An intertidal bioflim is a biofilm that forms on the intertidal region of bodies of water. Bacteria and various microorganisms, including algae and fungi, form communities of adhered cells called biofilms. A matrix of extracellular polymeric substances (EPS) within the biofilm forms sticky coatings on individual sediment particles and detrital surfaces. This feature protects bacteria against environmental stresses like temperature and pH fluctuations, UV exposure, changes in salinity, depletion of nutrients, antimicrobial agents, desiccation, and predation. Particularly, in the ever-changing environments of intertidal systems, biofilms can facilitate a range of microbial processes and create protective microenvironments where cells communicate with each other and regulate further biofilm formation via Quorum Sensing (QS)., While biofilm formation is advantageous to bacteria and other microorganisms involved, the attachment of microorganisms to ship hulls can increase fuel consumption and emission of greenhouse gases, as well as introduce Non-Indigenous Species (NIS), potentially resulting in harmful economic and ecological impacts on the receiving ecosystems.
Formation and development
Biofilm formation Biofilm formation begins with the initial attachment of microorganisms to a substrate, such as rocks, shells, or sand in the intertidal zone. This process occurs during the reversible attachment phase, in which the microorganisms only lightly adhere to the substrate. In this phase, the bacteria are encompassed in small amounts of EPS; they are still capable of individual movement and may return to planktonic life., Microorganisms may attach to the surface of substrates by weak Van der Waals forces and hydrophobic effects. A study of Pseudomonas aeruginosa mutants showed that twitching motility by type IV pili contributes to the organism's ability to aggregate on substrates. Another mechanism by which bacteria may adhere to surfaces is the binary division of attached cells. Similar to colony formation on agar plates, as cells divide, the daughter cells spread expansively, forming cell clusters. In all cases, adhesion depends on the microorganisms involved, the nature of the substrate, and the chemical and biological conditions of the environment. The next stage is the irreversible attachment stage, in which microbes start producing EPS. This process creates a three-dimensional polymer network that acts as the biofilm matrix and encloses the bacteria. In this stage, EPS prevent bacterial cells from moving, keeping them in long-term close contact and allowing interactions such as cell-to-cell communication and horizontal gene transfer to occur. In most biofilms, the microbes constitute less than 10% of the dry mass, while the EPS matrix can comprise over 90%.
Biofilm development
Maturation Following the irreversible phase, the next phase of the biofilm life cycle is maturation. In this stage, EPS play a critical role in protecting the biofilm from environmental fluctuations such as oxidative damage, antimicrobials, and host immune system response. Microcolonies are formed as a result of the aggregation of microbial cells and the increase of microbes with accessible nutrients. With the increase in cells, the biofilm matures and develops into a "tower" or "mushroom" like structure with a complex architecture of fluid-filled channels and pores.,,
Detachment Detachment, also known as dispersal, is the final stage of the biofilm life cycle. In this stage, cells are released from the biofilm matrix, individually or in clusters, and either resume planktonic life or attach to another surface., Various factors can lead to cell detachment, including insufficient nutrients, competition, lack of oxygen, and environmental factors.
Features
Taxonomic diversity Marine biofilm communities have rich and diverse taxa, with Cyanobacteria and Proteobacteria being the dominant phyla. Actinobacteria, Bacteroidetes, and Planctomycetes are also considered to be dominant phyla but their relative abundances differ between locations. Site-specific differences also arise within intertidal biofilms. For instance, intertidal biofilms in Río de la Plata contained high amounts of Betaproteobacteria from the Thauera genus, whereas intertidal biofilms along the Pearl River Estuary contained Alphaproteobacteria and Gammaproteobacteria as the most prominent taxa.
Extracellular Polymeric Substances (EPS)
Diatoms are a major component of intertidal biofilms, and they excrete EPS that support many functions, such as desiccation resistance, motility, and metabolite exchange. The EPS produced by microalgae also enhance biofilm growth and help other members of the biofilm with adhesion and migration. EPS are mostly composed of polysaccharides, but may also include proteins, nucleic acids, lipids, and low-molecular-weight, non-carbohydrate compounds.
Seasonal variation Intertidal biofilms exhibit stratification, where different microorganisms arrange themselves in layers based on factors like seasonality. Microalgae are found on the lower shore but their distribution can change. During the winter, a greater abundance and biomass of microalgae are found on the upper shore compared to the lower shore. Seasonal variability is also observed in the relative abundance of microalgae in intertidal biofilms. Specifically, microalgae in tropical and temperate intertidal biofilms are most abundant during winter and spring, with abundance decreasing in the warmer months. Cyanobacteria are relatively less affected by seasonal variation. This may be attributed to their greater tolerance to stressors such as temperature and insolation.
Interactions within the biofilm Interactions within biofilms are bidirectional. They can be affected by negative and positive feedback loops, as well as indirect effects. These interactions contribute to the resilience and adaptability of intertidal biofilms.
Trophic interactions Within intertidal biofilms, trophic interactions exist between microphytobenthos and bacteria. EPS, which are produced by microphytobenthos, act as a trophic resource, but their large size makes them difficult to break down and assimilate. Bacteria secrete various enzymes like β-glucosidase to break down complex carbohydrate compounds in EPS. These carbohydrates serve as a nutrient source for heterotrophic bacteria and sulfate-reducing bacteria (SRB), as well as a carbon source for consumers such as marine invertebrates.
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