Biofilms are a mixture of complex communities of organisms mostly composed of diverse bacteria that vary depending on the surrounding environmental conditions induced by physical and chemical factors. In biofilms, symbionts play major roles in the relationship among organisms by the production of bioactive molecules involved in quorum sensing signaling. A cohesive structure of a multi‐layer of extracellular polymeric substances (EPS) such as polysaccharides and proteins is the base of biofilm structural organization. Biofilms can be found in a variety of habitats, on free‐living, on the surface of other organisms or inert surfaces, both in aquatic as well as terrestrial environments.
- quorum sensing
Biofilms are complex, highly dynamic, structured ecosystems formed by a community of different microorganisms living attached to inert or living surfaces and embedded in a matrix designated extracellular polymeric substance (EPS). The colonization of a surface begins with EPS production by the initial bacterial colonizers through the formation of weak, reversible bonds called van der Waals forces and production of transparent exopolymer particles and their precursors by macroalgae that set the basis for the first bacterial colonizers settlement . Several other microorganisms come after entering in this very competitive ecosystem where quorum sensing and antibiotic resistance are determinants in the development of the community. Observation of biofilms dates back to the seventeenth century when Antonie van Leeuwenhoek observed bacteria from the plaque biofilm of his teeth under his primitive microscope . However, it was only in the 1940s that the concept of biofilm begun to arise with the works of Heukelekian and Heller  and of Zobell  whereas the first publication referring to the word biofilm appeared just in 1975 . Due to the invisibility of microbial biofilms and inexistence of adequate methodologies for their study during many years, comprehension of this ecosystem is still scarce. The best studied biofilm systems are the ones associated with human body surfaces and technical surfaces like sensor heads or reverse osmosis membranes of desalination plants  and references therein. More recently, increasing attention has been paid to other systems like the epiphytic community on macroalgal surface. Several scientific groups are presently dedicated to the study of these biofilms with a consequent boost in the number of publications (see  for the increase in bacteria‐macroalgae related publications). Since the description of the algal epiphyte bacterium,
The idea of considering biofilms as an extra “tissue” on the surface of eukaryotic organisms is based on the analogy between these two systems [6, 18]. We can thus consider the existence of an extra coat outside the macroalgal epidermis that gives an extra buffering between the host cells and the surrounding environment. In the biofilm, although cells are genetically different and variable, what does not happen in tissues, they interact functionally through nutritional exchange, communicate through quorum sensing and reproduce.
2. Macroalgal colonization and chemical interactions
Macroalgae inhabit an environment prone to epibiotic colonization (Figure 1) due to a constant pressure from the surrounding microbial community . The relationship between bacteria in biofilm and their planktonic counterparts is of 1 to 2 orders of magnitude higher [6, 20]. However, Chan and McManus  in their study with
As perceived by these cell density numbers, marine macroalgae are one of the most important eukaryotes that provide excellent conditions for microbial colonization on their surfaces in the marine environment. Several factors are determinant for colonization. These include (1) the microtexturing (size and surface features) of the surface , (2) the production by the macroalgae of natural compounds with antifouling properties which include antimicrobials and quorum sensing disruptors [7, 26, 29–32], (3) the production of organic carbon compounds that trigger the chemotactic behavior of bacteria [33–34] and (4) the releasing of certain substrates that fulfill the nutritional needs of the epiphytic microbial community. On the other hand, macroalgae also benefit from the presence of this rich community as their growth and development are somehow dependent essentially on their bacteriome [14, 35].
In the microhabitat of the biofilm and on its interface with the macroalgae, complex chemical interactions occur. Both basibiont (macroalgae) and epibiont (microorganisms in the biofilm) contribute to this myriad of compounds. The macroalgae supply bacteria with oxygen and fixed carbon which is released as extracellular exopolysaccharides such as alginate, cellulose and mannitol [36–38]. Bacteria, through the mineralization of organic compounds released in the biofilm, supply the macroalgae with CO2, minerals, vitamins and growth factors [39–42]. In the mineralization process, many other molecules are formed which enrich the biofilm microenvironment and contribute to its biomass formation.
Growth factors produced by bacteria are phytohormones and biostimulators of growth and development . Strains isolated from
Bacteria are fundamental for the growth and morphogenesis of several macroalgae. Provasoli  observed that
Furthermore, bacteria are also sources of fixed nitrogen and detoxifying compounds [7, 57, 58]. Nitrogen‐fixing cyanobacteria are known to provide fixed nitrogen to macroalgae. These include
Another advantage of microbial community in macroalgal biofilms is their ability of scavenging of heavy metal [42, 58] or crude oil . Many bacteria also play a fundamental role in biotransformation and nutrient cycling in the oceans due to the capacity to decompose the macroalgal cell walls [7, 65]. Bacteria, with the appropriate enzymatic machinery, contribute to the decay process of seaweeds .
Macroalgal epiphytic colonization is very uneven . Macroalgae inhabiting the same environment or closely related species like
Macroalgae are a rich source of bioactive compounds against colonizing organisms. They are assisted on this task by the many antimicrobials produced by microorganisms on their biofilms, production that is widespread among bacteria [72–75]. Members of the genera
The oxidative burst response is based on the production of large amounts of reactive oxygen species by the macroalgae inducing the death of undesired microorganisms like pathogens and also controlling bacterial growth on algal surfaces. Elicitors of oxidative burst, signals that mediate the activation of cell‐based induced defence responses, were recognized to be glycoproteins and glycopeptides, low‐molecular weight peptides in the red agarophyte
Bacteria, once thought to be silent, were discovered to have specific intra‐ and inter‐species signaling mechanism of communication that has been named quorum sensing (QS). They communicate via production of chemical signals with multifunctional activity due to their interacting QS gene regulatory ‘modules’ which are able to produce several different molecules, from the same or different chemical class that interact in hierarchies [81–83]. These molecules act as gene regulators of the population behavior in food uptake or common defence or escape when the survival of the community is at risk [81, 84]. In addition to communication with other microbes, bacteria also perceive molecules from eukaryotes that are known to be key factors in host‐epibiont interaction [15, 83].
QS communication was discovered in the 1990s and proved to fulfill different ecological purposes like the induction of biofilm formation, movement of bacteria and the production of bioluminescence, antibiotic and virulence factors [85–87]. Similar to the QS signals that balance the equilibrium of the community, quorum quenching (QQ) signals are inhibitors of QS and also have impact on biofilm communities.
Halogenated furanones are structural analogues to N‐acyl homoserine lactones (AHLs) and interfere with AHL‐regulated processes and impair biofilm formation [88–90]. The first compound with QS disruptor capacity isolated from a marine source, the red alga
Since the discovery of these QS inhibitors, other studies lead to the isolation of more compounds able to block QS signal like the mixture of floridoside, betonicine and isethionic acid, isolated from the red alga
The production of QS signals not only affects bacterial responses but also the settlement of the green macroalga
AHLs also have effect on diatoms in the biofilm matrix as was observed by Yang et al. . 3,4‐dibromo‐2(5)H‐furanone,4‐nitropyridine‐N‐oxide and indole were able to decrease significantly the growth of two marine diatoms,
Although biofilms are important for macroalgae biology, they can also bring on detrimental effects as their members compete for nutrients, interfere with gaseous exchange, form a barrier to light that is fundamental for macroalgal photosynthesis and can lead to disease and degradation of algal tissue [95–97].
Similar to humans and plants, macroalgae possess their own defence mechanisms and immunity adaptations developed to survive and coexist with pathogenic or phycocolloids degrader's organisms [18, 98]. It is suggested that, since microbes are more predominant in water than in air, macroalgae evolved to more resistant forms by elimination of more susceptible individuals and resistance of the ones capable of producing chemicals for self‐defence [99, 100]. Actually, few are the reports on the study of pathogenic microorganisms in macroalgae. Although, studies on algal diseases have risen due to the increase of the use of algae in food industry, seaweed mariculture and to the consequences of global warming and its impact on ocean temperature [96, 101]. The most studied pathogen‐macroalgae systems studied are the host‐specificity infection by
The biofilm community present on macroalgae not only has influence on the host life but also on other eukaryotes living nearby. Effects on sea urchin larval settlement by coralline algae biofilm communities  and on
3. The diversity of microbial community on macroalgae
In marine environments, biofilms are mainly formed by bacteria but also by several different eukaryotes such as diatoms, fungi and protozoa  in a ratio of 640:4:1 of Bacteria:Diatoms:Flagellates .
3.1. Bacterial diversity
Hollants and collaborators in 2013  did an exhaustive analysis of the macroalgae‐bacterial diversity compiling information from 55 years and 159 seaweed species (36 green, 72 red, and 51 brown marine macroalgae). They concluded that bacteria associated to macroalgae belong to the phyla
During many years, the study of macroalgal biofilm diversity was based on organism isolation in pure cultures with their subsequent taxonomic characterization. With the development of molecular and new generation sequencing techniques, a much more precise and detailed assessment of diversity has been possible. Presently, our knowledge, essentially at bacterial level, has been extended to many groups of organisms known for their difficulty to be brought into culture.
Longford et al. , using 16S rRNA gene library analysis, compared the bacterial community of the red macroalga
Tujula et al.  in a catalyzed reporter deposition fluorescence
The brown alga
The epiphytic bacteria on the macroalga
Hengst et al.  studied the composition and structure of bacterial communities on three macroalage from two coastal areas in the Northern Chile varying in copper concentration in seawater. They found that the bacterial communities’ structure was determined by the algal host and time dependent. Significant changes in the bacterial community structure induced by copper were observed in
In a DGGE and clone libraries study, Lachnit et al.  verified that the macroalgae
The bacteriome of the kelp
The effect of temperature on the bacteriome of the brown macroalga
Martin et al.  showed that
Using 16S rRNA gene clone libraries, Wu et al.  observed a host‐specific but temporally and spatially variable epibacterial community on the surface of the four red macroalgae,
The composition of
Previous studies have shown that planctomycetes are common inhabitants in macroalgal biofilms [111, 115, 122]. In 2014, Bondoso et al.  analyzed the
In the various studies of bacterial diversity associated to macroalgae, several patterns were observed. Seasonal and geographical (Baltic Sea and North Sea) differences in bacterial communities of
Although changes occur over season, life span and macroalgal thallus parts, specific association seems to exist between bacteria and macroalgae [25, 110, 112]. However, Burke et al.  verified that
3.2. Viriome studies
Recently, attention started to be paid to the viruses associated with macroalgae . The viriome associated with the red macroalga,
3.3. Fungal diversity
Singh et al.  did a comprehensive revision on marine fungi associated with the three groups of seaweeds. Their relationship towards the macroalgae can be of parasitism, saprotrophic nature or symbiosis, being pathogens and parasites the dominant ones . Many macroalgae of the three lineages can harbor a diverse assemblage of marine fungi. Endosymbiotic representatives are members of the genera
Using 28S rRNA gene PCR‐DGGE and real‐time PCR analyses, Zuccaro et al.  studied the filamentous fungi present in healthy and decaying
3.4. Algal diversity
The epiphytic microalgal community on macroalgae has been studied by a restricted number of authors [27, 31, 134–138]. This community is mostly dominated by benthic diatoms and some few centric species possessing an attached mode of life. The composition of benthic diatoms on macroalgal biofilms can be modulated by several environmental conditions including nutrients, salinity, light conditions and hydrodynamic regime as well as by biological factors like grazing, adhesive capacity of diatoms and chemical interactions with the host  and references therein.
In a study performed by Al‐Handal and Wulff , of the 50 epiphytic diatoms identified,
Based on molecular data and/or SEM characteristics, two abundant diatoms epiphytic on the assimilation hairs of the brown macroalga
Three macroalgae, the brown alga
Diatoms epiphytic on red macroalgae living under the sea ice showed a species pattern with depth in which
Totti et al.  in their study of the epiphytic communities on macroalgae from Iceland detected that erect growth forms of diatoms represented 50% of its community (
Tanaka  studied the adhesive capacity of diatoms and verified no close correlation with cell size, their cell form, motility, and mucus secretion. Also, no macroalgal species specificity existed in diatom colonization which was composed preferentially by
4. Biotechnological potential of macroalgae biofilms
The identification of thousands of microbial species and the increase in knowledge on macroalgal biofilms diversity and functioning lead to the valorization of its diversity with the development of several products in a wide variety of fields. The communities living on the surface of macroalgae benefit from a mutualist relationship with their host. The macroalgae are a reliable source of nutrients and on the other hand epiphytic bacteria and fungi help their hosts by producing bioactive molecules that protect all the community from unwanted invaders . The microorganisms in a biofilm community compete against each other and protect themselves from other surrounding pelagic microorganisms by working together as a team producing different kinds of chemicals such as antifungal, antiprotozoal, anti‐settlement and antibiotic molecules [39, 74, 126, 140]. It is the high competition in these communities that induce microorganisms to produce allelochemicals that can be applied in industries such as pharmaceutics, cosmetic industry or even in agriculture .
Globally, hundreds of new natural molecules produced by marine organisms are discovered in a temporal pattern wherein the last discovery peak of new metabolites happened 10 years ago . These authors tried to explain this effect with the need of improvement of the techniques used to discover new compounds. The genomic data now available, the high‐throughput assays for cytotoxicity in cell‐based screening and the automation in nuclear magnetic resonance (NMR) and mass spectroscopy assist the discovery, the accurate identification and validation of new leads to treat human diseases . This review emphasizes the importance of the searching of new compounds from macroalgae and their associated microorganisms. The highest number of bioactive hits found was provided from marine invertebrates, although in the last decades it was discovered that the compounds were actually produced by the associated/symbiotic microorganisms. Even though marine microorganisms provided the highest percentages of bioactive compounds, microorganisms associated to algae are still a minority .
Nowadays, the boom of reports that address the search of bioactive compounds produced by macroalgae‐associated microorganisms reflects the importance and novelty of the compounds obtained from these sources. Singh et al.  described in a review several reports regarding antimicrobial compounds from seaweeds‐associated bacteria and fungi published until 2014. Furthermore, they provided details on the bacteria and fungi associated with macroalgae that are producers of bioactive molecules. It is worth mentioning the ecological role of several new compounds such as haliangicin, korormicin, thallusin or violacein (antifungal, antibiotic, morphogenesis and photosynthetic activities, respectively) in the defence response in macroalgal biofilm [53, 142–144]. In the communities associated with green, brown or red macroalgae, 12% to 50% of strains were able to produce antimicrobial effects in one or more target microorganisms [72, 74, 75, 140]. Remarkable was the discovery of the antidiatom activity produced by 80% of the strains isolated from
The most recent publications concerning the biotechnological potential and bioactivity production of microorganisms living in macroalgal biofilms will be referred below.
Two species of
Striking is the lack of reports of epiphytic fungi regarding bioactive production, in disparity to bacteria and endophytic fungi. Godinho et al.  isolated 148 fungi from the Antarctic macroalgae,
Additional to the pharmacological applications of compounds produced by members of the macroalgal biofilm, another potential application is the usage of anti‐settlement compounds produced by some strains in paints used in aquatic environments that will inhibit the adhesion and settlement of algae in the surface of boats and other objects.
Although in this genomic era there is an incredible increase of information about microbial communities, it is foreseen that only 1–5% of the microorganisms are able to be cultivated. However, several advances in the search for genes encoding secondary metabolites biosynthetic pathways by culture‐independent methods, like metagenomics analysis and metabolomics, and application of this information in synthetic microbiology is increasing the possibilities to reveal new drugs impossible to discover until now. Unexpectedly, as opposed to what is found in sponge's bioactive studies, few are the genomic searches for genes that encode for polyketide syntethases or nonribosomal peptide synthases in macroalgae associated microorganisms.
The discovery of new compounds and new bioactive producers open us possibilities to fight against emergent and still incurable diseases and provide new clues to the understanding of the ecological role played by the complex macroalgal biofilm communities that live under constant societal and environmental pressures.
Macroalgae are protected microniches prone to epibiosis by microorganisms where complex and highly dynamic interactions occur. Firstly colonized by bacteria, many other microorganisms which include microalgae like diatoms, fungi and protozoa constitute these biofilms.
Awareness of the importance of macroalgae and their biofilm has risen recently and, in the last years, we are gaining knowledge on its diversity, especially on the bacteriome, on the multiple functions played by both components of the holobiont, on macroalgal diseases and on the biotechnological potential of these communities. As only a low number of bacteria have been cultivated, we still have a relevant ecological potential to discover in many unknown bacteria. Furthermore, only very recently the world of macroalgal associated viruses started to be revealed. New methodological advances, metagenomics associated with metabolomic/proteomic studies will certainly foster our comprehension of the community structure and functioning of the microbial–macroalgal system. As only a very small part of the more than 35,600 different known species of macroalgae have been studied, we still have a long way to discover the hidden microbial diversity in their biofilms and its biotechnological potential, to understand all potential interactions between algal host and its microbial community, and the regulatory mechanisms in the extra coat of macroalgae.
We are grateful to Manuela Portugal d'Oliveira and Aires Oliva Teles for the critical review of this chapter. This research was partially supported by the Structured Program of R&D&I INNOVMAR – Innovation and Sustainability in the Management and Exploitation of Marine Resources (reference NORTE-01-0145-FEDER-000035, Research Line NOVELMAR), funded by the Northern Regional Operational Programme (NORTE2020) through the European Regional Development Fund (ERDF).