energies Review Immobilising Microalgae and Cyanobacteria as Biocomposites: New Opportunities to Intensify Algae Biotechnology and Bioprocessing Gary S. Caldwell 1,* , Pichaya In-na 2 , Rachel Hart 1, Elliot Sharp 3, Assia Stefanova 4, Matthew Pickersgill 2,5, Matthew Walker 1, Matthew Unthank 3 , Justin Perry 3 and Jonathan G. M. Lee 2 1 School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK; [email protected] (R.H.); [email protected] (M.W.) 2 School of Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK; [email protected] (P.I.-n.); [email protected] (M.P.); [email protected] (J.G.M.L.) 3 Department of Applied Sciences, Northumbria University, Newcastle upon Tyne NE1 8ST, UK; [email protected] (E.S.); [email protected] (M.U.); [email protected] (J.P.) 4 School of Architecture, Planning & Landscape, Newcastle University, Newcastle upon Tyne NE1 7RU, UK; [email protected] 5 Northumbrian Water Ltd., Bran Sands, Tees Dock Road, Middlesbrough TS6 6UE, UK * Correspondence: [email protected]; Tel.: +44-(0)1912086660 Abstract: There is a groundswell of interest in applying phototrophic microorganisms, specifically microalgae and cyanobacteria, for biotechnology and ecosystem service applications. However, Citation: Caldwell, G.S.; In-na, P.; there are inherent challenges associated with conventional routes to their deployment (using ponds, Hart, R.; Sharp, E.; Stefanova, A.; raceways and photobioreactors) which are synonymous with suspension cultivation techniques. Pickersgill, M.; Walker, M.; Unthank, Cultivation as biofilms partly ameliorates these issues; however, based on the principles of process M.; Perry, J.; Lee, J.G.M. Immobilising intensification, by taking a step beyond biofilms and exploiting nature inspired artificial cell im- Microalgae and Cyanobacteria as mobilisation, new opportunities become available, particularly for applications requiring extensive Biocomposites: New Opportunities to deployment periods (e.g., carbon capture and wastewater bioremediation). We explore the rationale Intensify Algae Biotechnology and for, and approaches to immobilised cultivation, in particular the application of latex-based polymer Bioprocessing. Energies 2021, 14, 2566. https://doi.org/10.3390/en14092566 immobilisation as living biocomposites. We discuss how biocomposites can be optimised at the design stage based on mass transfer limitations. Finally, we predict that biocomposites will have Academic Editor: Jaakko Puhakka a defining role in realising the deployment of metabolically engineered organisms for real world applications that may tip the balance of risk towards their environmental deployment. Received: 1 April 2021 Accepted: 28 April 2021 Keywords: bioreactor; carbon capture; carbon dioxide; eutrophication; immobilization; latex poly- Published: 29 April 2021 mers; process intensification; wastewater Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Two of the three dominant mass microalgae and cyanobacteria (hereon microalgae) cultivation systems (ponds and photobioreactors) focus on maintaining the cells as a colloidal suspension, equivalent to the microalgae living within the planktonic state, i.e., free floating within the water column with minimal physical cell–cell or cell–substratum Copyright: © 2021 by the authors. interactions. The third main cultivation system (biofilm bioreactors) exploits surface Licensee MDPI, Basel, Switzerland. attachment, equivalent to the cells living within the benthic or substratum-associated state, This article is an open access article defined by more or less continuous cell–cell and cell–substratum interactions (Figure1a,b). distributed under the terms and There are pros and cons for each approach, particularly when attempting to culture at conditions of the Creative Commons industrial scale. Ponds are advantageous in terms of their simplicity (both to build and Attribution (CC BY) license (https:// operate) and their low capital costs, but they consume large tracts of land, are inefficient creativecommons.org/licenses/by/ with water use [1] and, if used for remediation services (e.g., wastewater treatment), 4.0/). Energies 2021, 14, 2566. https://doi.org/10.3390/en14092566 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 18 cells, i.e., the active biomass, are subject to washout from the process (hydraulic retention Energies 2021, 14, 2566 time, i.e., how long cultures are retained within the pond, is a critical operational param-2 of 18 eter [2]). Further, their dependence on ambient light and temperature, combined with their vulnerability to contamination from non-target organisms (predators, pathogens, competitors)the cells, i.e., makes the active achieving biomass, consistent are subject performance to washout challenging from the[3–7]. process (hydraulic retentionPhotobioreactors time, i.e., how ameliorate long cultures many are of retained the drawbacks within the of pond,open isponds a critical (reduced operational land andparameter water consumption, [2]). Further, theirimproved dependence control onof ambientculture conditions, light and temperature, and substantially combined re- ducedwith their threat vulnerability from non-target to contamination organisms) fromculminating non-target in greater organisms biomass (predators, yield [8,9]; pathogens, how- ever,competitors) these gains makes come achieving at the cost consistent of higher performance capital and operating challenging costs [3– 7[10].]. FigureFigure 1. 1. ComparingComparing cell cell distribution distribution and approachesapproaches for for cell cell retention retention in in mass mass microalgae microalgae cultivation: cultiva- tion:(a) Schematic (a) Schematic of suspension of suspension culture—typical culture—typical of ponds, of ponds, raceways raceways and photobioreactors. and photobioreactors. The cells The are cellsfree are floating free floating in the growth in the medium,growth medium, maintaining maintaining spatial separation spatial separation through electrochemicalthrough electrochemical repulsion repulsionwhich limits which cell limits density; cell density; (b) artificially (b) artificially illuminated illuminated tubular tubular photobioreactor photobioreactor using suspensionusing sus- pensioncultivation cultivation for wastewater for wastewater treatment; treatment; (c) open (c) biofilmopen biofilm culture—cells culture—cells attach attach to a to substratum a substratum and andbiofilm biofilm cohesion cohesion is maintained is maintained through through the naturalthe natural production production of extracellular of extracellular polymeric polymeric substances sub- stances (EPS). Biofilms are prone to failure leading to biomass loss; (d) cyanobacteria biofilm within (EPS). Biofilms are prone to failure leading to biomass loss; (d) cyanobacteria biofilm within a small a small raceway used for wastewater remediation; (e) encapsulated biofilms—cells are embedded raceway used for wastewater remediation; (e) encapsulated biofilms—cells are embedded within an within an artificial EPS, typically a hydrogel. Encapsulated biofilms are vulnerable to failure from desiccationartificial EPS, of typicallythe hydrogel a hydrogel. and subsequent Encapsulated loss biofilms of integrity; are vulnerable (f) microalgae to failure encapsulated from desiccation within of kappa-carrageenan.the hydrogel and subsequent Inset shows loss the of integrity;chlorophyll (f) microalgaefluorescence encapsulated of the cells using within imaging kappa-carrageenan. pulse am- plitudeInset shows modulated the chlorophyll fluorometry; fluorescence (g) latex biocomposites—cells of the cells using imaging are immobilised pulse amplitude within modulated materials otherfluorometry; than hydrogels, (g) latex such biocomposites—cells as latex; and (h) cyanobacteria are immobilised in suspension within materials as a biocoating other than in hydrogels,wet latex (uppersuch as left), latex; the and latex (h) without cyanobacteria cells is in shown suspension for comparison as a biocoating (upper in wetright). latex The (upper biocoating left), thewill latex be appliedwithout to cells a loofah is shown sponge for scaffold comparison (lower (upper left is right). uncoated, The biocoatinglower right will is coated). be applied Biocomposites to a loofah havesponge longer scaffold service (lower lives left that is other uncoated, immobilisation lower right systems is coated). and, Biocomposites depending on have the longernature serviceof the lives that other immobilisation systems and, depending on the nature of the binder, can deliver orders of magnitude improvements in performance compared with the other cultivation systems. Energies 2021, 14, 2566 3 of 18 Photobioreactors ameliorate many of the drawbacks of open ponds (reduced land and water consumption, improved control of culture conditions, and substantially reduced threat from non-target organisms) culminating in greater biomass yield [8,9]; however, these gains come at the cost of higher capital and operating costs [10]. Both approaches share some drawbacks, such as challenges around maximising carbon dioxide (CO2) mass transfer [11]. However, other shortcomings are more explicitly linked with the colloidal suspension, notably limitations in cell density and difficulty in harvesting. The planktonic