Diversity, distribution, and dynamics of coral reef bacterial communities across the Hawaiian archipelago and wider Pacific Ocean

Abstract

A comprehensive investigation characterizing the diversity, distribution, and dynamics of coral-associated bacterial communities was carried out through a series of targeted observational and experimental studies. A focus was placed on four overarching research themes deemed fundamental to understanding these complex communities: method optimization, biogeography, natural variability, and environmental effects on bacterial community structure. Protocols for genomic DNA extraction and Polymerase Chain Reaction (PCR) amplification of bacterial 16S rRNA genes were optimized for high-throughput analyses of coral-associated bacterial communities associated with common Hawaiian corals. Successful gene amplification and community profiles varied with sample pre-treatment/nucleic acid extraction method combination and differed significantly between Porites lobata and Porites compressa corals. A baseline assessment of coral-associated bacteria associated with Porites lobata throughout the Hawaiian Archipelago and Indo-Pacific Ocean revealed a weak, but significant isolation by distance pattern, indicating that geographic isolation is partially responsible for driving observed divergences at the archipelagic scale. However, some geographic groupings were not distance-based, indicating that additional environmental factors may also affect the distributions of these communities. The natural variability of bacterial communities associated with colonies of Porites compressa was investigated over a five-month period at two sites around Molokaʻi, Hawaiʻi. Terminal-restriction fragment length polymorphism analysis of PCR-amplified 16S ribosomal RNA genes revealed that bacterial communities did not differ between colonies within a site but differed between sites and fluctuated from month-to-month. Analysis of environmental data revealed site differences in environmental parameters (seawater salinity, temperature, turbidity, fluorescence, chlorophyll a, and nutrients), but for the most part, did not significantly correlate with bacterial community structure (with the exception of silicate at one site). To further investigate the effects of environmental parameters on associated bacteria, an array of experimental aquaria were used to test the effects of increased temperature on bacterial community structure and photophysiology of Porites compressa corals. While the temperature treatment rapidly impacted the photophysiology of the coral host, it did not elicit a statistically significant shift in bacterial community structure. Information gleaned from this research has expanded our limited knowledge of coral-associated bacterial communities and has brought us closer to understanding the roles of bacteria in coral health.