Specific microalgae species are an adequate source of EPA and DHA and are able to provide a complete protein, which makes them highly interesting for human nutrition. However, microalgae cultivation has also been described to be energy intensive and environmentally unfavorable in pilot-scale reactors. Moreover, production in cold temperature zones has not been sufficiently investigated. In particular, the effects of tube materials and cultivation season length have rarely been previously investigated in the context of a comparative LCA of microalgae cultivation. A computational “top-down” model was conducted to calculate input flows for Nannochloropsis sp. and Phaeodactylum tricornutum cultivation in a hypothetical tubular photobioreactor. Cultivation processes were calculated according to detailed satellite climatic data for the chosen location in Central Germany. This model was applied to a set of different scenarios, including variations in photobioreactor material, tube diameter, microalgae species, and cultivation season length. Based on these data, a life cycle assessment (LCA) was performed following ISO standard 14040/44. The impact assessment comprised the global warming potential, acidification, eutrophication, cumulative energy demand, and water scarcity. The results showed that a long cultivation season in spring and fall was always preferable in terms of environmental impacts, although productivity decreased significantly due to the climatic preconditions. Acrylic glass as a tube material had higher environmental impacts than all other scenarios. The cultivation of an alternative microalgae species showed only marginal differences in the environmental impacts compared with the baseline scenario. Critical processes in all scenarios included the usage of hydrogen peroxide for the cleaning of the tubes, nitrogen fertilizer, and electricity for mixing, centrifugation, and drying. Microalgae cultivation in a tubular photobioreactor in a “cold-weather” climate for food is sustainable and could possibly be a complement to nutrients from other food groups. The added value of this study lies in the detailed description of a complex and flexible microalgae cultivation model. The new model introduced in this study can be applied to numerous other scenarios to evaluate photoautotrophic microalgae cultivation in tubular photobioreactors. Thus, it is possible to vary the facility location, seasons, scale, tube dimensions and material, microalgae species, nutrient inputs, and flow velocity. Moreover, single processes can easily be complemented or exchanged to further adjust the model individually, if, for instance, another downstream pathway is required.

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The production of food for a growing world population is a great challenge. In particular, protein and the long-chain n-3 fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which exert a series of potential health effects, are scarce resources in the context of global food security. Fish from wild capture and aquaculture production cannot meet the current demand for EPA and DHA; therefore, a supplementation with alternative sources is crucial. Specific microalgae species have been shown to be a lucrative source of EPA, DHA, and protein, in particular, the oleaginous microalgae Nannochloropsis sp. and Phaeodactylum tricornutum. This study aimed to compare different cultivation scenarios of Nannochloropsis sp. and P. tricornutum with the production of aquaculture and capture fish as traditional sources of EPA and DHA in terms of environmental impacts. Scenarios included borosilicate glass and acrylic glass as photobioreactor (PBR) materials, two different tube diameters, and three different cultivation seasons. In these scenarios, carbon dioxide was modeled as an avoided burden. Additionally, all scenarios were modeled with the burdens resulting from carbon dioxide production. Environmental impacts of selected fish species were obtained from systematic literature research. Life cycle assessment following ISO 14040/44 was used to analyze the global warming potential, acidification, eutrophication, cumulative energy demand, water footprint, and land use. The system boundaries were set from “cradle-to-store,” where the target store is located in Germany. Microalgae biomass as a source of EPA, DHA, and protein was found to have similar or lower environmental impacts than fish fillet from wild capture and aquaculture production when carbon dioxide was modeled as an avoided burden. Microalgae production that included the full burden of carbon dioxide production still caused similar or lower environmental impacts than aquaculture fish. It was found that the distinct microalgae species can significantly influence the results if the comparison is conducted based on nutritional values. Regarding the recommended daily intake of 250–500 mg EPA+DHA, microalgae are an advisable source of nutrients to lessen the environmental pressure on marine ecosystems.

Fish as the primary source for the essential n − 3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) cannot cover the global demand for these important nutrients resulting in a supply gap of currently 1.1 million tons of EPA + DHA annually. A further exploitation of natural fish stocks is linked to great damage to ecosystems. Oleaginous microalgae are a natural source for EPA and DHA and could possibly contribute to closing this gap. The cultivation in photobioreactors (PBR) in a ‘cold-weather’ climate showed that microalgae compare favorably to aquaculture fish. The present study assesses the economic potential of microalgae for food in such system model. Techno-economic assessment was conducted on the basis of a dynamic system model for the cultivation of Nannochloropsis sp. in industrial scale in Central Germany over a time span of 30 years. The net present value (NPV) and return-on-investment (ROI) were obtained for a number of scenarios in which technic and economic parameters were altered. Taking the size of the PBR considered into account, the cultivation of Nannochloropsis sp. yielded a positive NPV of EUR 4.5 million after 30 years which translates to an annualized ROI of 1.87%. The sensitivity analysis overall resulted in annualized ROIs between 1.12 and 2.47%. Major expenditures comprised the PBR infrastructure, maintenance and labor cost. An extended cultivation season by four weeks was responsible for an NPV surplus of almost one third (32%). An increase in the selling price by 15% was responsible for a 47% higher NPV. In comparison with Atlantic salmon (Salmo salar) raised in aquaculture, EPA from Nannochloropsis sp. resulted in about halved cultivation costs (− 44 to − 60%). In this study we could show that microalgae from photoautotrophic cultivation not only have the potential to supply humans with essential nutrients, but they are also a lucrative investment, even in a ‘cold-weather’ climate where cultivation cannot take place year round. Graphic abstract

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