## 1 Introduction  
Cyanobacteria are Gram-negative microbes widespread in numerous habitats, such as terrestrial, freshwater, and hypersaline environments [1]. All species belonging to this phylum are mostly obligate oxygenic photoautotrophs, capable of using carbon dioxide (CO\({}_{2}\)) as their main carbon source and light as their main energy source [2]. To support these functions, they contain phycobiliproteins, which act as accessory pigments in photosynthesis [3].  
Due to their inherent ability to fix nitrogen and CO\({}_{2}\) and their excellent adaptability to various abiotic stress conditions [4], cyanobacteria can be considered excellent biofertilizers since they improve soil fertility and texture, resulting in increased crop productivity. Their ability to release antimicrobials makes them ideal candidates for use as biocontrol agents and as an alternative to chemical pesticides [5]. Moreover, some cyanobacteria can detoxify or degrade various pollutants, both natural and xenobiotic, and can uptake heavy metals through the use of metallothioneeins [6; 7]. In addition, cyanobacteria play an important role in rhizosphere engineering [8]. These organisms are capable of secreting biologically active compounds such as phytohormones, amino acids, sugars, phenols, and vitamins, which act as signaling molecules for the synthesis of a series of plant secondary compounds [9]. Biocrusts are cyanobacteria-dominated biofilms that develop on the soil surface and comprise a complex community of other microorganisms. These biocrusts improve water-holding capacity, increase infiltration, and decrease erosion while maintaining soil moisture for a longer period [4; 10; 11]. The cyanobacteria are largely responsible for the formation of extracellular polymeric substances (EPS), and their composition varies according to the species, nutrient availability, growth stage, and environmental conditions [12; 13; 14; 15]. Such substances consist mainly of polysaccharides and may remain associated with the cell surface or be released into the surrounding environment [15]. Besides playing a fundamental role in cell aggregation and adhesion, they can perform different functions, including protection from desiccation, viral attack, UV radiation, or sequestration of nutrients and metal ions [16; 17].  
Knowledge of EPS production in cyanobacteria is limited. The synthesis typically starts in the cytoplasm, where monosaccharides are converted into sugar nucleotides. These are then transferred by specific glycosyltransferases to transporter molecules in the plasma membrane. The subsequent steps of polymerization, assembly, and export are usually fairly conserved in bacteria [18]. Three main mechanisms have been described: the Wzy-dependent pathway, the ABC-dependent transporter pathway, and the synthase-dependent pathway [19]. Analysis at the phylum level shows that most cyanobacteria carry genes encoding proteins involved in one of the three pathways, but often not the full set that characterizes it. Moreover, EPS may be located loosely outside the cells (soluble fraction) or, in flocs or biofilms, closely associated with the cells [20]. Other important elements of EPS are lipopolysaccharide (LPS), which also play a key role in the stabilization of biocrusts [4; 10]. Lipopolysaccharides are large and complex amphipatic molecules consisting of lipids and saccharide residues that constitute the outer leaflet of the cell membrane of Gram-negative bacteria [21].  
_Halomicronema_ sp. is a cyanobacterium found in brackish water and microbial mats, where it forms biofilms. In this study, the isolate _Halomicronema_ sp. CCY15110 was considered; this strain was isolated from coastal microbial mats on the North Sea beach of Schiermonnikoog Island in the Netherlands [22] and its genome has been sequenced [23]. This study aimed to better characterize the biofilm formation of this bacteria, to identify chemicals that can inhibit this process, and to study gene expression regulating biofilm formation, including EPS genes.