## 1 Introduction  
Constructed wetlands (CWs) have been widely used in the last decades as an ecological technology for treating different types of wastewater [1, 2, 3]. These biological systems constitute an environmentally friendly and sustainable alternative to conventional treatments while providing a plethora of ecosystem services with huge impacts on nature and human wellbeing [4, 5]. Additionally, CWs are considered natural biodiversity hotspots favoring the establishment of plants, microorganisms, and animals [6, 7, 8].  
Macroophytes are among the most-commonly used plants in CWs since they can adapt to harsh edapoloclimate conditions, or several stresses derived from the wastewater composition, such as high levels of salinity, loadings of organics, nutrients, and pollutants [9, 10]. The use of polycultures has been reported as an attractive strategy that provides additional benefits like improving the landscape aesthetic and biodiversity, especially if they include ornamental plants [1, 4, 11]. One of the most important functions of vegetation is the oxygenation of the CWs bed, as oxygen influences redox potential and microbial activity [12].  
The efficiency of CWs for wastewater treatment relies on microbial-mediated processes, like the transformation and mineralization of organic matter and/or the degradation of pollutants [11, 13, 14], that occur in the biofilm formed around plant's roots and/or attached to the surface of bed substrate. Deeping the knowledge on the diversity and composition of microbial communities dwelling CWs and on the potential impact of environmental factors on their activity, is pivotal to understand the functioning of these biological systems [15]. Several authors have reported the influence of vegetation, the composition of wastewater, as well as the type of filter bed substrates on microbial populations abundance, structure, and diversity using culture-independent methods [16; 17; 18; 19; 20; 21; 22]. For instance, [PERSON] et al. [23] reported higher bacterial diversity and richness in a CW mesocosm (for hexachlorobenzene removal) colonized by _Typha angustifolia_ than in the mesocosm colonized by _Phragmites australis_. [PERSON] et al. [20] concluded that the type of substrate greatly influences the structure and diversity of bacterial communities in a CW system for surface water treatment. The bed substrate of a CW is thus, of utmost importance due to its role on plant support, microbial biofilms establishment, also influencing the hydraulic processes [14].  
The ultimate goal of CWs is the efficient treatment of wastewater, with a view to its potential reuse, including the use of treated water for irrigation purposes [24; 25]. However, depending on the origin of the wastewater it is important to evaluate the prevalence of some bacterial pathogens, including total and fecal coliforms, and _Escherichia coli_ found on the water, substrate, and plant tissues [25; 26; 27; 28].  
The present work aimed (i) to assess the influence of season variations across three consecutive years on the abundance of fecal indicator organisms (total coliforms and _E. coli_) in wastewater and associated with the substrate, and roots of plants inhabiting the inlet and outlet zones of a CW for domestic wastewater treatment, and (ii) to evaluate the influence of season variations and CW zones on structure and diversity of bacterial communities associated to the bed substrate.