3.4 Comparison with previous studies under natural light regimes
Several previous studies have investigated Phaeodactylum photophysiology under a sinusoidal light regime, using either bench-top or outdoor cultures. Past bench-top studies employed 3 cm-deep rectangular photobioreactors for culture maintenance, to reduce self-shading effects. These studies observed higher levels of NPQ capability and modeling of photophysiology suggested a high degree of AET in excess light [26] and [63]. This scenario represents what could occur in shallow, non-turbid natural waters. When fluctuations of light intensity with a period of 30 min were applied to mimic vertical mixing in natural systems, much smaller NPQ values were observed close to those reported in our work [26]. The outdoor studies were conducted either in open ponds or tubular photobioreactors [68], [69] and [70]. In open ponds, almost no decrease of Fv/Fm was observed around solar noon and NPQ induction levels were small, reminiscent of our results [68] and [69]. Mid-day photoinhibition was observed at low cell densities in tubular photobioreactors, but increasing Phaeodactylum cell densities ameliorated this [68], [69] and [70]. The low degree of photoinhibition found in outdoor cultures is likely due to the rapid mixing of cultures between excess light and sub-saturating intensities and the net time spent in excess light is not sufficient to induce NPQ or induce significant damage to PSII. This supports earlier suggestions that cell density and mixing regime are important considerations for maximizing productivity [28] and [71].