Microheroes at the rescue of coral reefs

Written by Sara Gagliardi
Edited by Cassie Wilson

In my first publication “The incredible world of coral-associated microorganisms”, we talked about the diversity of microorganisms, especially bacteria, living in close relationship with corals. These organisms play several key roles among the coral host, such as supplying energy (nutrient cycling and production), defending against pathogens (antimicrobial production, niche occupation, infection), driving the metamorphosis, etc1–3. Therefore, they are crucial for the coral’s adaptation and acclimation to environmental stresses.However, since 1970, coral reefs have suffered a substantial decrease in their global populations due to bleaching and disease outbreaks, which are caused by a switch in the microbial community composition associated with coral tissues. These changes respond to stressors both from the environment – i.e. thermal stress and water acidification – and the anthropogenic activities – e.g. water pollution, increasing greenhouse gas, overfishing, physical destruction. Some of the known agents of coral diseases are Vibrio shilonii and V. coralliilyticus, resulting in  the tissue lysis of Oculina patagonica and Pocillopora sp., respectively (bacterial bleaching); Aspergillus sydowii (Fungi), causing purple lesion rings that degrade Octocoral tissues (aspergillosis); and a microbial mat dominated by cyanobacteria and sulphide-reducing bacteria, provoking a black/reddish, anoxic and sulphide-rich lesion band that leads to coral tissues death (black band disease)4. As the duration and intensity of these stress events increases, likely increasing the mortality among corals, researchers are trying to find a solution to enhance coral resistance and resilience to these harsh conditions.

Figure 1: Diseased corals presenting signs of (a) bleaching, (b) aspergillosis and (c) black band disease

An idea was proposed by Dr. Rosado (2019), who decided to treat coral diseases manipulating the microbial communities living in coral tissues. Indeed, as it is done on land with the sustainable agricultural strategy called smart farming —where consortiums of Plant Growth-Promoting Rhizobacteria are used as biological control for plant pathogens and/or insects and improvers of plant development—, microbiome engineering techniques were first introduced to marine environments5. Early attempts to manipulate Beneficial Microorganisms for Corals (BMCs), defined as (specific) symbionts that promote coral health, were carried out in aquaria. On day 9, corals were infected at 30°C (thermal stress) with Vibrio coralliilyticus. Half of the specimens were inoculated the day after (day 10) with a consortium of BMC, including five Pseudoalteromonas sp., a Halomonas taeanensis and a Cobetia marina —related species strains isolated from the lace coral Pocillopora damicornis and its surrounding seawater. To infect coral hosts with the bacteria, fragments were removed from the tanks and placed in a Petri dish, inoculated with the pathogen and/or the BMC consortium.

Figure 2: The lace coral Pocillopora damicornis in an aquarium. Source: https://jadwigamorska.pl/pocillopora-damicornis-purple-foto-22-10-2020-sklep-l4-i-rozmiar-6-cm.html

Results showed that when corals were infected with the pathogen alone, bleaching appeared rapidly, causing the loss of pigmentation on coral tissues. However, if the BMC consortium was inoculated, tissues were not faded (see Figure 2). Moreover, the BMCs consortium avoided the Vibrio populations to invade and degrade the coral tissues. To do that, it likely produced antibiotics to actively wipe out the pathogen and used catalytic activities to protect the coral from reactive oxygen species produced during the disease progress, causing cell death through oxidative stress. Moreover, the colonization of coral tissues by the BMCs excludes Vibrio coralliilyticus, the pathogen, from tissue attachment, therefore indirectly preventing the disease from developing.

Figure 3: Coral response to the introduction of the pathogen Vibrio coralliilyticus without (left) and with (right) treatment by pBMC inoculation at 30°C. “Before” corresponds to the initial time control (day 9), where temperature was increased and inoculations initiated. “After” corresponds to the end of the experiment (day 26). D# shows the pigmentation rate. *original photographs are shown. (Rosado et al., 2019)

Despite the important results, the technique developed by Dr. Rosado cannot be used in the natural environment. However, scientists have continued to study several methods to inoculate BMCs on coral tissues. The latest (to my knowledge) was proposed by Dr. Assis in 20206. In her research at the Rio de Janeiro Marine Aquarium Research Center (AquaRio), she uses the predatory character of corals to her advantage to naturally track rotifers fed with the BMC consortium drawn up by Rosado. Indeed, even though corals live in symbiosis with many organisms that help them uptake nutrients, this relationship is not sufficient for their sustenance. Their diet requires among others rotifers, which is a common organism used in aquaculture because of the easy growth and nutrient delivery. Moreover, rotifers were used to provide a probiotic treatment to the western white shrimp. In her experience, Dr. Assis forced the rotifer to feed on the BMCs consortium after starving them, which caused a decline in the digestion rate, thus accumulating living bacteria in the rotifers’ gut and surface. Afterwards, the rotifers were introduced in the surrounding water of corals, which started to freely feed on them (you can see a video here).

Figure 4: The rotifer fed with the BMCs consortium (left) and the coral polyps catching the rotifers in the surrounding water (right). (Assis et al., 2020)

This technique presents a promising way to directly deliver bacteria with beneficial function to threatened corals in the ocean. However, it still remains to be discovered whether the coral establishes a stable symbiosis with the delivered BMCs, or if it is a temporary relationship. Moreover, as it was said by my colleague Fedra Herman, “the only way to really ensure the survival of coral reefs in the long-term is by reducing our greenhouse gas emissions”, as they are the major drivers of coral bleaching and diseases. Taking action to both reduce environmental and anthropogenic stressors, and increase coral resistance and resilience to threats is important to cease or even reverse the decline of coral populations. In this aim, microbiome engineering lays out an interesting tool that, in addition to restoring local coral populations and selecting thermal-resistant specimens, could save coral reefs from their decline.

References:

1. Cavalcanti, G. S., Alker, A. T., Delherbe, N., Malter, K. E. & Shikuma, N. J. The Influence of Bacteria on Animal Metamorphosis. Annu. Rev. Microbiol. 74, 137–158 (2020).

2. Efrony, R., Loya, Y., Bacharach, E. & Rosenberg, E. Phage therapy of coral disease. Coral Reefs 26, 7–13 (2007).

3. van de Water, J. A. J. M., Coppari, M., Enrichetti, F., Ferrier-Pagès, C. & Bo, M. Local Conditions Influence the Prokaryotic Communities Associated With the Mesophotic Black Coral Antipathella subpinnata. Frontiers in Microbiology 11, (2020).

4. Mohamed, A. R. & Sweet, M. Current Knowledge of Coral Diseases Present Within the Red Sea. in Oceanographic and Biological Aspects of the Red Sea (eds. Rasul, N. M. A. & Stewart, I. C. F.) 387–400 (Springer International Publishing, 2019). doi:10.1007/978-3-319-99417-8_21.

5. Rosado, P. M. et al. Marine probiotics: increasing coral resistance to bleaching through microbiome manipulation. ISME Journal 13, 921–936 (2019).

6. Assis, J. M. et al. Delivering Beneficial Microorganisms for Corals: Rotifers as Carriers of Probiotic Bacteria. Front. Microbiol. 11, (2020).https://www.microbiologiaitalia.it/batteriologia/le-barriere-coralline-microrganismi-in-soccorso/#Microrganismi_e_barriere_coralline

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