Received: 14 December 2020 Accepted: 15 March 2021 DOI: 10.1111/ppl.13405 SPECIAL ISSUE ARTICLE Physiologia Plantarum The cell wall of green microalgae and its role in heavy metal removal Olivia Spain | Martin Plöhn | Christiane Funk Department of Chemistry, Umeå University, Umeå Abstract Heavy metals in industrial wastewaters are posing a serious threat to the environ- Correspondence Christiane Funk, Department of Chemistry, ment and to human health. Microalgae are increasingly being seen as potential solu- Umeå University, 901 87 Umeå, Sweden. tions to this problem as they can remove pollutants through biosorption. This Email: [email protected] process offers certain advantages over other more traditional metal removal tech- Funding information niques as it is simple, inexpensive, eco-friendly, and can be performed over a wide Energimyndigheten, Grant/Award Number: 2018-017772; NordForsk, Grant/Award range of experimental conditions. Biosorption is possible due to the unique and com- Number: 82845; Svenska Forskningsrådet plex structure of the microalgal cell wall. The variety of functional groups on the sur- Formas, Grant/Award Number: 2019-00492; Umeå Universitet; Vinnova, Grant/Award face of the cell wall (such as carboxyl or amino groups) can act as binding sites for the Number: 2017-03301 heavy metals, thus removing them from the environment. This review focuses on the Edited by P.-E. Jensen cell wall composition and structure of the most commonly used microalgae in heavy metal removal and shows the role of their cell wall in the biosorption process. This review also aims to report the most commonly used models to predict the velocity of microalgal biosorption and the removal capacities. 1 | INTRODUCTION can be seen either as an obstacle (e.g., for extraction of valuable com- pounds from within the cells) or as an opportunity (e.g., for bio- Microalgae are a diverse group of unicellular and simple multicellular sorption of heavy metals or the production of carbohydrates). A photosynthetic microorganisms that are present in all existing ecosys- precise and in-depth knowledge of the cell wall properties is therefore tems on Earth (Mata et al., 2010). Due to their very limited growth required to optimize microalgal industrial processes and thus to requirements and high adaptability, they can be grown in an extensive reduce their overall costs. However, surprisingly little is known about variety of environmental conditions and do not require fertile land algal cell wall properties, probably because their composition, struc- (Benedetti et al., 2018). Microalgae only need sunlight, simple nutri- ture, and thickness depend greatly on the strain, growth phase and ents including nitrogen, sulfur, phosphorous, and carbon dioxide environmental conditions in which the algae are grown. Due to its (Pignolet et al., 2013) and can complete a full growth cycle within high costs and energy requirements, cell wall disruption is considered hours. Their inexpensive growth requirements as well as their advan- to be one of the most challenging bottlenecks in algal industrial appli- tage of being utilized simultaneously for multiple technologies cations. Gaining knowledge of the composition of algal cell walls (e.g., carbon mitigation, biofuel production, and bioremediation) have would enable the enhancement and/or design of sustainable disrup- made microalgae more and more popular for various biotechnological tion techniques and a better understanding of processes which applications (Suresh Kumar et al., 2015) (Figure 1). The algal cell wall involve the cell wall such as heavy metal removal. has important implications in these biotechnological applications, and Heavy metals are natural elements that can be found in every ecosystem on Earth. Even though they are essential for basic bio- Olivia Spain and Martin Plöhn contributed equally to this study. chemical and physiological functions in both the animal and plant This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2021 The Authors. Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society. Physiologia Plantarum. 2021;1–10. wileyonlinelibrary.com/journal/ppl 1 2 SPAIN ET AL. Physiologia Plantarum FIGURE 1 Schematic view of the product value chain of microalgae kingdoms, some heavy metals can be toxic to living cells. In certain applications such as heavy metal removal. In this review, we mainly concentrations and environmental conditions, metal ions can damage focus on the strains that have been reported in both cell wall studies DNA, causing conformational modifications that can then lead to car- and heavy metal removal studies. cinogenesis or cell death (Tchounwou et al., 2012). Due to anthropo- Chlorella and Scenedesmus are reported to be two of the most genic activities, the quantity of heavy metals in the environment has widely used algae for heavy metal removal, due to their high bio- significantly risen over the past decades. This increase in concentra- sorption capacities (Dwivedi, 2012). The genus Chlorella includes tion has inevitably led to an increase of our exposure to the metals single-celled, spherical green microalgae of about 2–10 μm in diame- and by consequence, an increase in heavy-metal related diseases. The ter. Chlorella is currently the most cultivated microalga worldwide, metals that are currently considered to be the most problematic are mainly due to its rapid growth rate, high photosynthetic efficiency copper (Cu), cadmium (Cd), chromium (Cr), magnesium (Mn), iron (Fe), and high nutritional value (Masojídek & Torzillo, 2008). Chlorella cells zinc (Zn), mercury (Hg), and lead (Pb) as these metals are toxic even in can contain up to 70% of protein (in dry weight), making the biomass low concentrations (Javanbakht et al., 2014). Industrial wastewaters very valuable to the food industry (Liu & Hu, 2013). Chlorella vulgaris from mining, agriculture, battery manufacturing etc. are often highly is one of the most commonly reported Chlorella species for heavy polluted with heavy metals. Conventional adsorbents (e.g., activated metal removal. carbons, zeolites, clays), nanostructures (reviewed in [Burakov The genus Scenedesmus includes the species Scenedesmus, et al., 2018]), as well as biosorbents (plant material, fungi) are used to Desmodesmus, and Acutodesmus, colonial green microalgae, which fre- remove heavy metals and other contaminants from the effluents quently exist in coenobia of four or eight cells inside a mother wall. (reviewed in Ajiboye et al., 2021). Found across the world, Scenedesmus is one of the most common In this review we will show that, due to the biochemical proper- freshwater algae and is commonly farmed for applications such as oil ties of microalgal cell walls, biosorption via microalgae has the poten- production or waste-water treatment (Pignolet et al., 2013). The pres- tial to offer a new eco-friendly, efficient, and cost-effective solution ence of a very thick cell wall makes Scenedesmus species very resistant to remove heavy metals from wastewater. As it is estimated that there to digestion and predation. are more than 50 000 species of microalgae (Richmond, 2003), each This review will also mention Nannochloropsis and Neochloris spe- with different cell wall structures and compositions, this review will cies, genera of unicellular green microalgae, as their cell wall struc- focus on a selected few industrially valuable and well-reported strains tures are well reported. Both Nannochloropsis and Neochloris are of Chlorophyceae. efficient lipid producers, making them extremely interesting for bio- fuel production (Rashidi & Trindade, 2018; Scholz et al., 2014). Certain species of Nannochloropsis have also been shown to be very effective 2 | CURRENT KNOWLEDGE OF CELL WALL biosorbents of heavy metals (Kaparapu & Krishna Prasad, 2018; STRUCTURE AND COMPOSITION OF GREEN Sjahrul, 2013). MICROALGAE 2.1 | Most frequently used strains for heavy metal 2.2 | Cell wall architecture removal As the interface between the cell cytoplasm and the outside environ- The cell wall composition of microalgae has only been studied for a ment, the cell wall is the first barrier between the cell and heavy very limited number of strains (Table 1). These strains do not always metals. The cell wall is mainly composed of carbohydrates, proteins, coincide with those that are currently used in biotechnological and lipids, who each offer negatively charged functional groups at SPAIN ET AL . TABLE 1 The cell wall structure and composition of different microalgal strains Genus Strain Cell wall structure (layers) Cell wall composition References Chlorella Chlorella vulgaris One phospholipidic inner layer Glucosamine-rigid wall (composed of galactose and rhamnose) Takeda (1991), Abo-Shady et al. (1993) 30% polysaccharides, 2.46% proteins, 15% lipids, 52.54% unknown substances Chlorella zofingiensis Inner layer and trilaminar 70% glucose and 30% mannose in its “rigid cell wall” and 65% Rodrigues and Da Silva Bon (2011) outer layer mannose, 30% glucose, plus minor amounts of rhamnose and galactose in its matrix cell wall Chlorella homosphaera 85% glucose 15% mannose in its “rigid cell wall” and 70% mannose, 20% glucose, and 10% galactose in its matrix cell wall Chlorella fusca Inner layer and