Effects of Ocean Acidification on Coral Endolithic Bacterial Communities in Isopora palifera and Porites lobata

Sung-Yin Yang,Kazuhiko Sakai,Shan-Hua Yang,Sen-Lin Tang,Chih-Ying Lu,Hideyuki Yamashiro,Rocktim Ramen Das

doi:10.3389/fmars.2020.603293

Abstract

Endolithic microbes in coral reefs may act as a nutrient source for their coral hosts. Increasing atmospheric CO2 concentrations are causing ocean acidification (OA), which may affect marine organisms and ecosystems, especially calcifying organisms such as reef-building corals. However, knowledge of how OA affects marine microbes remains limited, and little research has been done on how coral endolithic communities respond to shifting environmental baselines. In this study, the endolithic communities of two common shallow water coral species, Isopora palifera and Porites lobata, were examined to investigate the microbial community dynamics under OA treatments. The colonies were placed in an environment with a partial pressure of carbon dioxide (pCO2) of 1,000 or 400 ppm (control) for 2 months. Several I. palifera colonies bleached and died at 1,000 ppm pCO2, but the P. lobata colonies remained unaffected. Inversely, the endolithic community in P. lobata skeletons showed significant changes after OA treatment, whereas no significant dynamics were observed among the I. palifera endoliths. Our findings suggest that the skeletal structures of different coral species may play a key role in corals host and endoliths under future high-OA scenarios.

Highlights

Endolithic communities in the coral skeleton are a group of microorganisms including cyanobacteria, fungi, algae, and bacteria that live in harsh, dim-light environments; they face drastic diurnal fluctuations in pH and oxygen and play an important role in the biogeochemical cycles of the coral reef ecosystem
The highly synergistic relationship between coral hosts and their endosymbiotic zooxanthellae is considered the main contributor to primary production in the coral reef system; the endolithic community plays an important role [reviewed in Pernice et al (2020)]
Previous studies on the endolithic community predominantly focused on the aerobic algae Ostreobium, cyanobacteria, and fungi, which may contribute photoassimilates (Fine and Loya, 2002; Tribollet et al, 2006) to the coral host

Summary

Introduction

Endolithic communities in the coral skeleton are a group of microorganisms including cyanobacteria, fungi, algae, and bacteria that live in harsh, dim-light environments; they face drastic diurnal fluctuations in pH and oxygen and play an important role in the biogeochemical cycles of the coral reef ecosystem (Chazottes et al, 1995; Radtke et al, 1996; Ghirardelli, 2002; Ocean Acidification on Coral EndolithsDe Los Ríos et al, 2005). Anaerobic photoautotrophic bacteria—like green sulfur bacteria (GSB) and sulfur reducing bacteria—were dominant and formed a visible green band in the skeleton of the coral Isopora palifera. These endolithic communities might be indispensable for nitrogen cycling and provide numerous primary products for coral (Shashar et al, 1994; Yang et al, 2019; Pernice et al, 2020). Rising global CO2 emissions in the atmosphere lead to increases in seawater pCO2, which reduces oceanic pH and carbonate ion concentrations, a process known as ocean acidification (OA) (Caldeira and Wickett, 2003)

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Conclusion