Differential responses of diatom and phototrophic dinoflagellate species to five oxygen concentrations: growth, photosynthesis, and respiration rates

<p style="line-height: 1.6;"> While phytoplankton are a major component of coastal ecosystems and experience a wide range of oxygen conditions, the survival and physiological responses of common phytoplankton species under hypoxia remain insufficiently understood. To explore these effects, cell densities of the two dinoflagellate species <i>Prorocentrum triestinum</i> and <i>Amphidinium carterae</i>, and the two diatom species <i>Cylindrotheca closterium</i> and <i>Chaetoceros curvisetus</i>— isolated from waters with a dissolved oxygen (DO) concentration of 1.5 mg O₂ L^-1—were measured under target DO concentrations of 0.5, 1.0, 1.5, 4.0, and >7.0 mg O₂ L^-1. Using these data, the growth rate, net photosynthesis rate, and dark respiration rate of each species were determined. While <i>A. carterae</i>, <i>Cy. closterium</i>, and <i>Ch. curvisetus</i> survived at all tested DO concentrations, <i>P. triestinum</i> survived only at ≥1.0 mg O₂ L^-1. The two dinoflagellate species examined here, which possess active swimming ability, exhibited the highest growth rates at >7.0 mg O₂ L^-1, whereas the two diatom species examined here showed peak growth rate at 1.5 mg O₂ L^-1. Net photosynthesis rates of all four species peaked at 1.5 mg O₂ L-1, while dark respiration rates were highest at >7.0 mg O₂ L^-1. These findings suggest that although these phytoplankton species can grow under hypoxic conditions, dinoflagellate species achieve maximal growth at higher oxygen concentrations, possibly due to the energetic demands of motility. This study provides a useful basis for understanding species-specific responses of phytoplankton to varying oxygen conditions in coastal environments. </p>