# Microalgae Water Bioremediation: Trends and Hot Topics
## 1 Introduction
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Microalgae Cultivation** & **Description** \\ \hline
**System** & \\ \hline \multirow{6}{*}{Open Culture System} & Ponds, tanks, lakes and raceway ponds are examples of open culture systems. These outdoor systems were the first and are the most used for large-scale production of microalgae biomass due to their simple construction, easy operation and low energy demands [32]. However, open culture systems not only are affected by climate conditions and exterior contamination as they also show low productivity and loss of nutrients by evaporation [33]. \\ \hline \multirow{6}{*}{Closed Culture System} & Closed systems are usually denominated by photobioreactors and are characterized by not allowing exchanges between the microalgae culture and the external environment [20], presenting different designs, such as tubular, plastic bags, flat-plate and bubble-column [34]. Closed-culture systems overcome some of the challenges faced by open systems, such as higher productivity, less nutrient evaporation and contamination [35]. Nevertheless, these systems present a high energy demand, being their implementation expensive [34]. \\ \hline
**Method** & \\ \hline \multirow{6}{*}{Co-Culture Method} & Microalgae exist in nature as a part of a community, benefiting from the interaction among microorganisms [36]. In a co-culture method, more than one species is grown in the same medium, so it should be taken into account that the species selected have similar growth requirements [37]. Nowadays, co-culture of microalgae with yeast or bacteria has shown potential to enhance the phycoremmediation and biomass yield [26]. In this cultivation method, microalgae synthesize higher contents of exopolysaccharides to support the growth in hostile conditions [38]. \\ \hline \multirow{6}{*}{Immobilized Method} & When compared with microalgae free-cell cultivation, the immobilization process can overcome the biomass-harvesting challenge, ensuring that metabolic substances can diffuse through the polymer gel matrix [39]. Nevertheless, the costs of the polymer and the immobilization process, accumulation of metabolic products and the efficiency of the bioremediation are some of the drawbacks of this cultivation method [6]. Still, previous studies show the potential of immobilized microalgae in wastewater bioremediation and metal recovery [40]. \\ \hline
**Strategy** & \\ \hline \multirow{6}{*}{Batch} & This is a simple closed system and low-cost strategy, since it does not require much control. This strategy is characterized by no renewal of culture medium, so the microalgae culture grows until it reaches the decline phase [20]. In practice, the culture could crash for several reasons such as nutrient or oxygen depletion, self-shading, pH variations or contamination [41]. \\ \hline \multirow{6}{*}{Semi-Continuous} & This strategy is like a batch system but, in this case, the culture medium is renewed periodically, while effluent is removed at the same time [20]. \\ \cline{1-1}  & Nevertheless, this strategy could not be appropriate for microalgae cultivation because light is a limiting-factor that affects biomass productivity [42]. \\ \hline \multirow{6}{*}{Continuous} & A continuous strategy that consists in the constant renewal of growth medium, in which the volume of culture medium that is supplemented is the same as the volume of culture that is removed [20]. The advantage of continuous renewal of medium is the achievement of high biomass productivity [43]. Also, it is easy to scale-up for industrial microalgae production due to the simple operation of the cultivation system [44]. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of different systems, methods and strategies of microalgae cultivation commonly used. up-scale production, so the technology is still being developed in terms of research and development [46]. Current research lines focus on four strategic alignments: feedstock, process, products, and market uptake [30; 47].