Physiological responses of the freshwater N₂‐fixing cyanobacterium Raphidiopsis raciborskii to Fe and N availabilities

Abstract

Summary The cyanobacterium Raphidiopsis raciborskii is of environmental and social concern in view of its toxicity, bloom‐forming characteristics and increasingly widespread occurrence. However, while availability of macronutrients and micronutrients such as N and Fe are critically important for the growth and metabolism of this organism, the physiological response of toxic and non‐toxic strains of R. raciborskii to varying Fe and N availabilities remains unclear. By determining physiological parameters as a function of Fe and N availability, we demonstrate that R. raciborskii growth and N2‐fixing activity are facilitated at higher Fe availability under N2‐limited conditions with faster growth of the CS‐506 (cylindrospermopsin‐producing) strain compared with that of CS‐509 (the non‐toxic) strain. Radiolabelled Fe uptake assays indicated that R. raciborskii acclimated under Fe‐limited conditions acquires Fe at significantly higher rates than under Fe replete conditions, principally via unchelated Fe(II) generated as a result of photoreduction of complexed Fe(III). While N2‐fixation of both strains occurred during both day and night, the CS‐506 strain overall exhibited higher N2‐fixing and Fe uptake rates than the CS‐509 strain under N‐deficient and Fe‐limited conditions. The findings of this study highlight that Fe availability is of significance for the ecological advantage of CS‐506 over CS‐509 in N‐deficient freshwaters.

Comparative proteomics of the toxigenic diazotroph Raphidiopsis raciborskii (cyanobacteria) in response to iron

“…In contrast to CYN + , almost all energy proteins including those involved in PSI and PSII, as well as ATP synthase, were severely reduced in abundance in CYN − at exponential phase, appearing to be severely inhibited in response to the stress, a result confirmed by Fu et al . (2019) who reported that CYN − underwent chlorosis under the same growth conditions.…”

“…However, all detected transporters and binding proteins were decreased in abundance regardless of strain or growth condition, with the only exception being the large increase in a possible porin protein during exponential growth by CYN + . These results are in agreement with the increased capability of CYN + to uptake 55 Fe compared to CYN ‐ (Fu et al ., 2019), possibly mediated by the increased abundance of the porin protein. The increased ability of CYN + to uptake 55 Fe, likely through the increased abundance of the porin protein, further supports the increased fitness of CYN + compared to CYN − .…”

“…Concentrations of Fe were selected based on the 1000 nM Fe (unmodified medium) providing similar unlimited growth of both strains. Moderate (200 nM Fe) and severe (50 nM Fe) limitations of Fe were selected based on observed decreases in cell function and for comparison to previous Fe limitation experiments previously described (Fu et al ., 2019). Strains were harvested during the exponential growth phase after monitoring cell growth using chlorophyll a measurements as described previously (Meeks and Castenholz, 1971).…”

“…Further supporting limitation of nitrogen fixation by Fe availability is the simulated global ecosystem model that hypothesizes diazotrophy is controlled by the nitrogen:Fe ratio in marine systems (Mark Moore et al ., 2009). While numerous studies have focused on the ability of R. raciborskii to respond to nitrogen and phosphorous conditions, Fe limitation has largely been ignored, with the exception of a recent physiological study by our group (Fu et al ., 2019), which revealed an ecological advantage of toxic strains over non‐toxic strains under diazotrophic and Fe‐limiting conditions.…”

“…iTRAQ is a proven proteomic tool for comparing the protein abundance in response to altered external stimuli in a range of toxic and non‐toxic cyanobacteria (D'Agostino et al ., 2016; Yeung et al ., 2016). Utilizing the available genome sequences of Australian cylindrospermopsin‐producing (CYN + ) R. raciborskii CS‐506 and non‐toxic (CYN ‐ ) R. raciborskii CS‐509, we compared the proteomic response of each strain under different levels of Fe availability during diazotrophic growth conditions and at different growth stages, to complement previous physiological studies (Fu et al ., 2019). This is the first comparative proteomic study of the toxigenic R. raciborskii in relation to diazotrophic growth and Fe stress and provides a functional framework for how R. raciborskii responds to critical nutrient limitation.…”