Development of a free radical scavenging bacterial consortium to mitigate oxidative stress in cnidarians

Abstract

Corals are colonized by symbiotic microorganisms that profoundly influence the animal's health. One noted symbiont is a single-celled alga (in the dinoflagellate family Symbiodiniaceae), which provides the coral with most of its fixed carbon. Thermal stress increases the production of reactive oxygen species (ROS) by Symbiodiniaceae during photosynthesis. ROS can both damage the algal symbiont's photosynthetic machinery and inhibit its repair, causing a positive feedback loop for the toxic accumulation of ROS. If not scavenged by the antioxidant network, excess ROS may trigger a signaling cascade ending with the coral host and algal symbiont disassociating in a process known as bleaching. We use Exaiptasia diaphana as a model for corals and constructed a consortium comprised of E. diaphana-associated bacteria capable of neutralizing ROS. We identified six strains with high free radical scavenging (FRS) ability belonging to the families Alteromonadaceae, Rhodobacteraceae, Flavobacteriaceae and Micrococcaceae. In parallel, we established a consortium of low FRS isolates consisting of genetically related strains. Bacterial whole genome sequences were used to identify key pathways that are known to influence ROS.

Linking photoacclimation responses and microbiome shifts between depth-segregated sibling species of reef corals

“…Alteromonadaceae belong to a diverse group of heterotrophic gammaproteobacteria known to associate with marine hosts and nutrient-rich environments. Members of this group tolerate relatively high temperatures and have been used in coral probiotic studies as coral-associated bacteria capable of scavenging free radicals [ 66 ] and therefore could provide similar benefits in shallow, high-light environments. Similarly, Acidimicrobiales are known to be planktonic free-living photo-heterotrophs found in both temporal and tropical photic zones [ 67 ] and are associated with dissolved organic matter (DOM) in marine environments [ 68 ].…”

Exploring microbiome engineering as a strategy for improved thermal tolerance inExaiptasia diaphana

“…These approaches may address the issues of dilution and inadequate uptake of putative beneficial bacteria by coral and warrant further investigation. Furthermore, of the nearly 1000 bacteria isolated from E. diaphana (Dungan et al 2021a) and the thousands of bacterial cells they can carry (Costa et al 2021), only 12 were used in this study. More work is needed to explore the potential beneficial roles of the other bacteria, which may be possible with advances in metagenomic sequencing and the assembly of bacterial metagenome-assembled genomes.…”

“…Individual isolates were selected and grown to log phase in 50 ml of modified R2A broth (mR2A; Table S1). The FRS phenotype of each high and low FRS bacterial strain was similar when incubated at 26°C or 37°C (Table S2; Dungan et al 2021a). Since the bacteria grew more rapidly at 37°C, cultures were incubated at 37°C for 48 h at 150 rpm in an orbital incubator (OM11, Ratek).…”

“…To assess the ability of introduced bacteria to form a stable presence in the cnidarian microbiome and evaluate their influence on host health and bleaching, we inoculated the sea anemone Exaiptasia diaphana with a bacterial consortium consisting of host-derived free radical scavenging (FRS) bacteria (Dungan et al 2021a) prior to thermal stress. Metabarcoding of bacterial 16S rRNA genes was used to track the incorporation of the bacteria into the E. diaphana microbiome and changes in bacterial community structure across time and treatments.…”

Intracellular Bacterial Symbionts in Corals: Challenges and Future Directions

“…Microbiome manipulation is one of several avenues that are currently being pursued in order to mitigate coral bleaching [40,53,54,[167][168][169][170]. As excess ROS production by Symbiodiniaceae is one of the main causes of coral bleaching [34,[36][37][38], it was postulated that treating corals with bacteria with a high ROS-scavenging ability could mitigate coral bleaching [171]. Proof of concept studies have confirmed that the coral microbiome can be modified [172,173], without specifically targeting ROS scavenging, but only three studies so far have actually trialed probiotics to mitigate coral bleaching [174][175][176].…”