Microalgae: A Sustainable Source of Nutrients?

As the food and dietary supplement industries and consumers seek nontraditional sources of nutrition, microalgae have drawn some attention. Green microalgae, for example, represent more than 16,000 diverse species, some of which may provide materials for biofuel and an array of potential nutrients, including polyunsaturated fatty acid and protein. Some of the substances produced by microalgae include a spectrum of carotenoids and a collection of phytosterols (Yao et al. 2015, Le Goff et al. 2019).

Terms such as Crypthecodinium, Arthrospira, Chlorella, Nannochloropsis, and Phaeodactylum may not be familiar, yet these microalgae are just a few genera and their respective variations being studied for their nutrient profiles and potential contributions to human health (Wild et al. 2018). For example, Crypthecodinium cohnii is a rich source of docosahexaenoic acid (DHA) typically found in infant formulas (Wynn et al. 2010). Arthrospira platensis, formerly known as Spirulina, continues to be studied for its potential impact on gene expression that modulates cellular apoptosis related to specific cancerous cell lines (Czerwonka et al. 2018). Chlorella vulgaris has been popularized for its potential beneficial properties due, in part, to its innate substances, such as polysaccharides, glycoproteins, and b-1,2-glucans (Barkia et al. 2019). Nannochloropsis oculata produces eicosapentaenoic acid (EPA) conjugated to phospholipids and glycolipids and no DHA (Kagan et al. 2013). Phaeodactylum tricornutum, which produces the marine carotenoid fucoxanthin, has been implicated in decreasing intracellular lipid contents in a culture of select adipocytes (Koo et al. 2019).

The biomass of some microalgal sources contains 25% to upwards of 70% protein, on a dry weight basis, but the general protein quality of these sources is low, primarily due to low levels of tryptophan and lysine, poor digestibility, and incorrect nitrogen-to-protein conversion factor for this source of protein. The potential nutritional and functional ingredients in different microalgae include a broad range of carotenoids, fatty acids, proteins, vitamins, and phenolic compounds. Some of these substances may upregulate and downregulate numerous genes that impact human health. For example, fucoxanthin extracted from Chaetoceros calcitrans appears to modulate cellular proliferation of HepG2 cells, a human liver cancer cell line, via numerous genes involved in cell signaling, apoptosis, and oxidative stress (Foo et al. 2018).

As with any potential food source, there are innate toxins in microalgae that may contribute to adverse events. Common biogenic toxins in microalgae include algal toxins and nonbiogenic environmental contaminants, such as heavy metals (Matos 2017). Fresh-water and marine microalgae produce toxins that may not be detected by our normal sensory or analytical processes and may not be eliminated through thermal processes or kitchen cooking or inhibited via freezing.

On the other hand, microalgae such as Arthrospira and Chlorella vulgaris do not produce toxins. In fact, many microalgae in the Chlorophyta, Cyanobacteria, Dinophyta, Haptophyta, Heterokontophyta, and Rhodophyta phylla do not produce algal toxins and are considered safe for human consumption.

Considering that there are so many variables among microalgae, one must consider a collection of quality standards for these potential food ingredients. A brief review of standards advanced by the International Union of Pure and Applied Chemistry, World Health Organization, United States, Japan, France, and Brazil indicates some agreement and some inconsistencies.

Microalgae then, seem to carry the promise of renewable, sustainable, and potentially economical sources of biofuels, bioactive medicinal products, and food ingredients. However, the challenges of enhancing growth rate and product synthesis, dewatering algae culture for biomass production, pretreating biomass, and optimizing the fermentation process in case of algal bioethanol production remain significant.

As with any emerging technology, much more research is needed to fully understand their benefits and possible concerns. The overall costs of production, including cultivation systems and maintenance, are not trivial. While open ponds are seen as the most economical choice for large-scale cultivation, they are more prone to contamination. Harvesting and refining the biomass can also be difficult and cost prohibitive. Traditional methods for the extraction of high-value metabolites often rely on the use of organic solvents, and residual solvents in the products can compromise health.

Thus, greener technologies, like supercritical fluid extraction and enzymatic hydrolysis, are presented as follow-on emerging techniques for the isolation of high-value algal products. Despite these challenges, there appears to be an escalating appreciation of the potential of microalgae ingredients and a palpable demand for them.