Research > Symbiosis

An essential component to the health, productivity and growth of a coral reef is the cnidarian-dinoflagellate symbiosis. The unicellular dinoflagellate alga, Symbiodinium sp. is the symbiont that is found within cnidarian hosts, such as corals and anemones and also molluscs and foraminiferans, is key to light driven calcification process that leads to reef formation. Although the diploblastic tissue structure of the cnidarian host is basic compared to higher metazoans, the relationship with Symbiodinum sp. is highly complex and to a large extent, still unknown. We aim to resolve questions are key to understanding this symbiosis that underpins the formation, health and survival of coral reefs under the effects of climate change. Specifically, our research covers the following three phases of the symbiosis:

1) Onset

The developing cnidarian host acquires symbionts through vertical and/or horizontal transmission. Vertical transmission is a closed system where symbionts are passed directly between host generations. Horizontal transmission is an open system, where symbionts are newly acquired from the environment by each host generation.

Focus: The mechanisms and processes that are active in symbiont acquisition and uptake, the host selection specificity of the symbiosis between one or more specific algal types or clades and the negotiation of the immune systems as part of a ‘winnowing’ process leading to an established mutualism between partners.

2) Maintenance, Health and Growth:

Once established, the symbiosis is dependent upon the translocation of carbon, nutrients, amino acids, sugars and other metabolites between partners. The metabolic demands of each partner may vary according to diurnal cycle or changes in environmental conditions.

Focus: The mechanisms and capacity of translocation on metabolic substrates and products between partners, the source of translocated products, the role of the products within the symbiosis and how changes in environmental conditions affect the stability and capacity of the symbiosis, symbiont populations, growth and ultimately health of the reef.

3) Breakdown, Bleaching, Mortality or Recovery:

The symbiosis may be disrupted by availability of nutrients, environmental change and stress, pathogens and disease. If the translocation of organic carbon and metabolic products is disrupted, reduced or fails due to a physiological stress, then this may result in the metabolic demands of the tissues and cells not being met resulting in a breakdown of the symbiosis, loss of symbionts and bleaching. Residual symbionts may switch to a parasitic relationship within the host. Equally, a disruption of cross talk between partners from the stress could lead to non-self recognition immune response and a termination of the ‘dysfunctional’ symbiosis. The breakdown of the symbiosis is then reflected in reduced or retarded growth to the coral and to the reef. This in turn sets a negative balance against reef erosion and damage and the reef declines.

Focus: Elucidating the cellular mechanisms the operate during bleaching as a result of the breakdown in the symbiosis, the capacity of the physiological thresholds to infection and changes in environmental conditions, and how these can lead to mortality or recovery and regrowth