Two outdoor open thin-layer cascade systems operated as
batch cultures with the alga Scenedesmus obliquus were
used to compare the productivity and photosynthetic acclimations
in control and CO
supplemented cultures in
relation with the outdoor light irradiance. We found that
the culture productivity was limited by CO
2
availability. In
the CO
2
2
supplemented culture, we obtained a productivity
of up to 24 g dw.m
−2
.day
−1
and found a photosynthetic
efficiency (value based on the PAR solar radiation energy)
of up to 5%. In the CO
limited culture, we obtained a
productivity of up to 10 g dw.m
2
−2
.day
−1
while the photosynthetic
efficiency was up to 3.3% and decreased to 2.1%
when the integrated daily PAR increased. Fluorescence
and oxygen evolution measurements showed that ETR and
oxygen evolution light saturation curves, as well as lightdependent
O
uptake were similar in algal samples from
both cultures when the CO
2
limitation was removed. In
contrast, we found that CO
2
limitation conducted to a
decreased PSII photochemical efficiency and an increased
light-induced heat-dissipation in the control culture compared
to the CO
2
2
supplemented culture. These features
are in line with a lower light use efficiency and may therefore
contribute to the lower productivity observed in absence
of CO
supplementation in outdoor mass cultures of
Scenedesmus obliquus