## 3 Results
### Nitrate and Phosphate Uptake  
Considering initial nitrate content as 100%, almost the total amount of nitrate was taken up in the control, 75%-N, and 50%-N cultures by the end of the experiment in the hybrid reactor (Figure 6a). The dynamic of nitrate uptake was in accordance with growth or at least with the total biomass measured on the 7 th day of the experiment: A slow nitrate uptake was observed in the first four days in control culture, then more than 95% of nitrate was taken up by the 7 th day. There was faster nitrate uptake in the nitrate-depleted cultures: more than 80% of the initial nitrate content was taken up in the first four days in 75%-N culture. There was slower, but still accelerated nitrate uptake in the 50 and 25%-N cultures than in others. Interestingly, only ~70% of initial nitrate content was taken up in the 25%-N culture (Figure 6a).  
Figure 4: Chlorophyll concentrations in the control (C) and nitrogen depleted (75, 50 and 25%-N) _Monorapidium pusillum_ cultures (**a**) in the hybrid reactor (HR) and (**b**) in the tank reactor (TR). Means (n = 3) and standard deviations are plotted. Different lowercase letters at curves mean significant differences among the last day values (_p_ < 0.05).  
Figure 5: Dry masses on the last (7 th) day of the experiment in the control (C) and nitrogen depleted (75, 50, and 25%-N) _Monorapidium pusillum_ cultures in the hybrid reactor (HR) and in the tank reactor (TR). Means (n = 3) and standard deviations are plotted. Different lowercase letters represent significant differences (_p_ < 0.05).  
Trends in nitrate uptake were much more in line with expectations in the tank reactor: a significantly higher proportion of initial nitrate content (\(p<0.001\)) was taken up on the first two days in nitrate-depleted cultures, than in the control (Figure 6b). From the 3 rd day, nitrate uptake trends were similar in the control and 75%-N, and in 50%-N and 25%-N cultures, respectively (Figure 6b). Generally, 95% of nitrate was taken up in all cultures by the end of the experiment.  
In comparing nitrate uptake in the different reactor types of the same treatments, a significantly higher proportion of initial nitrate content was taken up in the tank reactor than in the hybrid reactor (\(p<0.05\)).  
There were no significant differences in phosphate uptake between the cultures exposed to different treatments in the hybrid reactor. It should be noted that only 20% of the initial phosphate content was taken up in the control, and only 10-13% in nitrate-depleted cultures (Figure 6c).  
A significantly higher proportion of the initial phosphate content was taken up in control cultures than in nitrate-depleted ones in the tank reactor between the 2 nd-5 th days of the experiment (\(p<0.001\); Figure 6d). A slow phosphate uptake could be observed in nitrate-depleted cultures in the first four days, an increasing trend occurred afterward with significant differences on the last three days, but there seemed to be no connection between  
Figure 6: Nitrate (**a,b**) and phosphate (**c,d**) uptake in control (C) and nitrogen depleted (75, 50, 25%-N). _Monoraphidium pusillum_ cultures in the hybrid reactor (HR) and in the tank reactor (TR). Means (n = 3) and standard deviations are plotted. Different lowercase letters at curves mean significant differences among the last day values (\(p<0.05\)).  
the extent of phosphate uptake and nitrate depletion (\(p<0.05\); Figure 6d). A 63-90% of the initial phosphate content was taken up by the end of the experiments in the tank reactor.  
Comparing phosphate uptake in the different reactor types of the same treatments, similarly to nitrate uptake, a significantly higher proportion of initial phosphate content was taken up in the tank reactor than in the hybrid reactor (\(p<0.05\)).