Impacts of nursery-based propagation and out-planting on coral-associated bacterial communities

Efforts to manage coral reef declines are increasingly turning towards in situ propagation of corals to aid reef recovery. Understanding the factors that influence 'success' throughout the propagation process is therefore critical to ensure efforts are viable and cost-effective, yet the extent to which propagation practices potentially impact the underlying coral biology remains unknown. Given growing evidence for the importance of the coral microbiome, we examined the influence of nursery-based propagation and out-planting on the bacterial communities of two coral species-Acropora millepora and Pocillopora verrucosa-increasingly propagated on the northern Great Barrier Reef (Opal Reef). Bacterial communities of coral fragments were characterised over four months of nursery propagation (sampling points: zero, seven and 125 days) and one month of subsequent out-planting (sampling points: zero, one and 30 days). Bacterial community structure differed between A. millepora and P. verrucosa throughout the experiment and species-specific temporal dynamics were observed during the transplantation of corals into nurseries and subsequent out-planting back to the reef. P. verrucosa bacterial community structure remained stable over time in the natural reef environment and within the nursery. In contrast, A. millepora bacterial communities within the nursery significantly changed over time, whereas those associated with source colonies within the natural reef environment remained unchanged. However, after one month of out-planting, the composition, richness and diversity of A. millepora bacterial communities were not statistically different to those associated with the source colonies. We interpret the transient shift of A. millepora bacterial communities within the nursery as an impact of distinctive environmental conditions in nurseries compared to natural reef settings, and the greater responsiveness of A. millepora bacterial communities to environmental change. Our observations highlight that different coral species exhibit distinct microbial responses to coral propagation and out-planting, and we recommend that these should be considered when designing and scaling future coral management strategies.

Automated summary

The study found that different coral species exhibit distinct microbial responses during coral propagation and out-planting. Specifically, the bacterial community of A. millepora significantly changed within the nursery environment but returned to a similar composition as the source colonies after one month of out-planting. In contrast, the bacterial community of P. verrucosa remained stable throughout the experiment. These findings suggest that coral species have different microbial dynamics during propagation and out-planting.