Editorial: Plant cells and algae in bioreactors

Atanas Pavlov Five years after the success of the first published Special issue on “Plant cells and algae in bioreactors” [1] it is our pleasure to present the second one, providing recent achievement and the stateof-the-art in this emerging area of bioprocess engineering. During the past five years there was increasing interest in microalgae and plant cells as production systems for pharmaceuticals, foods and food additives, biofuels and more. Despite a significant progress, low yields and the complex scale-up of bioreactor cultivation remain major limitations for industrial implementation of microalgae and plant cell culture. The articles collected in this Special issue represent results of the most recent achievements and critical research focused to find different ways for solving these limitations in yield and bioprocessing conditions. The main focus in current research represented in this Special issue is now the development of integrated approaches for process optimization and improvement of bioreactor designs. Economically effective and sustainable processes, based on microalgae and plant cell, tissue and organ cultures technology, could only be developed considering integration of optimization procedures, such as strain selection, nutrient media and environmental conditions optimization, bioreactor design and downstream processes. A valuable review on integration in microalgal bioprocess development is presented by Fresenwinkel et al. [2]. The authors suggest grouping the steps of microalgae bioprocess development into several degrees of integration with respect to classical upstream, bioreaction and downstream processes [2]. This aspect is also addressed by several authors in this issue for particular production processes based on plant in vitro systems: Jeandet et al. [3] describe resveratrol production at large scale using plant cell suspension; Callego et al. [4] present centoellosides production by plant in vitro systems with different degree of differentiation and also scale-up the processes; Berkov et al. [5] summarize 10 years of research on galanthamine production by shoot type plant in vitro cultures, describing different steps of process optimization, development of suitable bioreactor design and peculiarities in cultivation of that complex in vitro system. Production processes based on photosynthetic cells strongly depend on light as a factor influencing both biomass and product yields. A valuable contribution in this area is presented by Wang et al. [6]. The authors outline the research progress on the improvement of light utilization in microalgae cultivation. This aspect is also addressed by Gutierrez-Wing et al. [7] who review the impact of light irradiance and wavelength on the growth, productivity and composition of microbial photosynthetic cultures. Kula et al. [8] further provide evidence for the influence of far-red light on biomass production of Chlorella vulgaris. An innovative study of cultivation of cells under illumination is presented by Socher et al. [9]. The authors outline the perspectives of a RAMOS cultivation system for investigating the growth of phototrophic organisms under controlled light conditions [9]. Biofuels and alternative energy sources are further important issues addressed by the competence of photosynthetic bioprocess engineering. A valuable contribution in that area is presented by Weber et al. [10]. The authors describe detailed aspects of phototrophic hydrogen production by bacteria, cyanobacteria and algae, highlighted the resent technological and research achievements in this field [10]. Downstream processing, including the extraction of target metabolites from the biomass is the key point in algaebased productions. This aspect of technology is addressed by Michalak and Chojnaska [11], who compare advantages and disadvantages of classical and novel extraction techniques. The authors review the production of algal extracts avoiding utilization of toxic solvents or aggressive extraction conditions [11]. Employment of low cost bioreactor systems is one of the options to reduce the cost of secondary metabolites production by plant in vitro systems. Temporary immersion systems offer such an alternative. Georgiev et al. [12] describe variations in the design and the operation among the most recently developed temporary immersion systems and their potential for large-scale propagation of plantlets, shoots, hairy roots, and production of secondary metabolites. Another alternative to the conventionally used stainless steel bioreactors is application of single used or disposable vessels. Werner et al. [13] study the advantages of these cultivation systems and their application for the needs of plant biotechnology. The authors present the results of their research in mass propagation of Helianthus annuus suspension cells in orbitally shaken bioreactors and clearly demonstrate that these cultivation systems are efficient alternative of wave-mixed and stirred systems [13]. Key points in development of commercial processes, based on plant in vitro systems, is the effective scale-up. In this Special issue Nohynek et al. [14] present for the first time the results of large scale cultivation of cloudberry (Rubus chamaemorus) cells. The described industrial process results in production of cloudberry biomass with consistent quality and defined chemical composition [14]. Studies on product release and in situ extraction of target metabolites is often an underestimated part of integrated approaches for optimization of plant cell based production processes. However, this is an essential step to increase the yields and purity of desired metabolites produced in complex mixtures by the in vitro grown plant cell cultures. To highlight the importance of these technological steps, Su and Wu [15] provide an