The ecology of cyanobacteria and their synergism with bacterioplankton in benthic mats under nutrient limitations in the Virgin River in Zion's National Park.

Nitrogen (N) and Phosphorus (P) are essential nutrients which support cyanobacterial growth in lakes (as suspended cells or flocs) and flowing waters (as benthic mats). However, some toxic cyanobacteria can thrive even under depleted nutrient conditions. In this study, we investigated benthic toxic cyanobacterial mats in the Virgin River, Zion National Park, at two sites where visible benthic mats were observed at three different times. Our study focused on phosphorus and nitrogen cycling within these mats. The reactive P was below detection limits (<0.05 mg/L as P). Likewise, the dissolved inorganic N varied from below the detection limit to 0.06 mg/L as N. The relative abundance (based on total sequenced reads) of cyanobacteria ranged from 16.87 % to 50.02 % for samples from the Visitor's Center site and 10.34 % to 32.68 % for samples from the North Creek site with the presence of genera Microcoleus, Pseudoanabaena, Oscillatoria, Microcystis, and Cyanobium. The Visitors Center and North Creek sites shared several key heterotrophic bacteria, with Sandarakinorhabdus, Planctomyces, Brevundimonas, Thiomonas, and Pseudomonas being the most prominent ones. Notably, a toxic Microcoleus strain was identified in the nutrient-limited conditions of the Virgin River. This strain harbored an anatoxin-a biosynthesis gene cluster but lacked the anaK gene, suggesting limited toxin variability. We investigated the role of heterotrophic bacteria in nutrient cycling and their interactions with cyanobacterial populations within benthic mats of the Virgin River. Microcoleus and other cyanobacteria employed the Pho regulon, expressing genes such as pstS and alkaline phosphatase, to efficiently acquire organic phosphorus in low-nutrient environments. Nitrogen cycling also played a crucial role, with coexisting heterotrophic bacteria contributing to nitrogen fixation (nif gene) and assimilation, as well as denitrification (nosZ, nir, and nar genes). This research signifies the importance of integrated molecular approaches in unraveling nutrient cycling dynamics and toxin production mechanisms in benthic cyanobacterial mats. These insights are critical for understanding harmful cyanobacterial benthic blooms in freshwater ecosystems.