Shizuoka University demonstrates rapid biomass growth using untreated seafood processing wastewater

A research group led by Graduate Student Ryo-Ken T. Kaga and Associate Professor Ryo Nagao from the Graduate School of Integrated Science and Technology at Shizuoka University has demonstrated that untreated raw wastewater from a skipjack tuna processing facility in Yaizu City, Shizuoka Prefecture can be used as a culture medium for a microalgae-bacterial consortium. When grown in this wastewater, the organisms increased their chlorophyll concentration approximately fivefold in just nine days. This finding suggests the potential for converting wastewater into resources through biomass production. The study was published in Bioscience, Biotechnology, and Biochemistry.

Seafood processing wastewater contains high concentrations of organic matter derived from proteins and lipids, as well as nitrogen and phosphorus, making it among the most difficult types of wastewaters to treat. Previous research has primarily focused on the removal of COD and nutrients, with treatment processes such as activated sludge methods being central. However, in recent years, there has been a growing movement to reconsider wastewater as a resource. Particularly, microalgae are expected to find applications as fertilizers, feed, and energy materials, as they produce biomass through photosynthesis while taking up nutrients from wastewater.

The symbiotic relationship between microalgae and bacteria is universally observed in nature. Microalgae supply organic matter through photosynthesis and bacteria handle organic matter decomposition and nitrogen cycling, resulting in a mutually stable metabolic network. Such microalgae-bacterial consortia may also demonstrate high adaptability in wastewater environments. However, there have been almost no reported cases of using untreated raw wastewater from skipjack tuna processing facilities as-is as a culture medium.

The research group conducted cultivation experiments using a native microalgae-bacterial consortium grown in untreated wastewater from a skipjack tuna processing facility in Yaizu City. They analyzed nutrient dynamics, community structure, and the potential for resource recovery from multiple perspectives.

When the microalgae-bacterial consortium was directly inoculated into the wastewater, the culture medium changed to a vivid green color within nine days. Chlorophyll concentration surged to approximately five times the initial level, and total suspended solids (TSS) also increased. This indicated nutrients in the wastewater were efficiently converted into photosynthetic biomass.

Regarding water quality dynamics, DOC (dissolved organic carbon) decreased rapidly from the initial stage; approximately 85% was removed over nine days. Phosphate ion concentration also decreased significantly (approximately 70% reduction), while ammonium ion among nitrogen sources showed a gradual decrease following a transient increase. This behavior reflects the functioning of organic nitrogen mineralization-assimilation coupling, where bacteria decompose proteins and peptides to release ammonium, which is then utilized by microalgae.

16S/18S rRNA gene analysis revealed that Chlorella, which performs photosynthesis, was dominant. The coexistence of bacterial groups involved in organic matter decomposition, such as Erythrobacter and Paracoccus, was also confirmed. These findings suggest that they form a functional symbiotic relationship: bacteria utilize oxygen and organic matter supplied by the microalgae, while resupplying nutrients in forms available to the microalgae.

The microalgae-bacterial biomass obtained from this cultivation system is expected to be used as fertilizer, feed, and even as a bioenergy source. Seafood processing wastewater is particularly rich in proteins and lipids. Given this, there is potential to obtain biomass containing high-value-added components, such as polyunsaturated fatty acids and useful proteins, compared with conventional culture media.

This demonstrates a new concept that goes beyond the conventional idea of wastewater as something to be treated, converting wastewater directly into a cultivation resource. Moving forward, it is expected that a sustainable wastewater utilization model will be established through component analysis of the biomass obtained, fertilizer and feed trials, and assessment of its potential introduction into the seafood processing industry.

Nagao commented: "We have demonstrated that wastewater from seafood processing, which is considered difficult to handle in its untreated state, can be converted into a culture medium through the symbiotic power of the microalgae-bacterial consortium without dilution or pretreatment. Process simplification is a practical advantage, and we believe this opens a new pathway for resource recovery and biomass production. Going forward, we plan to advance reactor design, scale up, and further the quality evaluation of the biomass produced for different applications."

Journal Information
Publication: Bioscience, Biotechnology, and Biochemistry
Title: Cultivation of a native microalgae-bacterial consortium in seafood processing wastewater primarily from skipjack tuna
DOI: 10.1093/bbb/zbaf155