Effects of nitrate and ammonium on assimilation of nitric oxide by Heterosigma akashiwo

Abstract

The harmful alga Heterosigma akashiwo possesses a hybrid nitrate reductase (NR) enzyme, NR2-2/2HbN, which has the potential to convert NO to nitrate for assimilation into biomass. In previous research, NR transcription in H. akashiwo was induced by nitrate while NR activity was inhibited by ammonium. Here, the capacity of H. akashiwo to use NO in the presence of nitrate and/or ammonium was investigated to understand the regulation of NO assimilation. Continuous cultures of H. akashiwo were acclimated to growth on nitrate, ammonium, or a mixture of both. Aliquots from these cultures were spiked with 15N-labeled NO. The expression of genes involved in nitrogen assimilation was evaluated, as well as nitrate reductase activity and assimilation of 15N-labeled nitrogen into algal biomass. Results showed that NO induced expression and activity of NR, and upregulated expression of GOGAT regardless of the presence of other inorganic nitrogen sources, while GS expression decreased over time. Furthermore, 15NO uptake and assimilation was significantly higher in cultures acclimated for growth on ammonium compared to cultures acclimated for growth on nitrate alone. Assimilation of NO may provide H. akashiwo with a competitive advantage in N-poor environments or areas with elevated NO.

Full-Length Transcriptome Analysis of the Ichthyotoxic Harmful Alga Heterosigma akashiwo (Raphidophyceae) Using Single-Molecule Real-Time Sequencing

“…Recently, Healey et al (2023) proposed that utilization of nitric oxide (NO) may give H . akashiwo a competitive advantage over other phytoplankton in regions with high NO concentrations [ 16 ]. According to the physiological strategies of HAB species, including nutrient acquisition styles, behaviors, and biological interactions with coexisting species, Jeong et al (2015) categorized the HAB dynamics into four generation mechanisms (GMs; GM1–GM4), and GM4 can be used to explain the bloom of H .…”

Nitrate reductase enzymes in alga Chattonella subsalsa are regulated by environmental cues at the translational and post-translational levels

“…Recent research identified novel NRs with alternative structures in C. subsalsa and Heterosigma akashiwo (Raphidophyceae) (Stewart and Coyne, 2011;Wang et al, 2018; Figure 1). NR2-2/2HbN (referred to hereafter as NR2) identified in C. subsalsa and H. akashiwo has a 2/2 hemoglobin (2/2Hb) domain inserted in the Hinge-2 region that has been proposed to confer nitric oxide dioxygenase activity to this enzyme (Stewart and Coyne, 2011;Healey et al, 2023). Following research identified another novel NR in C. subsalsa (NR3) (Wang et al, 2018).…”