Specificity versus flexibility in cnidarian-algal symbioses
Poland, Daniel Mark
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Many shallow water tropical marine invertebrates form obligate and ecologically invaluable symbioses with dinoflagellates in the genus Symbiodinium and among these hosts, corals and related cnidarians are the dominant reef fauna. In exchange for nutrients from algal photosynthesis the cnidarian host provides inorganic metabolic waste to the symbiont. Despite this intimate and ancient association, these symbioses are sensitive to environmental perturbations such as extremes in temperature or salinity, whereby symbionts are lost from the colony in a "bleaching" response. Research over the last two decades has uncovered great genetic and physiological diversity within the genus Symbiodinium. Thus, scientists are now examining if this physiological diversity can lead to altered partnerships to adapt to environmental changes, such as those expected with climate changes. If hosts only maintain one symbiont type over its geographical range, life history or with abiotic stress, this suggests that partnerships cannot rapidly change in response to environmental disturbances. Alternatively, diverse symbiont populations within single host taxa would suggest that there is diversity for natural selection to act upon and thus a potential mechanism to respond to environmental change. Here, I establish that the host model system Briareum asbestinum (Octocorallia: Alcyonacea: Scleraxonia) largely associates with a single type of clade B symbionts, namely the B. asbestinum -specific symbiont B178 (alphanumeric code based on clade [B] and the size [178bp] of a hypervariable region in chloroplast 23S rDNA). However, careful surveys in the Florida Keys uncovered Briareum asbestinum populations that associate with the octocoral-generalist symbiont B184, thus highlighting the need for more extensive sampling across geographic areas. Despite this biogeographic pattern, transplant studies using offspring that do not inherit symbionts from their parent showed that the parental symbiont type dominates the offspring upon maturation (4-5 years) regardless of the symbiont type harbored by local adults. Additionally, during initial acquisition, the symbiont populations in B. asbestinum juveniles were diverse and often dissimilar to the parental symbionts. For the first time, a long-term (8-11 months) experimental study demonstrated that physiological benefits to the host varied depending on the symbiont type initially acquired by the juveniles. Investigations also showed that adult B. asbestinum with either B178 or B184 symbionts exhibited similar physiological responses to heat stress of 1-3°C above ambient. Regardless of the symbiont type harbored, in two out three trials, colonies showed similar levels of symbiont reductions. However, in one trial, which coincided with the increased sea surface temperature of the 2005 Caribbean bleaching event, colonies with Symbiodinium B178 before the trial harbored B178+B184 or B184 symbionts by the end of the trial. In some cases these changes in symbiont algal populations were maintained during the summer of 2005. No changes were observed in colonies with B184 symbionts, and differential symbiont loss (bleaching response) was seen between colonies harboring the two different types at the start of the study. Together, these data show that despite diversity of symbiont types, many associations may be genetically predetermined between generations. Additionally, certain symbionts are necessary for survival of juveniles and that closely related symbionts in adults might differ only slightly in their response to stress. These attributes may slow the rapid adaptive changes necessary with the predicted increases in environmental stress brought by climate changes.