Citation: Pereira, M.I.B., Chagas, B.M.E., Sassi, R., Medeiros, G.F., Aguiar, E.M., Borba, L.H.F., Silva, E.P.E., Neto, J.C.A., Rangel, A.H.N., 2019. Mixotrophic cultivation of Spirulina platensis in dairy wastewater: Effects on the production of biomass, biochemical composition and antioxidant capacity. PLoS ONE 14, e0224294. https://doi.org/10.1371/journal.pone.0224294
Why the interest in microalgae?
Microalgae have great potential to be used as biofuels because they are extremely rich in lipids and carbohydrates. Certain species are already used in the food, pharmaceutical and cosmetic industries. Spirulina platensis is one antioxidant-rich species that has been used for its nutritional benefits.
Commercial-scale growing of algae has been occurring for over a decade. The largest demand for S. platensis is in the protein supplements market. However, using microalgae in biofuel production is a relatively new area of focus for researchers. Growing phytoplankton in mixotrophic conditions enables them to thrive on different food sources, not just one type of food as is the case with autotrophy. Previous studies have estimated that cell biomass yields can reach 3-30 times more than those produced under just autotrophic growth conditions.
Where does the cheese come in?
Cheese whey protein is the residue from manufacturing various types of cheese, yogurt, ice cream and butter. It is unfortunately a polluting byproduct of the dairy industry. Cheese, one of the world’s leading agricultural products, generates whey waste at four times the amount of the actual cheese product. Fortunately, previous work suggests that it’s possible to produce microalgae biomass in mixotrophic cultures with whey. This dairy industry waste product is actually very valuable for phytoplankton cells because it provides sugars as a source of fuel for microalgae species.
The goal of this study was to investigate using S. platensis under mixotrophic growth conditions (cheese whey) to reduce the cost in a sustainable production system. If S. platensis, a commercially important phytoplankton species, could be grown efficiently with whey byproduct, this could be a pretty neat outcome.
The research process…
Scientists in this study obtained a culture of S. platensis strain and grew it under sterile conditions. To obtain a mixotrophic culture status, buffalo mozzarella cheese whey was added to the culture in different concentrations: 2.5%, 5% and 10% whey. For a baseline to compare to, cultures were also grown under autotrophic conditions. When the cultures reached the stationary phase (about 17 days), meaning when they stopped dividing, the biomass was collected by filtration and washed to remove salt remnants. The scientists then determined the growth rate and productivity of the treated cultures.
What did they find?
Whey generally represents about 55% of the nutrients present in milk — such a shame it is often discarded as waste! Lucky for phytoplankton, whey is a great food source. Here, the researchers found that mixotrophic cultivation of S. platensis using 5% whey was favorable for biomass yield. The growth of the largest amount of biomass in the shortest amount of time occurred with a 10% addition of whey, suggesting that this treatment could be best from an industry standpoint.
Generally, the mixotrophic cultures showed higher growth rates when compared to either autotrophic or heterotrophic cultures. In addition, the biomass showed a higher percentage of total carbohydrates and lipids, instead of the high protein obtained from cells grown in autotrophic conditions. As the amount of whey increased in the cultures, the protein content decreased and the carbohydrate content increased. This is great for biofuel production because more carbohydrates allows for more sugars to be available for fermentation into ethanol as fuel.
This study demonstrated the potential of applying mixotrophic cultivation to produce high-carbohydrate microalgae. In addition to increasing biomass production, the mixotrophic culture of S. platensis using cheese whey may decrease the processing cost and produce low protein biomass which would in turn make it favorable for the production of biofuels.
Overall, the mixotrophic cultivation increased the biomass and carbohydrate productivity of S. platensis; however, it decreased its antioxidant capacity, protein and lipid productivity. The respective 70% and 76% increase in biomass production and carbohydrate production indicates that mixotrophic culture conditions could be a great option for biofuel production. Using cheese whey to grow phytoplankton kills two birds with one stone because it can reduce the cost of large-scale algae cultivation, while consuming a waste product that is harmful to the environment. This study further enhances our understanding of microalgal growth and how to best develop large-scale bioethanol production to help to reduce our carbon footprint.
I am a PhD student in the Rynearson Lab studying Biological Oceanography at the Graduate School of Oceanography (URI). Broadly, I am using genetic techniques to study phytoplankton diversity. I am interested in understanding how environmental stressors associated with climate change affect phytoplankton community dynamics and thus, overall ecosystem function. Prior to working in the Rynearson lab, I spent two years as a plankton analyst in the Marine Invasions Lab at the Smithsonian Environmental Research Center (SERC) studying phytoplankton in ballast water of cargo ships and gaining experience with phytoplankton taxonomy and culturing techniques. In my free time I enjoy making my own pottery and hiking in the White Mountains (NH).