Mutualism

Mutualism

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

Background

• “Reef-building corals have been on earth, presumably in partnership with algal symbionts, for at least 200 My.” (Saffo, Mary Beth. "Mutualistic Symbioses." ELS (n.d.): n. pag. - ELS. 15 July 2014. Web. 20 February 2015.)

Definition:

• “a symbiotic association that is beneficial to both (or all) symbiotic partners” (Saffo)
• more complex than definition: effects on symbionts are less clear than those on hosts (Saffo)
  • ex. clear benefits for reef-building corals, but benefits for Symbiodinium (photosynthetic dinoflagellates in the corals) are less clear (Saffo)
  • defining a clear benefit for the symbiont within an association is difficult as “these associations can be a shifting continuum which, as has been proposed recently, can under specific conditions shift into parasitism” (Yellowlees, David, V. Rees, and William Leggat. "Metabolic Interactions between Algal Symbionts and Invertebrate Hosts." Plant, Cell, and Environment 31.5 (2008): 679-94. Wiley Online Library. John Wiley and Sons, 27 February 2008. Web. 20 February 2015.)
    • environmental factors, such as increased temperature, dirty water, and XX, can strain the relationship and cause these costs to outweigh the benefits of the relationship (Saffo)

Corals are from phyla Cnidaria (Venn, A. A., J. E. Loram, and A. E. Douglas. "Photosynthetic Symbioses in Animals." Journal of Experimental Botany 59.5 (2007): 1069-080. Journal of Experimental Botany. Oxford Journals, 10 February 2008. Web. 20 February 2015.)

• large surface area to volume ratio means that the dinoflagellates on the corals will be able to absorb a lot of light for photosynthesis (Venn 1070)
• over 700 coral species have been found to be in symbiosis with dinoflagellates (Yellowlees 683)

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