Meeting Abstract
Intimate partnerships between invertebrates and symbiotic microbial communities are widespread across marine habitats. Such interactions are exemplified on tropical reefs, where sessile organisms survive in nutrient-poor environments due to their association with autotrophic symbionts. Caribbean reefs are under considerable pressure from anthropogenic stressors (e.g. eutrophication) that may disrupt these symbioses by reducing light availability and altering water quality. To test whether these factors impact host growth and microbial community dynamics, we conducted a six-week factorial experiment examining the effects of reduced irradiance and elevated nutrient concentration on growth, chlorophyll concentration and cyanobacterial density in the tropical ascidian, Trididemnum solidum. Holobiont growth demonstrated a stepwise reduction in biomass as light intensity decreased. Chlorophyll concentrations were not significantly different among light treatments, however, photosymbiont abundance decreased significantly under lowered irradiance. Although nutrients had minimal impacts under control irradiance, when coupled with reduced light, increased nutrients resulted in decreased cyanobacterial density and a two-fold reduction in holobiont biomass. Moreover, T. solidum colonies subjected to the lowest irradiance and highest nutrient concentrations exhibited significant pigmentation loss and tissue death, indicating that invertebrate-microbe symbioses may be unable to compensate for multiple stressors working in conjunction. Overall, these results emphasize the importance of microbial symbionts for the survival and proliferation of marine invertebrates and highlight the necessity of examining multiple factors in combination to understand how anthropogenic stressors can disrupt invertebrate-microbe symbioses.
