Coral-algal symbioses: Consequences of flexibility in symbiont uptake for newly settled coral recruits and juveniles
McIlroy, Shelby E.
MetadataShow full item record
Our understanding of coral reefs and their fate in a changing climate has been expanded by our ability to monitor the diversity and abundance of the algal endosymbionts, Symbiodinium, that sustain them. For many coral species, the obligate association with members of the phylogenetically and physiologically diverse genus Symbiodinium is established anew each generation whereby a diversity of Symbiodinium species are initially taken up, but winnowed down over time to a predictable assemblage found in adult corals. This dissertation research includes ecological and genetic studies to monitor patterns of symbiont uptake and retention for newly settled Orbicella faveolata and Briareum asbestinum and to consider the consequences of flexibility and diversity in symbiont uptake on host growth, survival, and nutrient acquisition. We found that: 1.) The timing of exposure to environmental symbionts altered Symbiodinium uptake in aposymbiotic and laboratory inoculated O. faveolata juveniles. This emphasized the beneficial role of early exposure to Symbiodinium, demonstrated a coral’s ability to continue symbiont uptake for many months following settlement, and highlighted the ability of natural strains to displace laboratory inoculates upon exposure to natural seawater. 2.) The identity of the Symbiodinium (A1 or B1) taken up by newly settled O. faveolata in the laboratory resulted in differences in host fitness proxies including growth but not survivorship or thermal tolerance. These experiments showed that Symbiodinium species have unique photosynthetic characteristics based on PAM Fluorometry data with Symbiodinium A1 shown to use the available light less efficiently relative to Symbiodinium B1. Despite this difference, coral polyps harboring Symbiodinium A1 grew more quickly and to larger sizes, an important demonstration that variation in symbiont quality goes beyond relative differences in photophysiology. 3.) Symbiont uptake in B. asbestinum varied with the presence of single vs. multiple symbiont types with evidence of competitive exclusion. The amount of available light modulated these interactions with some species of symbionts being excluded more often in low light environments. Overtime, the symbiont assemblages became more variable and symbionts seemed to loose their competitive edge suggesting that this initial difference in colonization ability may not have long lasting effects. 4.) The presence and relative abundance of two species of symbionts in B. asbestinum, Symbiodinium A1 and B1 influenced the acquisition rate of carbon and nitrogen by the symbionts and the translocation of those nutrients to the host. These results demonstrated physiological differences between symbiont species. Furthermore, there was some evidence of competitive interactions among symbionts with carbonate fixation being lower than expected for mixed in hospite Symbiodinium assemblages. However, complex ecological interactions among symbionts and between the host and symbionts were difficult to tease apart. Overall these findings confirm the importance of Symbiodinium identity on host phenotype. Flexibility and turn over in symbiont associations may provide long-lived hosts with an additional level of adaptation by which corals can alter their phenotype by altering their associated symbionts We found that transitions between dominant symbiont types can occur at various points in the early stages of the symbiosis but that these transitions may have negative consequences where hosts cannot prevent associations with sub-optimal or competitive species of Symbiodinium.