Urea Is Both a Carbon and Nitrogen Source for Microcystis aeruginosa: Tracking 13C Incorporation at Bloom pH Conditions

Abstract

The use of urea as a nitrogenous fertilizer has increased over the past two decades, with urea itself being readily detected at high concentrations in many lakes. Urea has been linked to cyanobacterial blooms as it is a readily assimilated nitrogen (N) - source for cyanobacteria that possess the enzyme urease. We tested the hypothesis that urea may also act as a carbon (C) source to supplemental growth requirements during the alkaline conditions created by dense cyanobacterial blooms, when concentrations of dissolved CO 2 are vanishingly low. High rates of photosynthesis markedly reduce dissolved CO 2 concentrations and drive up pH. This was observed in Lake Erie during the largest bloom on record (2015) over long periods (months) and short periods (days) of time, suggesting blooms experience periods of CO 2 -limitation on a seasonal and daily basis. We used 13 C-urea to demonstrate that axenic cultures of the model toxic cyanobacterium, Microcystis aeruginosa NIES843, assimilated C at varying environmentally relevant pH conditions directly into a spectrum of metabolic pools during urea hydrolysis. Primarily, 13 C from urea was assimilated into central C metabolism and amino acid biosynthesis pathways, including those important for the production of the hepatotoxin, microcystin, and incorporation into these pathways was at a higher percentage during growth at higher pH. This corresponded to increased growth rates on urea as the sole N source with increasing pH. We propose this ability to incorporate C from urea represents yet another competitive advantage for this cyanobacterium during dense algal blooms.

The “Neglected Viruses” of Taihu: Abundant Transcripts for Viruses Infecting Eukaryotes and Their Potential Role in Phytoplankton Succession

“…Silica may act as the constraining nutrient that catalyzes the transition of the community. Reductions in silica deposition are hypothesized to occur in association with increased pH conditions during Microcystis blooms, constraining diatom growth (Krausfeldt et al, 2019). In marine environments, silica limitation has been linked to increases in viral infection of diatoms (Kranzler et al, 2019).…”

Culture-independent and culture-dependent analyses of the bacterial community in the phycosphere of cyanobloom-forming Microcystis aeruginosa

“…The pH value ranged from 7.0 to 9.4, and the most alkaline condition was found in the DC sample. Higher alkaline conditions, such as pH 9.5, have been shown to promote the growth of cyanobacteria 30 . The dissolved oxygen (DO) levels in the DC sample was low (2.0 ± 0.22 mg/L); similarly, the other samples also presented with low DO levels (<1 mg/L).…”

Distribution of the Harmful Bloom-Forming Cyanobacterium, <i>Microcystis aeruginosa</i>, in 88 Freshwater Environments across Japan

“…However, for MG3-abundant populations, designated as S-group-2 and S-group-4 to -6, the IGS-PC gene was detected up to pH 10.4. Since pH increases as Microcystis blooms grow because of CO 2 consumption by photosynthesis (Paerl and Ustach, 1982;Krausfeldt et al, 2019), dominant morphotypes in the late bloom phase may be adapted to high pH. M. aeruginosa and M. wesenbergii formed large colonies in the late bloom phase between August and October in Lake Taihu (Li et al, 2016;Xiao et al, 2018).…”