Short communicationRelease of hydro-soluble microalgal proteins using mechanical and chemical treatments
Introduction
In the 9th century AD the Kanem Empire in Chad discovered the benefits of the cyanobacterium Arthrospira platensis and used it as food (called dihé) for human consumption [1]. Later on in the 14th century AD, the Aztecs harvested the same species from Lake Texcoco and used it to make a sort of cake called tecuilatl. They also used these microorganisms as fodder, fertilisers and remedies. Nowadays, additional species are being industrially and profitably marketed worldwide for the same purposes.
The microalgal industry has grown rapidly over the last decade. Primarily, this is due to the capacity of these micro-organisms to produce lipids suitable for the biodiesel industry, and to grow in a wide variety of geographical and environmental locations, thus precluding competition with arable lands as well as intensive deforestation. Therefore, the major part of microalgal studies has concentrated on enhancing this bioenergy production to the detriment of other high-value biomolecules, but forgetting ancient history and the other advantages of these species.
Today the microalgal bioenergy industry is struggling to find a place in the market due to its uncompetitive cost and its overall unsustainable production [2], [3], [4], [5], [6] sometimes leaving negative footprints on the environment, and public opinion.
Microalgae were originally considered as an important source of protein, a major fraction of their composition; on a dry weight basis the Cyanobacterium Arthrospira platensis is composed of 50–70% proteins [7], [8], the Chlorophycea Chlorella vulgaris 38–58% [9], [10], [11], the Eustigmatophyceae Nannochloropsis oculata 22–37% [12], the Chlorophyceae Haematococcus pluvialis 45–50% [7], and the Rhodophyta Porphyridium cruentum 8–56% protein [13], [14]. They have a profile composed of a set of essential and non essential amino acids [10], with relatively similar ratios between species and generally unaffected by growth phase and light conditions [1]. To the best of our knowledge, studies on microalgal proteins have generally either concentrated on finding and proposing the nitrogen to protein conversion factor [10], [15], [16], [17], [18], in order to avoid incorrect estimations of microalgal total protein content, or focused on determining the best method for protein quantification using colorimetric techniques [19], [20], [21]. However, for some species such as the green microalgae C. vulgaris, N. oculata and H. pluvialis, maximising the recovery of proteins requires a unit cell disruption operation to overcome the barrier of their rigid cell wall and release the intracellular biomolecules. Thus, many cell disruption methods were used to break the cell wall of these microalgae, such as bead milling, ultrasonication, microwaves, enzymatic treatment and high-pressure homogenization [22], [23], [24], [25], [26]. Conversely, fragile cell walled microalgae such as P. cruentum and A. platensis require milder techniques to enhance recovery.
The main objective of this study is to evaluate the effect of two different cell disruption techniques on aqueous phase protein extractability, in five microalgae with different cell wall characteristics, while simultaneously evaluating and comparing the profile of amino-acids subsequent to these two cell disruption methods.
Section snippets
Microalgae
The selected microalgae were supplied as frozen paste from Alpha Biotech (Asserac, France): the Cyanobacteria Arthrospira platensis (strain PCC 8005), two different Chlorophyceae Chlorella vulgaris (strain SAG 211-19), and Haematococcus pluvialis (unknown strain), one Rhodophyta Porphyridium centum (strain UTEX 161), and the Eustigmatophyceae Nannochloropsis oculata (unknown strain).
Each microalga was cultivated on a different culture media; Hemerick media was used for P. cruentum, Sueoka media
Results
The total protein content of crude microalgae was determined from the value of total nitrogen obtained through elemental analysis, and the conversion factor found for each crude microalga in a separate study (Safi et al. 2012b). In all cases, the total protein content was high and consistent with literature values, ranging from 49 to 58% dry weight (Table 1). The fraction of hydro-soluble proteins released into water after both cell disruption techniques is presented in Fig. 1, after
Discussion
This study used two different cell wall treatments on five different microalgae followed by quantification of the proteins [27] released in the aqueous phase, and then assessed the amino acid profile of these proteins for each treatment. The characteristics of the microalgal cell walls play an important role in the release of these biomolecules. Nonetheless, regardless of cell wall characteristics we have shown that at the 95% confidence level using three replicates for each microalga, all the
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