Fig. 6.

a A generalized photosynthesis-irradiance (P vs. E) curve, shown in dimensionless form: Photosynthesis has been scaled to its light-saturated rate, P _m, and irradiance has been scaled to the light-saturation parameter, E _k. The scaled rate of light absorption is also shown: The shaded area between the two curves represents light energy that is absorbed by the cell but not used in photosynthesis. This must be dissipated to avoid oxidative stress; incubating cells at irradiances ≫E _k is likely to result in reduction in photosynthetic rate (i.e., photoinhibition) and growth rate. b The temperature-dependence of E _k in 820 P vs. E curves from estuarine, neritic, and oceanic waters off North America (H.L. MacIntyre et al., unpublished data). The data have been fit (heavy line and equation) to a second-order polynomial. Note the dispersion of data around the central tendency: 94 % are between 0.5 and 2x of the fitted value. Incubating samples at the irradiance predicted by the equation (Table 2) should ensure than the overwhelming majority of samples are exposed to irradiances that are between 0.5 and 2x E _k, a range over which photosynthesis achieves 14–61 % of the light-saturated rate and over which oxidative stress is minimal. Growth rates are expected to scale directly with the achieved photosynthetic rate