What are the biofilms?
The marine biofilms are a microbial community that adheres to an inert surface, or tissue, included in a complex matrix. Their formation includes several phases: bacteria adhere to a surface, form a micro-colony, and produce a polymeric matrix around the biofilm. (Fig.1) Then, once the process is over, the biofilm releases bacterial cells that can colonize further niches where new biofilms will form. They represent a typical mode of bacterial growth, giving many advantages, including the ability to withstand adverse environmental conditions and an increase in resistance to antibiotics and biocides. In addition, bacterial biofilms protect bacterial communities from external mechanical and chemical damage.
Microorganisms look different if observed in biofilm-mediated infections as they are located close to each other. They are also surrounded by this matrix containing exopolysaccharides, proteins, nucleic acids and bacterial debris. In addition, according to transcriptomic studies, biofilms exhibit low metabolic activity with over-regulation of the genes needed for anaerobic growth. A type of activity under the control of the quorum-sensing process. In addition, in the specific case of infections, especially human infections, assist the processes of inflammation and tissue damage, putting in difficulty the immune system of the host.
Thanks to new sampling methods, new analytical techniques and the need to better know the vast underwater world, the marine microbiome is prone to continuous studies. Microorganisms are essential for the health and resilience of the marine ecosystem, for their fundamental role in biogeochemical cycles and interactions with other organisms. (Fig.2) In particular, surface interactions, or biofilm formations, can bring several ecological benefits; environmental protection, increased accessibility to new sources of nutrients and opportunities for interaction between organisms.
Microbial associations may be harmful (biofouling, biocorrosion), beneficial or neutral. Generally, the composition of marine microbial biofilms may differ. The differences are according to the characteristics of the water in which it develops and can be influenced by the location and the type of substrate. Just this variability depending on the substrate has allowed demonstrating in some cases that, some specific species grow on certain organisms.
The outer surface of marine organisms often represents an active exchange interface between host and biofilm. When they settle on living surfaces they can influence:
- Information flows between host and environment
- The flow of energy
- The flow of matter
From these, we understand who has a high ecological potential, able therefore to modulate the abiotic and biotic relationships of the host. Many studies present the ecological influence of marine biofilms (epibiotic); in particular, they present an enormous potential in reducing the access of the host to fundamental natural sources, such as light, gas and nutrients. They also modulate the interactions of microorganisms with other “foulers” (fouling organisms), consumers or pathogens. These effects can also lead to changes in the surroundings.
Normally, in the marine environment, the outer surface plays a key role for organisms. As we have already said, most physiological exchanges take place through it, such as breathing, the release of waste products of metabolism, absorption of irradiations, nutrients, gas, etc. The same interactions between host/parasite or also between prey/predator, are often linked and controlled by properties typical of the body surface of the organisms themselves. The non-trophic association between the host (basibiont) and the epibiont (organism growing on it) is called epibiosis. Microbial epibiosis is the basis of the formation of microbial marine biofilms.
Types of interactions
The concentration of the various forms of life may vary depending on the season, the environmental conditions and the depth, reaching a density (per ml of seawater) of 106 for bacteria, 103 for microalgae and variable percentages for other various organisms (often macro-organisms). The harmful consequences for the host can be various, among which, the increase of the weight and the friction, hindered trans-epithelial exchanges, colours, odours and contours altered. These phenomena are translated into:
- Loss of buoyancy
- Impediments to mobility
- Mating problems
These changes are thought to be the engine of evolutionary force leading to the development of anti-fouling (anti-fouling) strategies. After all, until the last decade, microbial marine biofilms were not the subject of many studies, for various reasons including the reduced presence of this type of bio coating, the lack of suitable molecular techniques, the high variability and dynamism of this phenomenon.
Planktonic bacteria (free form), which make up the biofilm in a colony, are attracted by sources of organic matter. In addition, according to some studies, when compared to their planktonic life form, they are more densely aggregated of about 1-2 orders of magnitude, communicate more intensely, show high enzymatic activity, growth and production, and use a more intense horizontal gene transfer. (Fig.4) They are also more prone to predation and infection. In addition, they seem to be able to modulate (reduce, increase, and select) the recruitment of other bacteria, diatoms, fungi, larvae or spores.
Microbial marine biofilms on animal organisms
Probably there are no marine organisms whose surface is free from epibiotic bacteria, only some of them like some colonial ascidians belonging to the family of the Didemnids have a sterile surface, the others are provided with marine biofilms, of varying density and composition. The phenomenon of microbial-incrustation leads first, to the replacement of the epidermis of the host, with a new “tissue”, functionally different and also representing the only functional interface between host and environment. The microbial cells of the biofilm interact with all the others, exchanging metabolites and information, multiply and produce propagules (called dispersants) when the internal or external conditions worsen. Cells in biofilms do not share the same genome if they belong to different species, so each of them will give rise to different products, often functionally identical.
Marine biofilms: sponges and others animals
Often, the microorganisms associated with animals, differ depending on the position of these in the water column, and the association that the animals themselves may have with additional substrates. Much of the research on microbial marine biofilms is based on the study of sponges and associated microorganisms.
It has been shown that the composition of some microbial communities can remain constant, as in the case of some porifers, or vary according to different factors in other animals of the same genus. In general, the composition of epibiotic bacterial communities associated with microorganisms is influenced by climatic factors. Some particular bacteria are specifically and persistently associated with marine animals, and are not present in free form in the water column or on other animals (demonstration of interaction specificity). For example, the bacterial species Candidatus Endobugula sertula is specifically associated with the larvae of bryozoans belonging to the species Bugula neritina (Fig.5) and protects them from predatory fish (through the production of toxic substances).
Some examples of marine biofilms
Some studies have evidenced that in some cases these epibiontic bacteria are transmitted through gametes or larvae from the adults (bacteria Bacillus sp. associated with the sponge Ircinia fusca in figure 6).
In other cases, instead, the interaction between the various vital phases of the organism guarantee bacterial colonization and the formation of the marine biofilm (in the case of some corals such as Acropora palmata (Fig.7), A. cervicornis, A. humilis, etc.).
Finally, the average density of microorganisms on different organisms has been verified, from sponges (which have sterile or completely encrusted surfaces) to corals (which may have low concentrations of epibiotic bacteria or high concentrations of the latter). It has been demonstrated how also on carapaces of animals of various nature the interaction with the bacterial biofilm is fundamental for the appearance of anti-fouling techniques useful to prevent the decay of the carapaces themselves.
In conclusion, the presence of microorganisms improves the standard of living of some marine animals, while in some cases may lead to disadvantages. Studies are currently underway on how the development of bacterial marine biofilms can influence and prevent the formation of encrustations, particularly by applying these innovations and studies on man-made materials.
Original article: Biofilm marini: una seconda pelle per gli organismi marini
Written and translated by Luigi Gallucci