Examining Short Temporal Changes in Intertidal Macroalgal Microbiomes at 'Ewa Beach, O'Ahu, Hawai'i: Some Hosts Varied While Others Remained Stable.

Understanding the temporal variability of microbiomes is crucial for predicting dynamics within macroalgal communities under future climate change scenarios, rising temperatures, and increased marine heatwave events. Short-term variation has been observed in human- and coral-associated microbes, but these dynamics are less understood in macroalgae. Intertidal macroalgal communities are ideal systems for investigating microbiome temporal variation due to their exposure to daily fluctuations in abiotic conditions. We characterised and examined the variability in the microbiota of seven macroalgal species, with representatives from three different phyla, across five sequential low tides in May 2021 at a single intertidal bench at 'Ewa Beach, O'ahu, Hawai'i, USA. Bacterial community compositions found in two red algae, Acanthophora spicifera and Laurencia dendroidea, and one brown alga, Dictyota sandvicensis, had variable abundances of several amplicon sequence variants contributing to significant dissimilarity through time. Two green macroalgae (Avrainvillea lacerata and Halimeda discoidea) were stable over time. Temporal stability of the macroalgal microbiotas, therefore, was variable amongst macroalgal species, but may be dependent on its horizontal or vertical positioning within the intertidal zone, which can vary the level of environmental stress (e.g., temperature, light, desiccation). Additional work exploring the drivers of the temporal variability observed herein is needed.