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

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 CO2 are vanishingly low. High rates of photosynthesis markedly reduce dissolved CO2 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 CO2-limitation on a seasonal and daily basis. We used 13C-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, 13C 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.

Midgut metabolomic profiling of fall armyworm (Spodoptera frugiperda) with field-evolved resistance to Cry1F corn.

Populations of the fall armyworm (Spodoptera frugiperda) have developed resistance to transgenic corn producing the Cry1F insecticidal protein from the bacterium Bacillus thuringiensis (Bt). Resistance in S. frugiperda from Puerto Rico is genetically linked to a mutation in an ATP Binding Cassette subfamily C2 gene (SfABCC2) that results in a truncated, non-functional Cry1F toxin receptor protein. Since ABCC2 proteins are involved in active export of xenobiotics and other metabolites from the cell, we hypothesized that Cry1F-resistant fall armyworm with a non-functional SfABCC2 protein would display altered gut metabolome composition when compared to susceptible insects. Mass spectrometry and multivariate statistical analyses identified 126 unique metabolites from larval guts, of which 7 were found to display statistically significant altered levels between midguts from susceptible and Cry1F-resistant S. frugiperda larvae when feeding on meridic diet. Among these 7 differentially present metabolites, 6 were found to significantly accumulate (1.3-3.5-fold) in midguts from Cry1F-resistant larvae, including nucleosides, asparagine, and carbohydrates such as trehalose 6-phosphate and sedoheptulose 1/7-phosphate. In contrast, metabolomic comparisons of larvae fed on non-transgenic corn identified 5 metabolites with statistically significant altered levels and only 2 of them, 2-isopropylmalate and 3-phosphoserine, that significantly accumulated (2.3- and 3.5-fold, respectively) in midguts from Cry1F-resistant compared to susceptible larvae. These results identify a short list of candidate metabolites that may be transported by SfABCC2 and that may have the potential to be used as resistance markers.

Integrated proteomics and metabolomics suggests symbiotic metabolism and multimodal regulation in a fungal-endobacterial system.

Many plant-associated fungi host endosymbiotic endobacteria with reduced genomes. While endobacteria play important roles in these tri-partite plant-fungal-endobacterial systems, the active physiology of fungal endobacteria has not been characterized extensively by systems biology approaches. Here, we use integrated proteomics and metabolomics to characterize the relationship between the endobacterium Mycoavidus sp. and the root-associated fungus Mortierella elongata. In nitrogen-poor media, M. elongata had decreased growth but hosted a large and growing endobacterial population. The active endobacterium likely extracted malate from the fungal host as the primary carbon substrate for energy production and biosynthesis of phospho-sugars, nucleobases, peptidoglycan and some amino acids. The endobacterium obtained nitrogen by importing a variety of nitrogen-containing compounds. Further, nitrogen limitation significantly perturbed the carbon and nitrogen flows in the fungal metabolic network. M. elongata regulated many pathways by concordant changes on enzyme abundances, post-translational modifications, reactant concentrations and allosteric effectors. Such multimodal regulations may be a general mechanism for metabolic modulation.