The parameter a is the initial slop

The parameter a is the initial slope of the light curve and reflects
the utilization efficiency of light energy. The rETR is the electron
transport rate, which is related to the overall photosynthetic performance
of the microalgae (Juneau et al., 2005). Ik is a measure
of the onset of light saturation and can be calculated by the ratio
between the rETRmax and the alpha (a).