The roles of bacteria in corals

Bacteria are among the smallest (just a few micrometers in size) and oldest organisms existing in our planet. They are unicellular beings, thus formed by a single cell, which are characteristic for not having a membrane surrounding their DNA. This last characteristic represents a major distinction to separate eukaryotic cells with nuclear membrane enclosing the linear chromosomes (the cells that form all pluricellular organisms, but also yeasts and protozoans), from the prokaryotic cells having a single circular chromosome and lacking a nucleus (Bacteria and Archaea).

 

 

 

As the oldest inhabitants on Earth, bacteria have developed extremely broad metabolic capabilities. They are able to obtain energy and nutrients by themselves from a suit of substrates, not profitable by any other living being, through chemical reactions by a process called chemosynthesis. The fact of being the most prolific inventors of metabolites and biochemical pathways has permitted bacteria to colonize almost any environment including soil, water, acidic hot springs, radioactive waste, deep portions of Earth's crust, and even have flourished in manned spacecrafts. Bacteria are by far the most abundant group of organisms, as an example, a milliliter of fresh water can contain about a million bacterial cells!

 

 

All these characteristics turn bacteria into the pioneer colonizers of any surface in nature, which create what is called a starting 'biofilm' that then allows other macroorganisms like corals establish in the sea bottoms. Without these pioneer biofilms, no animal or plant colonization would be possible.

 

But additionally, these amazing microbes can as well live in symbiotic or parasitic relationships with plants and animals, surpassing by far the number of own host cells. When the symbiotic community suffers disequilibrium with losses in the populations, the situation may drive to pathologic stages. In such cases, other 'bad' microbes that act as parasites can occupy the niche (space) of 'good' bacteria, displace favorable partner cells, and start a disease.

 

In symbiosis, bacteria provision the host with metabolic nutrients, essential vitamins, and also with bioactive products, often with antibiotic properties, that assist to avoid deleterious infections and afford fitness in the holobiont system.

Corals, as most organisms, harbor an impressive charge of associated bacterial cells. These comprise different communities performing specific roles within the diverse compartments of the coral host, in order to overall maintain the health of the whole coral holobiont. For instance, the symbiotic microorganism populations that inhabit the external coral mucus secretions have generally a defensive significance, and produce antibiotic compounds that protect from infectious agents, and avoid settlement of 'bad' bacteria. These populations found in the mucus exudates differ from those in the soft living tissue, which may also produce antiobiotics, but also provide nutrients and essential vitamins. Finally, there is a distinct associated community of microbes living in the coral skeleton with further defensive and nutritional activities.

 

As one can imagine, having the 'right' combination of symbiotic bacteria is like having a life insurance. But if such combination even more, provides with stress tolerance and resistance, then these 'winner' associated microbiomes turn into an ecological treasure for future generations. Corals, as other creatures, have understood this well, and often the symbiotic partners (the microbiome) are transmitted within the eggs from the mother coral to its daughter next generation of corals via 'vertical' transmission. In fact, coral larvae observed under the microscope can be seen with their 'bagage' of mother-transmitted microbes. This process reflects the will of the mother intending to assure the health and survival for its descendants, by trespassing its major and most precious ecological treasure, the microbiome.

 

 

In the picture above, we may see the adult coral in the left under fluorescence microscopy caused by host but also by microbial cells. In the upper right we can observe coral recruits also harboring the microbial fluorencence, and in the bottom right, a coral larvae harboring the microbiome transmitted by the mother.

The picture of what is known about coral microbiomes and their functionality is still quite general and in certain fields unprecise, and much needs to be disentangled about bacterial partnerships in corals. This is actually one of the major goals in our Experiment. Hopefully we will be able to reveal a significant portion of this crucial black box with your help!...