Fertilization with microalgae-biomass from wastewater treatment ponds can transmit fecal indicator bacteria into the agricultural system and has specific effects on the root microbiome composition of wheat plants

High rate algal ponds (HRAPs) are effective alternative wastewater treatment systems for sustainable nutrient recycling. The nutrient rich microalgae biomass (MB) can be used as efficient agricultural biofertilizer to replace mineral fertilizers.

Complex effects are expected for the plant microbiome due to MB absorbed organic compounds, pollutants and attached bacteria including wastewater derived pathogens and antibiotic resistant bacteria. We studied the release of potential pathogenic, antimicrobial resistant and biocide tolerant bacteria and effects on community structure of the plant microbiome after fertilization with dried MB.

Wheat plants in a greenhouse pot experiment were fertilized either with dried MB or mineral fertilizer supplying 60, 120, and 180 kg ha-1 N-equivalents. The root microbiome of fertilized plants was compared to that of non-fertilized plants.

Cultivation dependent methods detected E. coli, ESBL-E. coli, enterococci and biocide tolerant bacteria. Total DNA was extracted from rhizospheres and roots for 16S rRNA gene amplicon sequencing and quantification of bacteria, fungi, nitrogen cycling and antibiotic resistant genes (ARG). Compared to control plants, both fertilizers supported plant growth and yield (Mückschel et al. 2023). E. coli, ESBL-E. coli and enterococci were only cultured from MB and roots of MB-fertilized plants but not from potting soil and wheat seedlings used for the experiment. Biocide-tolerant Pseudomonas were cultured from all samples, while biocide tolerant E. coli, Klebsiella and Enterobacter were only cultured after MB application.

While the abundance of root colonizing bacteria, fungi and N cycling genes showed no significant difference after MB and mineral fertilizer application, clear differences were obtained for the diversities and composition of the rhizosphere and root microbiome. Plant colonization with MB derived bacteria was thereby indicated. In addition, higher abundance of ARGs was obtained after MB application.

Our data showed that fertilizations with HRAP derived MB has complex effects on agricultural ecosystems and still requires more detailed multidisciplinary research.