Occupants and surface types drive microbial dynamics in controlled indoor environments

Indoor microbial communities play a critical role in influencing indoor environmental quality and human health and are shaped by occupant activity, surface characteristics, and environmental conditions. While previous studies have examined these factors individually, systematic evaluations of their combined interactions, particularly involving Heating, Ventilation, and Air Conditioning (HVAC) and drainage systems, remain limited. This controlled, long-term (1.5-year) investigation assessed how human occupancy, surface moisture (dry vs. wet), aquaponics (soilless plant-aquarium systems), and environmental parameters (humidity, ventilation, and seasonal variations) influence bacterial and eukaryotic dynamics in tightly sealed residential units. Continuous air-conditioner operation without fresh-air intake led to elevated CO₂ levels during occupancy and pronounced seasonal humidity fluctuations, emphasizing the need for improved ventilation and adaptive humidity control in compact urban residences. Amplicon sequencing revealed higher microbial diversity on dry surfaces (aerosols, air-conditioner filter dust, and floor dust) than on wet surfaces (waste drains and showerheads). Wet environments supported biofilm-associated taxa adapted to moist conditions (e.g., Methylobacterium, Vermamoeba). Human occupancy significantly enriched air-conditioner filter dust with opportunistic bacteria (e.g., Finegoldia and Streptococcus), underscoring occupant-driven microbial accumulation via recirculated air. Additionally, the small-scale aquaponic system had minimal measurable influence on microbial composition at the room scale, suggesting limited aerosolization or dispersal under typical usage conditions. Indoor relative humidity was significantly correlated with microbial diversity in air systems, notably enhancing moisture-adapted taxa such as Sphingomonas during humid seasons. Seasonal variations markedly influenced eukaryotic communities (e.g., pollen influx), whereas bacterial communities were more strongly influenced by human occupancy. These findings highlight the critical role of human-driven microbial accumulation in air-conditioner filters and the distinct microbial profiles associated with dry and wet indoor surfaces. Although small-scale aquaponics demonstrated minimal room-wide microbial impact, its potential localized influence warrants further exploration. These insights offer practical guidance for targeted hygiene protocols, HVAC system maintenance, and building design strategies aimed at improving indoor microbial quality and supporting occupant health.