Reef location has a greater impact than coral bleaching severity on the microbiome of Pocillopora acuta

The composition of the microbial communities in the coral Pocillopora acuta were examined following the 2016 bleaching event on the Great Barrier Reef (GBR). Corals located within two reefs of the Palm Island archipelago on the Central GBR experienced moderate heat stress,

In-water bleaching surveys were conducted at two depths (2 m and 5 m) on the 4th and 6th of April 2016. Five 10 × 1 m belt transects were used with observers capturing 1 m × 1 m square continuous images. Observers counted each coral colony > 5 cm in diameter within each 1 m × 1 m image, recorded:

genus level identification categorical bleaching score based upon the proportion of living coral tissue visibly bleached white:

No bleaching

Pale

1–50% tissue bleached white

51–99% bleached white

100% bleached white with or without intense fluorescence

partial or complete mortality of the colony (recently dead regions of tissue).

Bleaching severity categories at the reef community level were based on the percentage of corals bleached as follows:

no bleaching (0% bleached corals);

minor bleaching (1–10%),

moderate (10–30%);

major (30–60%);

severe (> 60%)

Coral fragments were collected from 22 colonies at Pandora on the 4th April 2016 and from 25 colonies at Havannah on the 12th April 2016

Samples were prepared for DNA extraction and Symbiodiniaceae cell counts as described in (Botte et al. 2021). Surface area of the coral skeletons was estimated using established protocol (Veal et al. 2010). Statistical analysis of DNA sequencing data was conducted using R.

This study was conducted to identify the impact of bleaching severity on the microbiome of the P. acuta coral during the mass bleaching event of summer 2016, at two sampling sites within the Palm Island group (Central GBR). A significant impact of sampling site was found and no effect of bleaching severity, demonstrating the importance of fine-scale sampling when investigating large scale bleaching events.