Dissolved nitrogen and phosphorus trigger Euglena sanguinea blooms via Burkholderiaceae enrichment and extracellular polymeric substance stimulation

As representative landscape water bodies, urban park ponds are typically shallow and hydrologically isolated, making them highly susceptible to algal blooms. This study focused on recurrent summer-autumn blooms of Euglena sanguinea in Hefei Binhu Forest Park. These blooms form thin, red, oil-slick-like surface scums that severely suppress aquatic photosynthesis. We investigated phytoplankton community succession and its drivers by collecting surface biofilm, mid-depth water, and bottom sediment samples from three representative ponds during the 2024 bloom season. Results revealed extensive E. sanguinea blooms in July-August, with surface cell density reaching 9.86 × 10⁶ cells/L (42% of total) and biomass attaining 98.61 mg/L (94% of total). This bloom peak coincided with a 2.5-fold increase in surface dissolved total nitrogen (DTN) and phosphorus (DTP). Concurrently, the surface biofilm exhibited a peak extracellular polymeric substance (EPS) concentration of 43.92 mg/L and a film-forming rate of 90.73%, structurally supported by the predominance of large algal-bacterial aggregates (>64 μm), which accounted for nearly 80% of the particulate composition. The bounding EPS (BEPS), rich in tryptophan-like proteins, corresponded with peak biofilm hydrophobicity. Critically, this nutrient-enriched microenvironmental transformation selected for a low-diversity, high-dominance microbiome. Burkholderiaceae dominated the August biofilm (23%), contrasting sharply with sediment communities (dominated by Steroidobacteraceae, 7%) and post-bloom October biofilms (dominated by Sporichthyaceae, 21%). Mechanistic path analysis revealed that DTN and DTP stimulated bloom expansion not by directly promoting algae, but by enriching Burkholderiaceae and stimulating EPS production. These findings elucidate a microbially mediated pathway linking nutrient enrichment to E. sanguinea bloom formation, challenging the conventional direct nutrient-bloom paradigm. The study provides mechanistic blueprint for targeted, microbiome-informed management of urban landscape water blooms.