Nitrogen excess induces cytokinesis arrest and alters lipid metabolism in the bloom-forming desmid Cosmarium tinctum

Abstract

The desmid genus Cosmarium often contributes to freshwater blooms. However, its physiological response to nitrogen, particularly excess nitrogen, remains unclear. A systematic investigation was conducted to ascertain the effects of nitrogen availability on C. tinctum (KCTC AG60915), isolated from a bloom source. Cells were batch- cultured over 18 days at four nitrogen concentrations (2 to 250 mg N L-1), monitoring growth, life stages (FlowCam imaging), photosynthetic efficiency (Fv/Fm, Chlorophyll a contents by pulse-amplitude modulated fluorometry), and fatty acid profiles (gas chromatography), followed by multivariate data integration. Optimal growth occurred at 10 mg N L-1. High nitrogen levels (250 mg N L-1) significantly reduced growth rates and severely impaired photosynthetic efficiency (Fv/Fm values dropping to ~0.26), while inducing a substantial accumulation of Stage 3 daughter-cell pairs (up to ~15 % of cells), indicative of arrested cytokinesis confirmed by field emission scanning electron microscopy (FE-SEM). Total fatty acid (TFA) content peaked under nitrogen limitation (2 mg N L-1); however, under nitrogen excess, TFA content was not only the lowest but failed to show any net accumulation above initial levels, suggesting suppressed lipid biosynthesis. In contrast, fatty acid composition depended primarily on culture duration, revealing a decoupling from TFA content regulation. Multivariate analyses confirmed distinct nitrogen-driven physiological states. Nitrogen excess acts as a profound physiological stressor in C. tinctum (KCTC AG60915), markedly impairing photosynthesis, arresting cytokinesis, and severely suppressing net TFA accumulation, potentially involving carbon reallocation. These findings challenge simple nutrient-enrichment paradigms for desmids and reveal complex metabolic trade-offs relevant to ecophysiology and biotechnology.