Short communicationAqueous extraction of proteins from microalgae: Effect of different cell disruption methods
Introduction
Microalgae were first exploited for their capacity to accumulate proteins and, through time, interest in this biomass took a new course especially during the last two decades with increasing demand for sustainable energy. This biomass proved to be an important source of lipids suitable for biodiesel production. Hence, many of the studies were concentrated on lipid extraction for fuel purposes, neglecting the potential of microalgae to produce proteins and other high-value components [1]. However, until now all studies and estimates confirmed that costs of production of biodiesel from microalgae remain high [2], [3] and far from being competitive with fossil fuel. Researchers are therefore turning towards valuing other components present in the microalgae such as proteins, pigments, dyes, sugars, etc.
Extracting the totality of a specific component from microalgae is often prevented by the intrinsic rigidity of its cell wall. To overcome this barrier, an initial operation unit of cell disruption is required to permit complete access to the internal components and facilitate the extraction process. Hence, many cell disruption techniques have been tested to break the cell wall of microalgae such as bead milling [4], [5], ultrasonication [6], [7], [8], microwave radiation [9], enzymatic treatment [10], [11], cell homogenizer [12] and high-pressure cell disruption [13] to recover different components. The efficiency of cell disruption was usually evaluated by extracting a single component especially lipids before and after applying the treatment or by microscopic observation. To our knowledge, studies of microalgal proteins have been focused on: evaluating the nitrogen to protein conversion factor [14], [15], [16], [17], [18]; finding the best method to analyze proteins and differentiate between soluble and non-soluble proteins [19]; and analyzing the behavior of proteins at the air/water interface [20].
Therefore, the present study focuses on evaluating the effect of different cell disruption techniques on protein extractability in water of five different microalgae having different cell wall macrostructures. Namely, the Cyanobacterium Arthrospira platensis, which has a relatively fragile cell wall, composed mainly of murein and no cellulose [21], [22]. The Chlorophycean Chlorella vulgaris and the Eustigmatophyceae Nannochloropsis oculata, which have a cell wall mainly composed of cellulose and hemicelluloses [23]. Another Chlorophycean Haematococcus pluvialis has a thick trilaminar cell wall composed of cellulose and sporopollenin [12], [24], [25]. The composition of its cell wall, similar to that of spores, makes this microalga less permeable and extremely resistant to mechanical treatments [26]. Finally, the Rodophythe Porphyridium cruentum, which lacks a true cell wall, but instead is encapsulated by a layer of sulfurized polysaccharides [27], [28], [29], [30], [31], [32].
In addition, the microalgae selected in this study have a cytoplasm containing soluble proteins, and they all have a chloroplast except for A. platensis, which instead has thylakoids bundles circling the peripheral part of the cytoplasm with their associated structures, the phycobilisomes (containing the phycobiliproteins) present on the surface of the thylakoids like in the chloroplast of P. cruentum [21]. Furthermore, the chloroplast also contains soluble proteins and a central pyrenoid, which is a non-membrane, bound organelle composed of RuBisCO.
In this study, proteins released in the aqueous media were evaluated and discussed considering the cell wall macrostructure of each microalga along with the effect of each cell disruption technique used.
Section snippets
Microalgae
The microalgae selected are the Cyanobacteria Arthrospira platensis (strain PCC 8005), two different Chlorophyceaen Chlorella vulgaris (strain SAG 211–19), and Haematococcus pluvialis (unknown strain), one Rhodophyta Porphyridium cruentum (strain UTEX 161), and the Eustigmatophyceae Nannochloropsis oculata (unknown strain).
Each microalga was cultivated in a different culture medium. Hemerick medium was used for P. cruentum, Sueoka medium for C. vulgaris, Basal medium for H. pluvialis, Conway
Results
The total protein in crude microalgae was determined by obtaining total nitrogen through elemental analysis and converting it into protein percentage using the conversion factor found for each crude microalga in the study conducted by Safi et al. [18]. In all cases, the total protein content was high, ranging from 46 to 57% dw (Table 1).
The fraction of soluble proteins released in water after each cell disruption technique is presented in Fig. 1. The fraction of soluble protein in the total
Discussion
The goal of the present study was to highlight the release of protein into aqueous media after the application of different cell disruption techniques. The results do not only rely on the mechanical rigidity of the cell wall of each microalga but also on its chemical characteristics. Indeed, having a deep understanding of the macrostructure is necessary in order to evaluate the release of components after any treatment was conducted on the cells. This approach has been considered in a study
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