Mutualism

Mutualism

A form of symbiosis that provides an obvious benefit to both the symbiont and the host.

Background

• Reef-building corals, in partnership with their algae symbionts, have been on Earth for at least 200 million years.

Definition

• Mutualism is a symbiotic relationship in which both the host organism and the symbiont benefit. However, the coral and algae mutualistic relationship is a little more complicated than the definition because the effects on the symbiotic algae are less clear than the effects on the hosts.
  • The mutualistic relationship has clear benefits for the reef-building corals. However, the benefits for the photosynthetic dinoflagellates in the corals, Symbiodinium, are less clear.
  • It is important to remember that the conditions for a healthy relationship between the coral and the algae have fairly strict boundaries as far as water temperature, ocean acidity, and amount of sunlight.
  • Because the coral-algae mutualistic relationship is dependent on a variety of factors, it is difficult to define a clear benefit for the symbiotic algae. The relationship can be a "shifting continuum" that can "under specific conditions shift into parasitism."
    • As said previously, environment factors, such as increased temperature and dirty water, can strain the relationship between the coral and the algae. This strain can occasionall cause the costs of the relationship to outweigh the benefits.

Biology

• Corals are from the phyla Cnidaria
• Because corals have a large area to volume ratio, the algae on the corals are able to absorb a lot of light for photosynthesis, given ideal conditions.
• Over 700 species of coral have been found to be in symbiosis with dinoflagellate algae.

Subcellular Arrangement (Yellowlees 682)

• • “The dinoflagellate cell contents are surrounded by the algal plasma membrane and cell wall; this is in turn surrounded by multiple-layer host-derived membrane complex (the symbiosome), the coral endodermal cell and finally the host plasma membrane. This multi membrane architecture necessitates that metabolites that pass between the alga and host must pass through multiple cell membranes.”

Relationship

Autotroph-Heterotroph (Saffo)

• “The alga requires nutrients that are derived from the host or from the environment surrounding the host, and the host acquires photosynthetic products from the autotrophic alga” (Yellowlees 680)
• because mutual benefit occurs when the coral and the alga are together, studying the benefits in action is difficult (Yellowlees 680)

Benefits to Corals:

• dinoflagellates produce 90% of the carbon that corals need through photosynthesis (Saffo)
  • “In some coral species, much of the photosynthetic carbon is allocated to mucus, which is produced in copious amounts and is important in feeding and protection from mechanical damage and pathogens” (Venn 1074)
  • also remove respiratory carbon dioxide from the corals to use in photosynthesis (Yellowlees 680)
• dinoflagellates release glycerol, sugars, organic acids, and amino acids (Venn 1073)
  • these are all “mobile” compounds that are “compounds translocated to the animal tissues” (Venn 1072)
• dinoflagellates enhance growth of coral skeleton, giving it stability for health and “physical architecture” (Saffo)

importance seen in “coral bleaching” - corals often die when dinoflagellates leave, causing the coral to turn white (Saffo)

• • “impacts of bleaching included reduced growth and reproduction, increased susceptibility to disease and mechanical damage, and occasionally death of the host” (Venn 1074)

Benefits to Alga:

• corals help transport carbon from the surrounding seawater to the photysnethic dinoflagellates (Yellowlees 683)
  • The host has an enzyme that acidifies the boundary layer to convert carbon dioxide. “This CO2 subsequently diffuses into the host where it is trapped by conversion to bicarbonate and is then transported to the symbiosome by an unknown pathway.” (Yellowlees 683)
• corals provide alga with habitat
• again, hard to determine