Variability of toxic Microcystis blooms and bacterial communities in freshwater bodies with different anthropopressure and climate zones: a novel comparative study in Guatemala and Poland

Abstract The global rise of harmful cyanobacterial blooms (HCBs) in freshwater systems is driven by anthropogenic pressure and climate change, yet limited information exists on how these factors shape HCBs and their phycosphere-associated bacterial communities (PBCs). We examined the influence of eutrophication (nutrient concentrations and land use), climate zone, and seasonality on phytoplankton and PBCs across three freshwater bodies: hypertrophic Lake Amatitlán with urban and agricultural catchments, mesotrophic Laguna del Pino with a forested/agricultural catchment (both tropical, Guatemala), and eutrophic Sulejów Reservoir (temperate, Poland). Surface water was collected (2024) during the dry season (Guatemala) and summer (Poland). Phytoplankton was analyzed via qPCR and microscopy; bacterioplankton via 16S rRNA sequencing; and microcystins via HPLC. Lake Amatitlán was dominated by toxic Microcystis exceeding World Health Organization recreational microcystin thresholds, especially in the agricultural catchment. Sulejów Reservoir showed a seasonal bloom of Microcystis , Aphanizomenon , and Snowella . In contrast, the diatom Cyclotella dominated lower-nutrient Laguna del Pino. PBCs included Flavobacterium , Pseudomonas , and Limnohabitans in nutrient-rich waters, whereas Polynucleobacter , often associated with good water quality, was abundant in Laguna del Pino. Nutrient enrichment was associated with cyanobacterial dominance and PBC structure in tropical systems, while nutrient availability and temperature were associated with bloom dynamics in the temperate system.