Impact of low oxygen in diatom Nitzschia in light and dark conditions

Rising ocean temperatures is a major concern as it intensifies deoxygenation, threatening marine life and biogeochemical cycles. The lowering oxygen concentration can impact phytoplankton, a key primary producer reliant on O₂ for respiration, affecting marine productivity, fisheries, and food webs. Here, we examined how lowering oxygen concentration (hypoxia, anoxia) affect pennate diatom Nitzschia sp. under dim-light (∼20 μmol photons m^-2 s^-1) and dark conditions using isolates from anoxic benthic waters of the Western Indian Continental Shelf (WICS), the world's largest coastal hypoxic zone. In the in vitro experiment conducted under dim light, diatom growth decreased as O₂ concentration decreased, with lower growth rates under hypoxic (0.21 day^-1) and anoxic (0.24 day^-1) conditions compared to normoxic conditions (0.32 day^-1). While in the dark, diatom growth rates were lower compared to light under oxic (0.06 day^-1), hypoxic (0.10 day^-1), and anoxic (0.10 day^-1) conditions; however, cells remained viable under all lower oxygen conditions. Interestingly, under dark conditions, the highest nitrate uptake was observed under anoxic conditions (53 pmol cell^-1 day^-1), compared to oxic conditions (42 pmol cell^-1 day^-1; 82%). While, under 12 h light: 12 h dark incubation conditions, nitrate uptake was lower in oxic- L:D (6.9 pmol cell^-1 day^-1) and anoxic- L:D (5.9 pmol cell^-1 day^-1) conditions. The intracellularly stored nitrate (ICNO₃) was consumed only under anoxic-D conditions (0.3 pmol cell^-1 day^-1), compared to oxic-dark, anoxic-L:D and oxic-L:D, suggested the enhanced ICNO₃ utilization in the dark-anoxic condition. Similarly, higher NO₃^- uptake in antibiotic-treated (AT-Nitzschia, 39%) and antibiotic-untreated (UT-Nitzschia, 56%) cultures, with higher ICNO₃^- consumption (AT-Nitzschia, 84%; UT-Nitzschia, 69%) and increased extracellular ammonium (ECNH₄⁺) indicated diatom-mediated nitrate utilization under anoxic-dark condition.