marine drugs Review Microalgal Enzymes with Biotechnological Applications Giorgio Maria Vingiani 1, Pasquale De Luca 2 , Adrianna Ianora 1, Alan D.W. Dobson 3,4 and Chiara Lauritano 1,* 1 Marine Biotechnology Department, Stazione Zoologica Anton Dohrn, CAP80121 (NA) Villa Comunale, Italy 2 Research Infrastructure for Marine Biological Resources Department, Stazione Zoologica Anton Dohrn, CAP80121 (NA) Villa Comunale, Italy 3 School of Microbiology, University College Cork, College Road, T12 YN60 Cork, Ireland 4 Environmental Research Institute, University College Cork, Lee Road, T23XE10 Cork, Ireland * Correspondence: [email protected]; Tel.: +39-081-5833221 Received: 4 July 2019; Accepted: 1 August 2019; Published: 5 August 2019 Abstract: Enzymes are essential components of biological reactions and play important roles in the scaling and optimization of many industrial processes. Due to the growing commercial demand for new and more efficient enzymes to help further optimize these processes, many studies are now focusing their attention on more renewable and environmentally sustainable sources for the production of these enzymes. Microalgae are very promising from this perspective since they can be cultivated in photobioreactors, allowing the production of high biomass levels in a cost-efficient manner. This is reflected in the increased number of publications in this area, especially in the use of microalgae as a source of novel enzymes. In particular, various microalgal enzymes with different industrial applications (e.g., lipids and biofuel production, healthcare, and bioremediation) have been studied to date, and the modification of enzymatic sequences involved in lipid and carotenoid production has resulted in promising results. However, the entire biosynthetic pathways/systems leading to synthesis of potentially important bioactive compounds have in many cases yet to be fully characterized (e.g., for the synthesis of polyketides). Nonetheless, with recent advances in microalgal genomics and transcriptomic approaches, it is becoming easier to identify sequences encoding targeted enzymes, increasing the likelihood of the identification, heterologous expression, and characterization of these enzymes of interest. This review provides an overview of the state of the art in marine and freshwater microalgal enzymes with potential biotechnological applications and provides future perspectives for this field. Keywords: microalgae; enzymes; marine biotechnology; -omics technologies; heterologous expression; homologous expression 1. Introduction Water covers around 71% of the Earth’s surface, with salt water responsible for 96.5% of this percentage [1]. Due to its molecular structure and chemical properties, water includes (and often participates in) every chemical reaction that is biologically relevant [2]. In such reactions, enzymes cover a fundamental role: They are organic macromolecules that catalyze biological reactions (so-called “biocatalysts” [3]). Due to their substrate-specificity, enzymes are commonly used in several sectors (such as food processing, detergent, pharmaceuticals, biofuel, and paper production) to improve, scale, and optimize industrial production. For example, hydrolases, which are enzymes that catalyze the hydrolysis of chemical bonds, have applications in several fields. Examples of industrially relevant hydrolases are cellulases for biofuel production [4], amylases for syrup production [5], papain, phytases and galactosidases for food processing [6], and other hydrolases which have various pharmaceutical applications [7]. The demand for new enzymes is growing every year, and many financial reports Mar. Drugs 2019, 17, 459; doi:10.3390/md17080459 www.mdpi.com/journal/marinedrugs Mar. Drugs 2019, 17, x 2 of 20 industrially relevant hydrolases are cellulases for biofuel production [4], amylases for syrup production [5], papain, phytases and galactosidases for food processing [6], and other hydrolases Mar.which Drugs have2019, 17various, 459 pharmaceutical applications [7]. The demand for new enzymes is growing2 of 20 every year, and many financial reports expect the global enzyme market value to surpass the $10 billion mark by 2024 (Allied Market Research, 2018, expecthttps://www.alliedmarke the global enzymetresearch.com/enzymes-market;R market value to surpass the $10esearchandMarket.com, billion mark by 2024 (Allied2018, Market Research,https://www.researchandmarkets.com/research/6zpvw9 2018, https://www.alliedmarketresearch.com/industrial?w=4),/enzymes-market;ResearchandMarket.com of which $7 billion alone , 2018,will https://bewww.researchandmarkets.com for industrial /researchapplications/6zpvw9 /industrial?w(BCC =Research,4), of which $72018, billion alonehttps://www.bccresearch.com/mark will be for industrial applicationset-research/biotechnology/global (BCC Research, 2018, https:-markets-for-enzymes-in-indus//www.bccresearch.com/market- researchtrial-applications.html)./biotechnology/global-markets-for-enzymes-in-industrial-applications.html ). MicroalgaeMicroalgae areare photosynthetic photosynthetic unicellular unicellular organisms organisms that can that be can massively be massively cultivated cultivated under undercontrolled controlled conditions conditions in photobioreactors in photobioreactors with withrelatively relatively small small quantities quantities of ofmicro- micro- and and macro-nutrientsmacro-nutrients [8[8],], andand cancan thus fit fit perfectly perfectly into into this this market market sector. sector. Microalgae Microalgae continue continue to be to beused used in in a a number number of of biotechnological biotechnological applications applications.. 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Considering interval between the full “1999–2018”, 20-year interval it is clear between that as of“1999–2018”, 2012, there has it is been clear a rapidthat as increase of 2012, inthere the has number been ofa rapid publications increase involving in the number both “microalgae”of publications and involving both “microalgae” and “biotechnology”, reaching a peak in the years 2015–2016. “biotechnology”, reaching a peak in the years 2015–2016. FigureFigure 1. 1.Microalgae Microalgae Biotechnology Biotechnology PubMed PubMed Search Search Results Results 1999–2018. 1999–2018. 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The literature regarding the biotechnological applications of microalgae is dominated by four main researchThe literature sectors: regarding (1) Direct the use biotechnological of microalgal cells,applications for bioremediation of microalgae applications is dominated and by asfour food supplementsmain research [9]; sectors: (2) Extraction (1) Direct of bioactivesuse of microalgal for different cells, applicationsfor bioremediation (e.g., cosmeceutical, applications and nutraceutical, as food andsupplements pharmaceutical [9]; (2) applications, Extraction andof bioactives for biofuel for production different applications [10,11]); (3) (e.g., Use ofcosmeceutical, microalgae as platformsnutraceutical, for heterologous and pharmaceutical expression applications, or endogenous and gene for editingbiofuel andproduction overexpression [10,11]); [12(3)]; Use (4) Use of of microalgaemicroalgae as sourcesas platforms of enzymes for heterologous for industrial applicationsexpression or [13 ].endogenous The latter fieldgene appears editing to beand less well-studiedoverexpression compared [12]; (4) to Use the of others, microalgae due to as the sour highces costs of enzymes currently for involved industrial in applications enzyme extraction [13]. The latter field appears to be less well-studied compared to the others, due to the high costs and characterization, as well as the scarcity of annotated microalgal genomes. currently involved in enzyme extraction and characterization, as well as the scarcity of annotated Recent projects, such as those funded under the European Union Seventh Framework 2007–2017 microalgal genomes. (EU FP-7), e.g., BIOFAT (https://cordis.europa.eu/project/rcn/100477/factsheet/en) and GIAVAP (https: //cordis.europa.eu/project/rcn/97420/factsheet/en), together with Horizon 2020 programs, e.g., ALGAE4A-B (http://www.algae4ab.eu/project.html) and VALUEMAG (https://www.valuemag.eu/), have resulted in an increase in –omics data (i.e., genomics, transcriptomics, proteomics and metabolomics data) available for microalgae,