The final chlorophyll concentration, measured at the end of the xperiment, strongly depended on the temperature (Fig. 2: about 1.5 mg/L at 15C and 2.6 mg/L at 25C).
The photoperiod had little influence on this parameter, although, higher biomass values were determined for the culture B15-18 compared with those of B15–12 after approximately 70 h and until 170 h. This could be attirbuted to the lower chlorophyll concentration at time zero for the culture B15–12.
The main differences between the experiments at the two photoperiods are the shorter exponential growth phase and the
presence of a stationary phase for B15–18. This may be explained by the pH profile of the cultures (Fig. 2) over time. After approximately 70 h, a significantly higher pH of the culture B15–18 is observed compared with B15–12.
This is expected to be linked to a higher CO uptake rate, related to increased microalgae concentration in the culture B15–18 (Fig. 2b).
Since pH has a strong impact in microbial growth and activity, this increase might have contributed to the halt of the exponential growth phase for B15–18 sooner than that for B15–12.
2 In the experiment B25–12 (Fig. 1), the exponential growth phase (until approximately 100 h) was followed by a phase of linear growth.
A possibility is that this linear phase resulted from a light and/or nitrogen limitation [27]; at 100 h, ammonium depletion
in the medium was observed, as previously-stated in Section 3.1.
In order to quantify the relative significance of non-photo- trophic microbial metabolism, experiments were also performed in obscurity (Fig. 4).
In darkness the pH of the cultures remained approximately constant over time at 8.5 for all the temperatures tested (data not shown).
No increase in the chlorophyll concentration (neither in the TSS or VSS concentrations, data not shown)
was observed, suggesting that there was no microbial growth and, in particular, no algal biomass production.
This suggested that the microalgae in our cultures were obligate phototrophs unable to grow or perform heterotrophic metabolism with the available organic carbon in the medium.
This may be also due to the presence of refractory organic carbon that are not biodegraded by heterotrophic bacteria or microalgae. Thus photosynthesis seems to be the predominant biological phenomenon in our cultures.