A. Catarina Guedes

Microalgae as sources of high added-value compounds—a brief review of recent work

Microalgae have found commercial applications as natural sources of valuable macromolecules, including carotenoids, long‐chain polyunsaturated fatty acids, and phycocolloids. As photoautotrophs, their simple growth requirements make them attractive for bioprocesses aimed at producing high added‐value compounds that are in large demand by the pharmaceutical market. A few compounds synthesized by microalgae have indeed proven to possess anti‐inflammatory, antiviral, antimicrobial, and antitumoral features; astaxanthin, a known antioxidant produced by Haematococcus pluvialis, is an illustrative example with important anti‐inflammatory and antitumoral roles. From a chemical standpoint, several such compounds are polysaccharides or long chain fatty acids, where the latter can be either saturated or unsaturated. Additionally, their chemical structures are often atypical, whereas their concentrations can exceed those found in many other natural sources. The productivity and biochemical composition of microalgae depend strongly on the mode of cultivation, medium composition, and nutrient profile. Consequently, numerous efforts aimed at elucidating the practical impacts of the aforementioned parameters have been developed. This review accordingly covers the knowledge produced in the last two decades on the uses of microalgae to obtain physiologically active compounds, and on the optimization of the underlying production and purification processes. It also identifies major gaps and opportunities in this field that should be addressed or exploited in the near future.

Nutritional Value and Uses of Microalgae in Aquaculture

Microalgae play indeed a crucial nutritional role with regard to marine animals in the open sea, and consequently in aquaculture. Most marine invertebrates depend on microalgae for their whole life cycle, so commercial and experimental mollusc or fish hatcheries have included a microalga production system in parallel to their animal production itself. Microalgae are utilized as live feed for all growth stages of bivalve molluscs (e.g. oysters, scallops, clams and mussels), for larval/early juvenile stages of abalone, crustaceans and some fish species, and for zooplankton used in aquaculture food webs at large. It should be emphasized that the productivity of any hatchery is directly related to the quantity and quality of the food source used therein.

Effects of temperature and pH on growth and antioxidant content of the microalga Scenedesmus obliquus

Reactive forms of oxygen can damage DNA (among other molecules), thus triggering, e.g., atherogenesis and carcinogenesis. However, such dietary antioxidants as lutein and β‐carotene can effectively inactivate them; these compounds were found to high levels in a novel strain (M2‐1) of the microalga Scenedesmus obliquus. The independent and combined effects of pH and temperature on its rates of growth and production of antioxidants were experimentally assessed, via a full factorial experimental design; the effects of each parameter independently, and of their interactions were accordingly quantified by ANOVA. Our results indicated that temperature plays a more important role on the maximum specific growth rate than pH; in terms of antioxidant content, pH and, to a lesser extent, temperature also have relevant effects. Consequently, the highest rate of biomass specific growth (0.294 ± 0.013 day−1) and biomass productivity (0.837 ± 0.054 mg L−1 day−1) were associated with relatively low pH (6) and relatively high temperature (30°C). Conversely, the antioxidant production rate increased with pH; hence, the highest productivity (0.638 mg L−1 day−1) was attained at pH 8 and 30°C. At the best operating conditions for antioxidant content, the levels of lutein and β‐carotene were 203.57 ± 1.41 and 18.20 ± 0.33 mg mL−1, respectively; the maximum production of either one occurred at the early exponential phase.

Microalgal compounds modulate carcinogenesis in the gastrointestinal tract

Gastrointestinal cancers rank second in overall cancer-related deaths. Carotenoids, sulfated polysaccharides, and polyunsaturated fatty acids (PUFAs) from microalgae exhibit cancer chemopreventive features at different stages of carcinogenesis. For instance, sulfated polysaccharides bear a prophylactic potential via blocking adhesion of pathogens to the gastric surface, whereas carotenoids are effective against Helicobacter pylori infection. This effect is notable because H. pylori has been targeted as the primary cause of gastric cancer. Recent results on antitumor and antibacterial compounds synthesized by microalgae are reviewed here, with an emphasis on their impact upon H. pylori infection and derived pathologies accompanying the progression of gastric carcinogenesis.

Optimization of ABTS radical cation assay specifically for determination of antioxidant capacity of intracellular extracts of microalgae and cyanobacteria.

A renewed interest in antioxidants has arisen in recent years; microalgae and cyanobacteria are potential sources thereof for use as food/feed ingredients. However, improved methods for comprehensive screening of antioxidant capacity specifically in intracellular extracts of marine microorganisms are required - encompassing lipophilic and hydrophilic compounds simultaneously. The original ABTS method was thus improved, and in particular the procedures of cell disruption and storage were optimized. The best solvent found was ethanol/water (1:1, v/v). The reaction to form ABTS(+) in said solvent was essentially complete by eight hours, and this radical cation was stable for at least 6 days; at room temperature, the ABTS(+) solution remained within an allowable analytical range for up to 13 h. Ultra Turrax was the best cell disruption method, and refrigeration was the best preservation method. This improved methodology was validated with four representative strains that respond poorly to cell disruption.

Applications of Spent Biomass

Abstract There are several difficulties in developing algal biofuels that can fully replace fossil fuels; the main challenges relate to lower environmental impact (including the possibility of CO2 sequestering and wastewater treatment) and economic feasibility. To date, most studies have focused on selection of species, coupled with optimization of cultivation, biomass harvest and fuel extraction; efforts on enhancing environmental benefits and upgrading spent biomass are thus in order. Industry should indeed adopt an integrated strategy inspired by algal-based biorefineries – toward maximizing the economic return of all products present in algal biomass, by recovering high value compounds prior to biofuel manufacture, and using the remainder for aquaculture/land animal feed and production of other biofuels via supplementary fermentation.

Application of microalgae protein to aquafeed

Fisheries are the most important sources of feedstock for fishmeal. Only a small percentage of global fish production is indeed channeled to human consumption, with the remainder being used for fish and animal feed. Fishmeal is a protein-rich food, and sets the basis for any balanced formulation used in commercial aquaculture. For several reasons there is a definite need for a new source of nutritious fish food. Therefore, the potential use of unconventional feed ingredients, such as microalgae, as feed inputs to replace high cost feed stuffs has been increasing. Microalgae are a more reliable and less volatile source of protein, and their availability is not dependent on fish captures. This provides industry with a better control of their costs, and supports a potential for future investment due to the reduction of risk in aquaculture farming operations.

Viability of dietary substitution of live microalgae with dry Ulva rigida in broodstock conditioning of the Pacific oyster (Crassostrea gigas)

ABSTRACT The current study evaluated the microalgae replacement by dry macroalgae (Ulva rigida) in the reproductive success and biochemical composition of the Pacific oyster (Crassostrea gigas) during broodstock conditioning. Five nutritional regimes were tested: 100% macroalgae (diet 1), 50% macroalgae+50% microalgae (diet 2), 25% macroalgae+75% microalgae (diet 3) and 100% microalgae (diet 4). An unfed group was used as a negative control. The microalgae blend was composed of 33% Isochrysis galbana and 67% diatoms (75% Skeletonema costatum+25% Chaetoceros calcitrans). Gonadal maturation was reflected in the physiological condition of the individuals. All treatments, except diet 1, showed an increase in condition index and were fully matured at the end of the trial, with the best physiological condition observed in oysters fed diet 3 and diet 4. Protein and total lipid content increased during the conditioning period, whereas glycogen content decreased. Oysters conditioned with diet 3 had higher protein and total lipid content and lower glycogen content than the other treatments. In addition, diet 3 showed the highest percentage of viable veliger larvae. The current study demonstrated that it is possible to replace 25% of microalgae with macroalgae in the broodstock conditioning, minimizing the operative cost in bivalve hatcheries. This article has an associated First Person interview with the first author of the paper. Summary: The use of an alternative diet with 25% microalgae replacement by dry macroalgae is beneficial during broodstock conditioning and allows the operation costs of bivalve hatcheries to be minimized.

Microalgal fatty acids—From harvesting until extraction

Abstract Microalgal biomass can be “energy rich,” but the growth of algae in dilute suspension around 0.02%–0.05% dry solids is a considerable challenge in achieving a viable energy balance in a microalgal biofuel process. Many researchers consider that efficient harvesting is the major challenge of microalgal biofuel commercialization but a single method or combination of harvesting methods suited to all microalgae does not exist. Additionally, lipid extraction continues to be a significant trial even though a multitude of extraction methods, especially in what concerns the presence of residual water in the microalgal biomass; its effects on lipid extraction or the need of cell disruption are still not well understood. In this chapter, these critical points on biofuel production—biomass harvesting and dewatering, biomass pretreatment, and fatty acid extraction methods—will be discussed.