# Integrated Carbonate-Based CO\({}_{2}\) Capture--Biofixation through Cyanobacteria
## 5 Conclusions  
One of the primary challenges in processes mediated by microorganisms is the requirement to operate in dilute solutions. This necessitates the construction of large-scale plants (incurring high CapEx) and significant energy consumption to manage the substantial liquid volumes.  
Our work has provided robust support for the suitability of Spirulina strains in ICCU systems, as depicted in Figure 1. We achieved high growth rates (P = 0.875 g\({}_{\text{biomass}}\)/L-d, R = 1.41 g\({}_{\text{CO2}}\)/L-d), and a substantial CO\({}_{2}\) utilization efficiency (U of 58%), along with ease  
Figure 6: Schematic of the PBR closed cycle design.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**n.**} & **Incolumn NaHCO\({}_{3}\)** & **P** & **R** & **U** \\  & **(mol/L)** & **(mg\({}_{\text{biomass}}\)/L-d)** & **(mgCO\({}_{2}\)/L-d)** & **(mol\%)** \\ \hline
1 b ** & 0.09 & 760 \(\pm\) 26 & 1320 \(\pm\) 45 & 46 \\
2 & 0.09 & 640 \(\pm\) 23 & 1030 \(\pm\) 39 & 43 \\
3 & 0.36 & 875 \(\pm\) 23 & 1410 \(\pm\) 40 & 58 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Closed cycle growth experiments a.  
of biomass harvesting. Replicating the performance observed in the laboratory-scale PBR, as shown in Figure 2, during scale-up can be challenging, especially for light irradiation. Fortunately, our data indicate that Spr02 can efficiently grow even under relatively low PPFD values.  
Nevertheless, before contemplating scaling up to industrial settings, further research needs to be conducted to reduce process volumes and/or further increase growth rates. Several topics can be suggested as possible advancements:  
* The employment of non-saline carriers in addition to carbonate should address the reduction in process volumes as not linked to an increase in ionic strength. For example, the use of organic amines compatible with algae cultivation can be considered [39].
* The implementation of an oxygen reduction/fixation method, even in the gas phase, could result in a growth boost [18], which would be especially useful in closed-loop systems.
* Our work has shown some advantages in the pre-adaptation of Spr02 strains to 0.36 M bicarbonate. A re-evaluation of the nutrient solution should be performed. For example, it was reported that the addition of small amounts of acetates could favorably improve the growth rate [20].
* Pulsed illumination should also be evaluated [35].  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app131910779/s1](https://www.mdpi.com/article/10.3390/app131910779/s1)]([https://www.mdpi.com/article/10.3390/app131910779/s1](https://www.mdpi.com/article/10.3390/app131910779/s1)), Table S1: Representative elemental analysis of cyanobacteria strains; Figure S1: Optical microscope photographs of cell morphology for (a) Spr01, and (b) Spr02; Figure S2: Schematic representation of the DoE; Figure S3: Pareto charts relative to Spr01 (a) or Spr02 (b) growth experiments; Figure S4: Additional representations of the influence of simultaneous variation of NaHCO\({}_{3}\) concentration and T on P, for Spr02 strain.  
Conceptualization, F.R. and A.U.; methodology, L.F.; software, B.A. and A.U.; validation, V.D., B.A. and F.R.; investigation, A.U.; resources, F.R. and A.L.R.; data curation, V.D. and A.U.; writing--original draft preparation, F.R. and A.U.; writing--review and editing, F.R.; visualization, A.U.; supervision, F.R. and L.F.; funding acquisition, F.R. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by Algae S.p.A. under the PON Ph.D. MUR program (DM 1061, 2021).  
Not applicable.  
The data presented in this study are available within the article itself or in the related supplementary material.  
We thank \"Vetreia Punte Vetro S.n.C.\", Vignola (MO), Italy, for the gifted glass plates, employed for the realization of the PBR of Figure 2.  
The authors declare no conflict of interest.