious, deleterious effects in Symbiodinium. The potential amelioration of these effects was investigated, testing the hypothesis that Symbiodinium cells find refuge from stress when cultured within sediment. An exclusively free-living clade A (not known to form symbioses) and the symbiosis-forming type A1 were grown with or without a microhabitat of carbonate sediment at 25°C, 28°C or 31°C. The exclusively free-living clade A was physiologically superior to symbiosis-forming A1 across all measured variables and treatment combinations: it iii reproduced faster within the sediment, exhibited high levels of motility and maintained a stable maximum quantum yield (F ...

Chronic disturbance modulates symbiont (Symbiodiniaceae) beta diversity on a coral reef

“…However, we view www.nature.com/scientificreports www.nature.com/scientificreports/ thousands of kilometers away may be limited 63,64 . This idea is supported by previous research demonstrating that Symbiodiniaceae tend to disperse locally 65,66 and endosymbiotic Symbiodiniaceae are competent in the water column for only approximately seven days 67 , while coral larval competency can be one month or more [68][69][70] .…”

Population genetics of reef coral endosymbionts (Symbiodinium, Dinophyceae)

“…When viewed from a symbiont-centric perspective, one can posit Symbiodinium employs a strategy of pathogen-like shedding, wherein cells actively released from current hosts are not related to density regulation but rather a means to reach new habitats, or in this case, additional host individuals capable of establishing a symbiosis (reviewed by Holt & Barfield 2006). Under this hypothesis, it is improbable that released cells need to be indefinitely free-living, which is consistent with longevity estimates for Symbiodinium of~7 days in the environment (Nitschke 2015). Instead, Symbiodinium sources are continually renewed and survival of individual cells is likely diminished due to decreased habitat suitability, increased uncertainty and limited access to nutrients.…”

Latitudinal Variation in the Molecular Diversity and Community Composition of Symbiodiniaceae in Coral From the South China Sea

“…Biogeographical and ecological studies have shown that the center of Symbiodiniaceae diversity overlaps with the “Coral Triangle” (LaJeunesse et al, 2012), which is close to the IR and LR and far away from the HC and HR. The physiological evidence suggests that Symbiodiniaceae swim just 3–10 m over the course of one day (Fitt and Trench, 1983), and have a lifespan of approximately seven days in their natural environment (Nitschke, 2015). They are most likely dependent on sea currents for dispersal (Wirshing et al, 2013; Thornhill et al, 2017).…”