Konstantinos Pispas

The depletion of fossil resources and increasing plastic-derived pollution have become major environmental concerns. In this context, biodegradable biopolymers such as polyhydroxyalkanoates (PHA) have attracted considerable scientific interest. Microalgae and cyanobacteria represent promising photosynthetic microorganisms for sustainable bioplastic production. In this study, ten microalgal species (Parachlorella kessleri, Chlorella vulgaris, Chlorella minutissima, Tetraselmis tetrathele, Chromochloris zofigiensis, Acutodesmus obliquus, Euglena gracilis, Chlorococcum oleofaciens, Botryococcus braunii, and Coelastrella vacuolata) and four cyanobacterial species (Arthrospira platensis, Nostoc muscorum, Chlorogloeopsis fritschii, and Synechocystis sp.) were screened for growth and PHA accumulation. Cultivation was performed under autotrophic conditions, mixotrophic growth with acetate, mixotrophic growth in anaerobic digestion effluent, and nitrogen-depleted conditions. Arthrospira platensis showed the highest PHA content under autotrophic growth, reaching 4.3% w/w. Most species adapted successfully to anaerobic digestion effluent, achieving total nitrogen and phosphorus removal above 90% in most cases, although PHA accumulation remained low (0.6–1.3% w/w). Acetate supplementation and nitrogen limitation markedly enhanced PHA accumulation. Mixotrophic cultivation with acetate yielded up to 10.2% w/w in cyanobacteria, whereas nitrogen starvation increased accumulation further, with Synechocystis sp. reaching 14.4% w/w. Overall, cyanobacteria outperformed green microalgae, supporting their potential for sustainable bioplastic production. These findings provide a basis for species selection and future process optimization.

Large-scale cultivation of cyanobacteria is often limited by the high cost of synthetic culture medium and the environmental impact of nutrient consumption. Cheese whey, a major agro-industrial waste product, is rich in organic and inorganic nutrients, making it a promising low-cost alternative for microbial growth while addressing waste bioremediation. This study investigates the growth performance and the biochemical composition of four different cyanobacterial species (Phormidium sp., Synechocystis sp., Chlorogloeopsis fritschii, and Arthrospira platensis), cultivated in cheese whey (CW). Pretreated CW was used at 20% and 100% v/v concentrations. All species grew satisfactorily in both concentrations, reaching biomass above 4 g L⁻¹ (in 100% v/v CW) and 2 g L⁻¹ (in 20% v/v CW). The highest μ_max value (0.28 ± 0.02 d⁻¹) was presented by Synechocystis sp. grown in 20% CW. Waste bioremediation of both 20 and 100% v/v CW demonstrated effective nutrient removal, with COD removal exceeding 50% for most species, while total nitrogen (TN) and total phosphorus (TP) removals reached up to 33% and 32%, respectively. Biochemical composition analysis revealed high carbohydrate and protein content, while lipid content remained below 15% in all cases. Interestingly, C. fritschii accumulated 11% w/w polyhydroxyalkanoates (PHAs) during the last day of cultivation in 20% v/v CW. These findings highlight the potential of C. fritschii as a valuable candidate for integration into bioprocesses aimed at sustainable bioplastic production. Its ability to synthesize PHAs from agro-industrial waste not only enhances the economic viability of the process but also aligns with circular economy principles. This study is a primary step towards establishing a biorefinery concept for the cultivation of cyanobacterial species in cheese whey-based wastewater streams.

Cyanobacterial phycocyanin pigment is widely utilized for its properties in various industries, including food, cosmetics, and pharmaceuticals. Despite its potential, challenges exist, such as extraction methods impacting yield, stability, and purity. This study investigates the impact of the number of freeze–thaw (FT) cycles on the extraction of phycocyanin from the wet biomass of four cyanobacteria species (Arthrospira platensis, Chlorogloeopsis fritschii, Phormidium sp., and Synechocystis sp.), along with the impact of five extraction solutions (Tris-HCl buffer, phosphate buffer, CaCl₂, deionized water, and tap water) at various pH values. Synechocystis sp. exhibited the highest phycocyanin content among the studied species. For A. platensis, Tris-HCl buffer yielded maximum phycocyanin concentration from the first FT cycle, while phosphate buffer provided satisfactory results from the second cycle. Similarly, Tris-HCl buffer showed promising results for C. fritschii (68.5% of the maximum from the first cycle), with the highest concentration (~12% w/w) achieved during the seventh cycle, using phosphate buffer. Phormidium sp. yielded the maximum pigment concentration from the first cycle using tap water. Among species-specific optimal extraction solutions, Tris-HCl buffer demonstrated sufficient extraction efficacy for all species, from the first cycle. This study represents an initial step toward establishing a universal extraction method for phycocyanin from diverse cyanobacteria species.

Over the past few decades, plastic-derived pollution has been recognized as a major environmental issue because the use of conventional plastics results in vast amounts of waste as well as in fossil-fuel depletion. Biodegradable and biobased polymers are a promising alternative to conventional plastics. In this context, polyhydroxyalkanoates (PHAs) are bioplastics with similar mechanical and thermal properties to petroleum-based plastics which can be used in a wide range of applications. Several studies have reported the accumulation of PHAs in the biomass of microalgae and cyanobacteria. Under optimal conditions for PHA accumulation, that is, nutrient limitation, and optimal light intensity, PHA content can significantly increase, achieving 85% of dry biomass weight. Downstream recovery of PHAs is also a critical step that affects the properties and the yield of PHAs. Bioplastic production from microalgae and cyanobacteria on a commercial scale is still limited due to its high cost, with the cultivation medium accounting for up to 50% of the total production cost. The use of wastewater as a growth medium can improve the economic feasibility and sustainability of PHA production from microalgae and cyanobacteria and contribute to a more circular economy.