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Important light protecting mechanism in reef symbiotic partnerships revealed

10 January 2017

Researchers from The University of Technology Sydney, studying the impact of light stress on the symbiotic partnership between reef building organisms and associated microalgae, have revealed a previously unrecognised mechanism by which the partners reorganise symbiont location to maintain the health of the relationship. The important insights gained may help bridge existing knowledge gaps about other important symbiotic partnerships, such as in corals, where the breakdown of this partnership, results in coral bleaching, the scientists say.

Light is the principle driver of productivity in coral reef ecosystems, due to the many symbiotic partnerships formed with photosynthetic algae. To maintain the health of these partnerships, the amount of light received by the algae must be tightly regulated, and it is generally assumed this regulation comes under the role of the algal partner. However the research team demonstrated that it was the host, in this case the foraminifera Marginopora vertebralis , that instigated the process to protect stressed algae and, ultimately, the symbiotic relationship.

Marginopora vertebralis is a large sediment dwelling microorganism that calcifies and is a major contributor to formation of reef sands. It lives in symbiosis with Symbiodinium  a photosynthetic  microalgae better known for its association with hard corals,” says Dr Katherina Petrou from UTS Science School of Life Sciences.

Symbiodinium (red) within individual chambers of M.vertebralis test (green)

Symbiodinium (red) within individual chambers of M. vertebralis (green)

Sequential whitening of M.vertebralis exposed to high light. K.Petrou/D.Nielsen

Sequential whitening of M.vertebralis, top left to bottom right, exposed to high light. K.Petrou/D.Nielsen

Dr Petrou said that the newly observed mechanism, recently described in The ISME Journal, plays an important and previously unrecognised role in protection from light stress in tropical reefs.

Using fluorescence microscopy and chemical inhibitors, video analysis of the algal symbionts showed that, when subjected to high-light stress, the symbionts moved deeper within the host’s cell structure.

“The host receives a signal from the algae indicating they are under stress, the infochemical causes the host to mobilise proteins that pull the algae to an area of lower light thus protecting the health of the partnership,” Dr Petrou says.

“It’s likely that the ability to change the intracellular position of the algal symbionts helps optimise productivity and minimize photosynthetic damage,” she says.

“It’s quite remarkable to observe and record this vertical migration away from high light. It demonstrates a link between the symbiont stress and the host’s regulation of symbiont positioning and this indicates direct communication between the partners. It’s not just the flagellated movements of the algae as previously thought,” says co-author Dr Daniel Nielsen.

The research team, that included UTS Climate Change Cluster Director, Professor Peter Ralph, believe the new insight can serve as a platform for understanding host-microbe communication in other important photo-symbiotic organisms, such as corals.

Publication details:

“A novel mechanism for host-mediated photoprotection in endosymbiotic foraminifera” Katherina Petrou, Peter J Ralph and Daniel A Nielsen. The ISME Journal (2016) doi:10.1038/ismej.2016.128