Nitrogen flux into metabolites and microcystins changes in response to different nitrogen sources inMicrocystis aeruginosaNIES‐843

Abstract

The over-enrichment of nitrogen (N) in the environment has contributed to severe and recurring harmful cyanobacterial blooms, especially by the non-N 2fixing Microcystis spp. N chemical speciation influences cyanobacterial growth, persistence and the production of the hepatotoxin microcystin, but the physiological mechanisms to explain these observations remain unresolved. Stable-labelled isotopes and metabolomics were employed to address the influence of nitrate, ammonium, and urea on cellular physiology and production of microcystins in Microcystis aeruginosa NIES-843. Global metabolic changes were driven by both N speciation and diel cycling. Tracing 15 N-labelled nitrate, ammonium, and urea through the metabolome revealed N uptake, regardless of species, was linked to C assimilation. The production of amino acids, like arginine, and other N-rich compounds corresponded with greater turnover of microcystins in cells grown on urea compared to nitrate and ammonium. However, 15 N was incorporated into microcystins from all N sources. The differences in N flux were attributed to the energetic efficiency of growth on each N source. While N in general plays an important role in sustaining biomass, these data show that N-speciation induces physiological changes that culminate in differences in global metabolism, cellular microcystin quotas and congener composition.

Nutrient limitation dynamics of western lake erie cyanoHAB biomass and toxin production

“…Furthermore, an adaptive management approach is needed to determine if the 40% reduction goal needs to be adjusted with changes in land use practices and climate change [99]. Additionally, future studies should focus on drawing direct functional links between nutrient enrichment and cyanotoxin production, e.g., Krausfeldt et al [36]. Lastly, anatoxin should be more closely monitored in WLE, as it is a potent neurotoxin with human health-associated implications [100].…”

“…There is an urgent need to determine the linkage between different bioreactive forms of N and P and the promotion of toxic CyanoHABs, to establish the necessary reduction in these nutrient forms to ensure the security of surface potable water. Nutrient reduction will likely need to be even greater as climate change increases the N and P reduction thresholds required for CyanoHAB mitigation [35,36]. The in situ bioassay-based study reported here is among the first to use an experimental approach to investigate the response of a natural CyanoHAB community dominated by either Microcystis (Maumee Bay) or Planktothrix (Sandusky Bay) to actual reductions in N, P or both, under natural conditions in Lake Erie.…”

“…The prominent cyanotoxins, microcystin and anatoxin, have molecular structures containing N, suggesting that their syntheses may be linked to N availability; hence, there is a need to investigate the potential roles N fertilizers (i.e., NH 4 , NO 3 , and urea) play in bloom dynamics and toxin production in Lake Erie [23,36,42]. A recent study showed that there are N concentration reduction thresholds at which bloom microcystin levels will decrease, leading to further evidence that N limitation may play a role in controlling cyanotoxin production in the WLE blooms [41].…”

Nitrogen Availability Affects the Metabolic Profile in Cyanobacteria

“…In addition, there are differences in gene expression associated with utilizing the nitrogen sources and changes in activity of enzymes involved in nitrogen assimilation when cyanobacteria are exposed to each of the nitrogen sources, NO 3 − , NH 4 + , and urea [14][15][16]. The physiological response of cyanobacteria to nitrogen sources also varies according to the species [13,17]. We previously reported that when Synechocystis sp.…”

“…For this purpose, we performed a combination of in vivo 15 N-labeling of metabolites and metabolome analysis. The 15 N-labeling technique is applied to detect metabolites of interest in cyanobacteria or to examine the metabolic behavior of a few metabolites [2,17,21,22]. This technique enables us to compare the metabolic turnover under different nitrogen sources by calculating the 15 N labeling rate at each time point.…”