Bacterial and algal symbiont dynamics in early recruits exposed to two adult coral species

Abstract

Corals live in a symbiotic relationship with various microorganisms including bacteria, some of which are essential for host health and survival. Bacterial assemblages are typically highly diverse in juveniles and are speculated to exert roles critical to coral ontogenetic development. However, knowledge about bacterial dynamics in coral recruits is scarce, especially in brooders. In this study, we investigated the temporal dynamics of bacterial communities associated with Pocillopora acuta early recruits exposed to different microbial environments. One week after settlement, recruits were grown in a flow-through system with 0.4 µm filtered seawater either alone, or in the vicinity of adult Pocillopora acuta or Platygyra daedalea coral fragments, which associate with different microbiomes. After 5 weeks, all corals were transferred to a mesocosm containing various marine organisms, where they were reared for another 4 weeks. In addition to observing the development of coral-associated microbial communities, we assessed whether exposing Pocillopora acuta recruits to different adult coral species influenced their bacterial communities. Our motivation was to examine whether this approach can be used to modify bacteria and Symbiodiniaceae associated with these early life stages. While adult Pocillopora acuta fragments had bacterial assemblages dominated by Endozoicomonadaceae, bacterial communities of Platygyra daedalea fragments were dominated by Flammeovirgaceae and Rhodospirillaceae. Bacterial communities associated with recruits were more diverse than those found in adults and contained larger relative abundances of Flavobacteriaceae, Rhodobacteraceae and Erythrobacteraceae. All recruits harbored the same Symbiodiniaceae type throughout the experiment (dominated by Cladocopium C1d sequences), and their bacterial communities did not differ across treatments. Instead, there was a significant change over time characterized by a decline in the relative abundance of Flavobacteriaceae and an increase in the abundance of Endozoicomonadaceae. Our findings indicate that Pocillopora acuta recruits harbor dynamic and diverse bacterial assemblages, which were not influenced by nearby adult corals in our experimental design.