jack c koch

scientist | creator | educator

Cnidarian-Algae Symbioses

Coral reefs are one of the ecosystems that are suffering the most from global climate change and the secondary effects that it brings including warming oceans, sea level rise, and ocean acidification (OA). Coral reef ecosystems are some of the most productive and diverse on the planet, yet they cover less than 1% of the Earth’s surface, making them extremely vulnerable to loss (Odum and Odum 1955).

 

The structural foundation of these ecosystems is constructed by scleractinian (calcifying) corals. Corals belong to the phylum Cnidaria, a large animal group including jellyfish, hydroids, sea anemones, and corals. Despite their importance, cnidarians are simple animals consisting of two tissue layers, an ectoderm and an endoderm (gastroderm). The mesoglea is a gelatinous matrix sandwiched between the two tissues. The ectodermis is the outer tissue layer and is in contact with the external environment. The endodermis lines the inside of cnidarians creating the gastrovascular cavity (GC), where digestion takes place.

 

The productivity of coral reef ecosystems is dependent upon a multi-partite relationship between corals and their resident microbe and microalgal symbionts. Corals however are not the only cnidarians to participate in symbiosis with microalgae. In fact, some jellyfish and sea anemones also engage in symbiosis with algal partners, contributing to primary production in tropical, sub-tropical, and temperate waters (Shick 1991). Often the term endosymbiotic is used to describe these microalgae because the smaller symbiont cell is housed within a larger host cell. Specifically, microalgal symbionts are housed intracellularly in host gastrodermal cells within host-derived vesicles called symbiosomes.

 

In cnidarian-algae symbioses, the host provides a stable light environment conducive to photosynthesis, enhanced protection from grazers, inorganic nutrients such as phosphorus and nitrogen, and dissolved inorganic carbon (DIC), which is a vital ingredient for photosynthesis (Harland and Davies 1995). In return, the algae provide photosynthetic reduced dissolved organic carbon (DOC) to the host for growth, metabolism, reproduction, and survival (Muscatine 1990).

 

One of the most important products transferred from the host to the symbionts is DIC. However, the location of symbionts deep within host tissues is not ideal for DIC transfer through the ectoderm. The more efficient means of moving exogenous DIC to intracellular symbionts is via the GC and through the endodermis. In addition, the preferred form of DIC for photosynthesis is CO2, yet the predominant form of DIC in seawater is bicarbonate (HCO3-). Although there are physiological mechanisms in place to convert HCO3- to CO2, namely carbonic anhydrases (CAs), the concentration of DIC forms in seawater can also be altered by pH. Thus, the importance of the pH microenvironment in the GC, to the success of the symbiosis.