Horizontal transmisson of Symbiodinium

Symbiodinium dinoflagellates establish symbiont relationships with a variety of hosts, including, Cnidarian coral. Symbiodinium can be free living in the water column or can be attained through symbiont acquisition. 85% of Cnidarian species host Symbiodinium through indirect transmission (Fournier, 2013) and Symbiodinium clades establish non-random relationships with a host species. This paper highlights there is little knowledge on how corals first acquire their symbionts, they look into the growing field of the ecology of free-living Symbiodinium types and those symbioses with juvenile corals to the benthos.

In order to fully appreciate this paper it is important to note the two modes of transmission for the uptake of Symbiodinium which can occur in cnidarian species; horizontal and vertical. This paper focuses mainly on horizontal transmission (indirect, from the water column), however vertical (Direct, from parent) is spoken about for comparison. Evidence for vertical transmission of Symbiodinium cells may not be a ‘closed’ circuit and corals using vertical transmission may uptake exogenous Symbiodinium cells may successfully colonise on adult hosts.

Nitschke et al used three species of newly settled aposymbiotic (symbiont-free) corals (Acropora millepora, Acropora selago and Isopora palifera) in an open aquarium system containing; (i) sterilised sediment and adult coral fragments combined; (ii) adult coral fragments alone; (iii) sterilised sediments alone or (iv) seawater at Heron Island, GBR. The colonies were placed within aquariums and monitored at Heron Island Research Station. The settlement for juvenile corals of the Acropora species were tested with filtered seawater to remove particulates and sediment before distribution in the aquaria. Symbiont acquisition was investigated through visual counting, with the aid of a light microscope, at six time points within a 12 day period. Days counted were determined based on which coral spp. was used, between Acropora spp and Isopora palifera.

Scleractinian corals expel photosynthetically active symbionts at a constant rate and in large quantities. Results found similar patterns of symbiont transmission across the two broadcasting coral species: A. millepora and A. selago. Symbiodinium was expelled originally in the coral alone treatment after 7days, after this symbiont acquision continued at a rapid rate.

Results showed that in seawater alone juvenille corals acquired Symbiodinium cells by the end of the experiment; A. millepora (19%), A. selago (14%) and I. palifera (18%). This suggests that seawater alone could be a potential source of Symbiodinium, however as the % values show there is low abundance of recruitment. Graham et al (2008) demonstrated that aposymbiotic coral larvae can survive in the water column for 200 days in vitro, showing that recruits may survive for extended periods without Symbiodinium cells. Other research concluded that Symbiodinium can colonise in sediment, this could show that sediment may provide another habitat for the symbionts to grow and survive until taken up by a juvenile host, although, it is important to understand if these colonies are clade specific.

It is understood that some scleractinian, reef-building corals inhibit nutrient poor waters, highlighting the importance of understanding horizontal transmission of Symbiodinium cells between adult and juvenile corals, as this symbiosis is necessary for nutrient recycling in oligotrophic waters. The knowledge in this area could be improved by further understanding into the density of Symbiodinium in the water column and to investigate their free living nature until chemotaxis occurs.

Paper reviewed:

Nitschke, M.R., Davy, S.K. and Ward, S., 2016. Horizontal transmission of Symbiodinium cells between adult and juvenile corals is aided by benthic sediment. Coral Reefs, 35(1), pp.335-344.

References:

Fournier, A. (2013) The story of symbiosis with zooxanthellae, or how they enable their host to thrive in a nutrient poor environment. BioSciences Master Reviews, 1-8.
Graham EM, Baird AH, Connolly SR (2008). Survival synamics of scleractinian coral larvae and implications for dispersal. Coral Reefs 27:529-539