Predatory Bacteria: The gate keepers of the holobiont?

This study assesses predatory bacteria, Halobacteriovorax, within the holobiont of three corals: Siderastrea siderea, Agaricia spp., and Porites spp. Within the microbiome there are various bacterial predators, Halobacteriovorax is one such predator and part of the Bdellovibionales group. These predators are thought to regulate the holobiont. Halobacteriovorax is a prevalent active predator on coral surfaces (Schoeffield and Williams, 1990). However, its role within the holobiont is poorly understood.

Interactions within the holobiont are important to coral health. In other systems, these interactions are regulated by predation. Although poorly understood, bacterial predation could influence microbial function by regulating community composition.

Delta-proteobacteria, such as Bdellovibionales and like organisms (BALOs), prey exclusively on other bacteria, including many known pathogens. In a small initial concentration, unlike viruses, BALOs can alter the bacterial composition of the host. Therefore, they have the potential to act as a microbial regulatory mechanism.

As Halobacteriovorax has a broad distribution range, it is a common and active predator with a biphasic life cycle, growth phase and attack phase. It was found in 79% of the samples taken across three genera, albeit with a low mean relative abundance. However, Siderastera and Agaricin accounted for the majority of BALOs interactions while Porites accounted for 8%, suggesting that its presence is not uniform across all hosts. Due to its high presence and predatory life style, it can be considered an important low-abundance member of the core microbiome.

By using the plaque assay technique, the study found that Halobacteriovorax predated Vibrio coralliiticus and Vibrio harveyii, known coral pathogens. It was also discovered that under laboratory conditions Halobacteriovorax preyed upon Vibrio fortis. V.fortis was isolated from a seemingly healthy Porites astreoides, which could be evidence that Halobacteriovorax maintained coral health by regulating the abundance of V.fortis.

Using taxonomic data from 16s rRNA sequencing, co-occurrence networks and networks for known drivers for holobiont shift were created, which were temperature and nutrient enrichment. The analysis showed that Halobacteriovorax interactions were influenced by temperature and nutrient supply. At the temperature extremes, the largest changes in interactions occurred. Other predatory bacteria, in this case Myxcoccales, were affected by temperature, implying there is a common response to microbial drivers. The nutrient response showed that three co-occurring taxa existed under nutrient enrichment conditions while only one occurred under control conditions. Only positive interactions were found in the analysis of the BALOS co-occurrence networks, displaying no mutual exclusions.

The Bdellovibionales increased with prey populations, suggesting that the presence of predatory bacteria is a method of microbial regulation. Since there were exclusively positive interactions, it suggests that the conditions are favourable to the co-occurring taxa and predators are removing the co-occurring taxa competitors. This would allow those taxa to increase their abundance, so this could be a method of microbial regulation within the holobiont and fits with the predator–prey model. As read depth and the ability to detect negative interactions were poorly corelated, it is likely the presence of the predator was beneficial to the co-occurring taxa. Some hosts displayed different interactions, such as Bdellovibionales co-occurring with Vibriionales on Angncia but not other hosts, this could be further evidence that BALOs in the holobiont are host specific.

This study showed that bacterial predators have an impact on microbiome dynamics and could be considered a regulatory tool of the holobiont. The highest level of interactions occurred at the temperature extremes, which could provide the host with a tolerance to environmental changes. In a review by Pernice et al. (2015), it was suggested that stressors increase the microbial diversity, potentially due to opportunistic pathogens invading the microbiome. Given that BALOs help defend against these invasions, further study could provide interesting opportunities regarding targeting conservation management.

References

McDevitt-Irwin, J.M., Baum, J.K., Garren, M. and Vega Thurber, R.L., 2017. Responses of coral-associated bacterial communities to local and global stressors. Frontiers in Marine Science, 4, p.262.

Schoeffield, A.J. and Williams, H.N., 1990. Efficiencies of recovery of bdellovibrios from brackish-water environments by using various bacterial species as prey. Applied and environmental microbiology, 56(1), pp.230-236.

Article Reviewed

Welsh, R.M., Zaneveld, J.R., Rosales, S.M., Payet, J.P., Burkepile, D.E. and Thurber, R.V., 2016. Bacterial predation in a marine host-associated microbiome. The ISME journal, 10(6), pp.1540-1544.