1. Threshold concentrations for Pi limitation: The solid circles represent the relative enhancement of N2 fixation rates in mixed-layer samples following Pi addition, plotted as a function of ambient Pi concentration.
Threshold concentrations for Pi limitation: The solid circles represent the relative enhancement of N2 fixation rates in mixed-layer samples following Pi addition, plotted as a function of ambient Pi concentration. The open circles are estimates of the minimum concentration required to saturate Pi uptake kinetics of the microbial assemblage—calculated as twice [Km] (i.e., the Pi concentration required to support one-half the maximum uptake rate derived from the Michaelis–Menten equation)—relative to the ambient Pi concentration, plotted as a function of in situ Pi concentration [data from Grabowski et al. (30) and Björkman et al. (27), respectively]. The observed median Pi concentration in the upper euphotic zone (0–45 m) for the period 1989–2015 is marked by the vertical dashed line; the 75th percentile range and median Pi concentration observed during positive and negative PDO phases are displayed in blue and red, respectively.
Ricardo M. Letelier et al. PNAS 2019;116:26:
©2019 by National Academy of Sciences