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 (Fv/Fm) 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 reproduced faster within the sediment, exhibited high levels of motility and maintained a stable maximum quantum yield (Fv/Fm), In contrast, A1 exhibited dramatic declines in cell concentration and cell motility when cultured in sediment, most severely at 31°C. These data suggest that symbiosisforming Symbiodinium types may live only transiently in sediment or outside coral hosts.
Most scleractinian coral species produce aposymbiotic juveniles that must acquire Symbiodinium from a free-living reservoir, but the nature of the source remains unclear. The final study examined whether juvenile corals derive Symbiodinium cells from a benthic population, and if the added presence of adult coral (a conspecific) enhances symbiont acquisition. This question was investigated at Heron Reef with two broadcast spawning species, Acropora millepora and Acropora selago, and the brooder Isopora palifera. Newly-settled, aposymbiotic corals were maintained in open systems containing: sediment + adult coral fragments; adult coral fragments; sterilised sediment or seawater. For the Acropora species, the first instance of symbiosis was apparent by day seven of exposure to treatments. By day 12, approximately 70% of juveniles exposed to the combined treatment of sediment + adult coral had acquired Symbiodinium, compared with only 19% of those exposed to only seawater. Separately, exposure to adult coral or sediment produced intermediate acquisition in juveniles. By comparison, an accelerated but similar pattern was observed for I. palifera in that symbiotic juveniles were apparent after only four days of exposure to treatments. It thus appears that the supply of Symbiodinium from a symbiotic conspecific is indeed advantageous for recruiting corals, and it therefore follows that juveniles recruiting to recovering or damaged reefs devoid of specific Symbiodinium types (and their sources), may be seriously limited. The presence or absence of suitable free-living Symbiodinium types may prove to be a new aspect of coral reef health worthy of monitoring.
This thesis examined Symbiodinium in situ, in vitro and in hospite, providing new insight into the dynamics between free-living cells, potential hosts and known microhabitats. Particularly noteworthy results include the apparently transient survival of expelled Symbiodinium cells and how reductions in growth and cell motility during elevated temperature is type specific. Future studies should aim to determine how benthic reservoirs of Symbiodinium and the process of symbiont acquisition, a critical period in the coral life history, may change during the environmental conditions predicted for the future.