People have run 24/7 light before, but whether or not that's ideal or leads to faster phyto growth depends on a ton of different factors: species, light spectrum, light intensity, temperature, nutrient levels, etc. (these listed ones are probably the most important, but others like pH, flow, and more will also play a role).
All of these factors influence growth rate, cell size, nutrition, sometimes color, etc. of a phytoplankton culture.
As an example to showcase this, low to moderate light intensity over a 24:0 (light:dark) photoperiod is likely going to show relatively higher growth than low to moderate light intensity over a 16:8 photoperiod, but high light intensity over a 24:0 photoperiod may actually result in slower growth than high light intensity over a 16:8 photoperiod.
These variables are why some people report higher yields/faster growth at one photoperiod and lower yields/slower growth at another, and why some people sometimes have conflicting reports on which photoperiod is better for a specific phytoplankton culture.
As a general rule, anywhere between 16:8 and 24:0 photoperiods are going to be the best, with 16:8 being the safer but possibly less productive option, dependent on the other factors. There are some exceptions to this rule, but they're the exceptions, not the norm.
