Coral reefs: micro-superheroes to the rescue

We all know, or so it should be, that climate change has done and will do more damage to our planet. Among the ecosystems most affected, we find that of the Great Coral Reef.

Coral reefs: a natural World Heritage Site

Who doesn’t know, or at least once in their life has seen a wonderful video of these submerged places full of colours and life? The Australian Great Coral Reef, is a natural World Heritage Site, for its great wealth in biodiversity and beauty. It is the largest existing coral extension in the world, it extends for 2300 km on about 344,400 km. So extensive that it can also be seen from space (Fig.1). Just this great treasure that our oceans offer us, is currently in danger. The biggest threats are:

  • over-exploitation of tourism;
  • climate change.

The latter, in particular through the phenomenon of coral bleaching, has led to the extinction in recent years of about 20% of the reef (2016).

Coral reefs
Figure 1 – A part of the Australian Great Barrier Reef photographed from space. [Source: wikipedia.com]

Climatic phenomena affecting the healthy state of corals

The impact of the climate crisis on coral reefs is increasing. Much of it is due to rising surface temperatures and the incidence of sea heat waves. The heat waves are nothing but anomalous phenomena of heating of the waters (Fig.2), with devastating effects on the ecosystems. More and more often it happens to witness phenomena of greater duration, compared to the more sporadic fluctuations in temperature to which even the researchers themselves were accustomed not to give the right importance. The longest wave ever recorded, called “The Blob” by experts, led to thousands of deaths of specimens of different marine species.

heat waves in coral reefs
Figure 2 – Schematic categorization for sea heat waves, which shows time series of observed temperatures (dashed line), long-term regional climatology (bold line) and climatology of the 90th percentile (thin line). Interpreting the data, each category corresponds to a magnitude of waves, increasing in value according to the colour and the number of the category. In this example, the dotted line shows that the phenomenon has reached the maximum degree, considered extreme, therefore very dangerous for the marine environment. [Source: Hobday et. al, Categorizing and naming marine heatwaves. Oceanograph 31(2):162–173, https://doi.org/10.5670/oceanog.2018.205]

As a result of the increased duration and intensity of these thermal events, high mortality rates have been recorded in coral populations around the world. To mitigate the adverse effects of climate change on coral populations, researchers are working to implement different bio-conservation strategies.

Microorganisms and coral reefs

One of the most recent approaches, according to a recent conservation study, would be to manipulate the resistance and resilience of corals through the use of microorganisms typical of the microbiome of the corals themselves or atypical. Coral-associated bacteria are able to fix nitrogen, degrade polysaccharides, and produce antimicrobial compounds that can inhibit pathogen growth. Thanks to previous studies that have well described the coral microbiome in many of its aspects, the selection and use of specific microorganisms with a “probiotic” function for corals is a whole new field of research. This type of approach was already proposed in 2017 in a study published on Frontiers (Peixoto et al., 2017).

P. damicornis of  reefs, improve through
Figure 3 – Coral polyps belonging to the species P. damicornis. [Source: wikipedia.com]

The bacteria are selected according to their ability to bring benefits through greater protection, maintenance of the health status of the coral and its vital development. To define this selected group of probiotic bacteria, the definition of Beneficial Microorganisms for Corals (BCM) was used. A term which was taken up from the analogous one, for the agricultural field, Plant Growth Promoting Rhizosphere (PGPR), meaning the use of microorganisms as probiotic bacteria in the rhizosphere in order to promote directly or indirectly vegetal development.

Vectors useful for application on coral reefs

They have been proposed over time, engineering systems useful in closed environments, not applied for lack of scientific evidence of the usefulness of this consortium of organisms in systems such as large aquariums. In an oceanic environment, it was thought, to use small organisms typical of the fauna (or even microfauna) of coral reefs. In particular, the use of small crustaceans (Artemia or better known as sea monkeys), rotifers, copepods to transmit probiotic bacteria to other animals, including corals. More specifically, the study has hypothesized the use of a rotifer, of the species Brachionus plicatilis, with subsequent monitoring and tracing of the absorption (Fig.4) of the bacteria by the corals belonging to the species Pocillopora damicornis.

Probiotic bacteria used for improve healthy states
Figure 4 – Some phases of the capture of the rotifer B. plicatilis by the coral P. damicornis. [Source: Assis JM et al. 2020,Front. Microbiol. 11:608506. doi: 10.3389/fmicb.2020.608506]

Composition of the consortium

The 7 strains chosen, have all been isolated from the microbiome of P. damicornis grown in both natural and artificial environments, and 5 of these belong to the genus Pseudoalteromonas, a strain of Cobetia marina (a species of marine gram-negatives) and a strain of Halomonas taenensis. All grown on land that’s selective for marine bacteria, like Marine Agar. These once selected and cultured were put in contact with rotifers, and checked through an electron microscope (SEM) scans for 16 hours. After ingestion and verification of interaction, rotifers were placed in an aquarium containing some specimens of P. damicornis. Through the use of digital microscopes, it has been verified that the polyps properly ingested the rotifers containing the consortium.

Conclusions and future research

Through this study, the researchers demonstrated that the use of small marine organisms as vectors and the consortium of probiotic bacteria, duly selected, can be useful and hopeful to combat the phenomenon of coral whitening, and generally enhance the health of these wonderful organisms. In particular, mitigate the effect of the pathogenic organism typical of corals Vibrio coralliilyticus in high-temperature scenarios. In addition, many strains of the genus Pseudoalteromonas, are able to produce a class of bacteriostatic and amphiphilic anti-Vibrio molecules, able to prevent the adhesion and growth of other microorganisms, potentially harmful.

Some macroscopic details of the surface  of P. damicornis coral reefs and probiotic bacteria
Figure 5 – Some macroscopic details of the surface of P. damicornis. [Source: wikipedia.com]

Furthermore, it seems that always the same bacteria, are able to protect the gastric region of corals and protect from reactive oxygen species (ROS) produced as a result of stress. The next step, according to the researchers would be to deepen the duration and the effective application of these benefits on larger populations of corals and how easy the modality of the application itself can be. In the meantime, to do our part, we can also make small gestures, try to improve the health of our oceans, protect them from pollution and always look with an eye to the increasingly growing climate crisis.

Original article: Le barriere coralline: microrganismi supereroi in soccorso written and translated by Luigi Gallucci

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