Characterization of selected microalgae and cyanobacteria as sources of compounds with antioxidant capacity
Introduction
Microalgal biomass has been traditionally used as food and food ingredient due to its considerable amounts of carbohydrates, proteins, lipids, and polyunsaturated fatty acids as well as the presence of pigments, phenolic antioxidants, vitamins, and minerals with potential health supporting effects [1]. The potential of algal biomass has been demonstrated for the production of high-value products, such as biopharmaceuticals, biostimulants as well as biochemicals [2].
Compounds of value can be present at low concentrations in algal biomass. For instance, phenolic compounds with antioxidant activity were found in Chlorella pyrenoidosa and Spirulina platensis at total contents of about 18 and 43 mg gallic acid eq. per gram biomass, respectively [3]. Phenolic compounds can act as antioxidants either through single electron or hydrogen atom transfer [4]. They are hypothesized to be the basis for an antioxidant defense mechanism preventing degenerative diseases, such as cancer, cardiovascular diseases, diabetes, osteoporosis, and neurodegeneration, which are related to an excess of free radicals or reactive oxygen species [5]. With regard to cosmeceutical applications, they can act as skin-whitening agents via oxidative/antioxidant bleaching [6].
Beside antioxidant phenolic compounds, also pigments show a certain antioxidant activity. Carotenoids are a large class of fat-soluble pigments which, because of their conjugated double bonds, can absorb short-wave light, and thus produce a characteristic color tone in plants and animals. It was reported that carotenoids can deactivate singlet oxygen by physical quenching, quench radicals by hydrogen atom transfer, or by accepting electrons from radicals [7]. Carotenoid contents in biomass can range from around 1–30 mg g−1 [8] depending on algal strain and culture conditions [9]. When being used as feed ingredients, accumulation of fat-associated components, such as chlorophylls, carotenoids, and other pigments, in different tissues of the animals is well documented [[10], [11], [12]].
The antioxidant capacity of biomass and extracts is not based on one single compound but on a mixture of different antioxidant aromatic compounds (e.g., phenolic compounds, aromatic amino acids), carotenoids as well as further non-classified compounds [4]. Therefore, even when the quantities of single bioactive compounds are known, the final antioxidant capacity might be higher or lower due to synergistic, additive, or antagonistic effects [13], making an investigation of biomass and obtained extracts necessary. However, the discussion on phenolic compounds in microalgae and cyanobacteria is still very controversial, as those compounds are more exclusively to be found in plants.
For the extraction of valuable compounds from algal biomass, several pretreatments and extraction solvents, depending on the cell wall strength and the components to be extracted, are available. Common methods for cell disruption include mechanical treatment with high pressure or grinding, chemical treatment using bases or acids in combination with heat, or physical treatment, such as the use of microwaves or ultrasound [14]. The most popular methods for cell disruption are mechanical and physical methods, although these can also be cost-intensive [15] and the costs for cell disruption should not exceed the additional profit from the higher yield [16]. Furthermore, when choosing a disruption method, it is important to ensure that the treatment is harsh enough to sufficiently destroy the whole cell wall but at the same time do not affect the valuable ingredients [17].
The aim of this study was to proceed towards a utilization of microalgal and cyanobacterial biomass for the formation of high-value compounds. Biomasses from 13 algal and cyanobacterial strains were extracted and biomasses as well as extracts were characterized for their total phenolic content (TPC), chlorophyll and carotenoids contents and yields, respectively, as well as studied for their antioxidant capacity. Furthermore, treatment of Acutodesmus obliquus biomass by cell disruption prior to extraction was investigated as technique to increase extraction yields. Results of this study do not only introduce to biomass treatment technique resulting in best yields of antioxidant-active compounds but shed light on the capability of algal and cyanobacterial strains belonging to the different divisions: Chlorophyta, Rhodophyta, Cyanophyta, and Diatomae to serve as sources.
Section snippets
Microalgal strains and cultivation
Investigated microalgal strains, sources of purchase, and cultivation conditions are shown in Table 1.
Aqueous extraction
Hot water extraction method was used as described in [6]. Here 7–10% (w/w) dry biomass was dissolved in water and rehydrated for at least 14 h before being boiled at 100 °C for 1 h. After a cooling phase, the extract was separated from the solid by centrifugation at 3000 ×g for 10 min.
Dry mass
Dry mass was determined by drying 3–4 g of the biomass for 50 min at 105 °C using an infrared moisture analyzer
Results and discussion
The results of the analyses of the antioxidant activity of water- and lipid-soluble components as well as total contents of chlorophyll and carotenoids, the TPC in the biomasses and extracts are summarized in Tables 2 and 3, respectively.
Conclusions
This study revealed the potential of the 13 tested microalgal strains as sources of bioactive compounds. However, no superior strain regarding its antioxidant capacity was identified. N. oleobundans, Phormidium sp., and W. murravi can be recommended as strains with highest contents in phenolic compounds. P. ambiquum and Phormidium sp. were the strains with highest ACW values, while N. oleobundans and A. obliquus were those with highest ACL values. Chlorophyll was the pigment present in highest
Statement of informed consent, human/animal rights
No conflicts, informed consent, or human or animal rights are applicable to this study.
CRediT authorship contribution statement
MA: Writing – Original draft, Visualization, Writing – Review and Editing; FS: Investigation, Writing – Original draft; SR: Writing – Review and Editing, Supervision, Resources; EK: Supervision, Resources, Methodology; MS: Supervision; DP: Writing – Original draft, Visualization, Writing – Review and Editing.
Declaration of competing interest
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgements
The authors acknowledge the German Federal Ministry of Economy and Energy for providing funding within the program INNO-KOM-OST, module “Market-oriented research” (MF140026).
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