The influence of ammonium to nitrate ratio on the thermal responses of early life stages of the giant kelp Macrocystis pyrifera

The distribution and productivity of seaweeds, including kelps, are strongly influenced by changes in temperature and inorganic nitrogen (Ni) availability. However, their tolerances, sensitivities, and capacity to adjust to these changes can vary across their complex life cycle (microscopic and macroscopic stages). For the early life stages of Macrocystis, this study investigated the effect of the nitrate: ammonium ratio on physiological traits and gametophyte development at a range of temperatures (6.5 to 26.5 °C). Thermal performance curves (TPCs) were utilized to determine and predict the effect of temperature on meiospore germination success (%), gametophyte size (μm 2 ), sex ratio and physiological traits (photosynthetic efficiency and nitrate and ammonium uptake rates). We found that the presence of NH 4 + led to negative NO 3 − uptake rates in Macrocystis pyrifera gametophytes and reduced growth rates, and results suggest that thermal tolerance windows might vary between developmental stages and physiological traits. Our findings indicate that reduced meiospore germination and gametophyte size induced by suboptimal temperatures and increased NH 4 + concentrations can lead to lower physiological performance that might subsequently affect adult individuals and populations. Knowledge of the Ni uptake kinetics and source preferences across different life cycle phases can be crucial for conservation and aquaculture purposes as it might increase species productivity and persistence in a changing ocean.