Phosphorus limitation heightens vulnerability of

Introduction: Phytoplankton rely on diazotrophs like the globally important Methods: We performed complete temperature curves (20-34 °C) under three nutrient conditions [iron (Fe)-limited, phosphorus (P)-limited, and Fe/P-replete] to compare physiological responses and test previously hypothesized temperature-nutrient interactions. Results: Across the viable temperature range, replete culture growth and fixation rates showed narrower, unimodal-like curves, contrasting the greater plateauing witnessed in nutrient-limited thermal curves. Under both limitations, nitrogen fixation was more impacted than carbon fixation. Fe-limited cultures performed better at higher temperatures (survival range: 22-34 °C) and shared thermal optima with replete cultures for growth rates (28-32 °C), while P-limited cultures performed better at lower temperatures (survival range: 20-32 °C) with a narrower optimum range (28-30 °C). Furthermore, extreme temperatures appear to outweigh and override nutrient limitation effects at the warmer end for Fe-limited cultures (34 °C) and the cooler end for P-limited cultures (20 °C), as no significant differences were observed between limited and replete cultures for both growth and fixation rates at these temperatures. Discussion: Given predicted sea temperature increases and the rising frequency and intensity of oceanic heat waves, our results suggest that