Mixotrophic cultivation of Chlorella prototheicoides using peach peel extract as an alternative source of nutrients for circular economy purposes

The Emilia-Romagna region (Italy) has a strong agricultural vocation with large tracts of lands dedicated to the cultivation of fruit trees, among which the apple, peach, pear, and apricot trees are prominent. Besides direct consumption, their fruits and false-fruits are widely processed for the production of juices and purees. Fruit processing generates high amounts of by-products (fruit processing matrices - FPMs), which consist in non-edible parts, that require post-treatments before their disposal, for example due to their still high content in organic matter. Driven by the need to reduce industrial wastes, the European Union is aiming for a transition to an increasingly circular economy by 2050. In this perspective and considering that FPMs represent a low-cost abundant renewable source of organic carbon, it is attractive the option of their valorization, instead of disposal, for example as a low-cost feedstock for microalgae cultivation. Microalgae are photosynthetic microorganisms well-known for their application in different biotechnological fields. Due to their high adaptability to different growth conditions (e.g., substrate, light intensity, temperature, etc.), microalgae can valorize various types of wastes by producing a biomass rich in lipids, proteins, and carbohydrates, for example for the pharmaceutical, energetic and food/feed sectors. Depending on microalgae species, the sugars and nutrients contained in FPMs can be metabolized in the absence (heterotrophic conditions) or in the presence of light (mixotrophic conditions) for microalgal biomass and biobased compounds production. The aim of this work was to assess the possibility of cultivating microalgae in mixotrophy using peach peel extract (PPE); for the present study, preliminary results on Chlorella protothecoides are reported. Results from tests using PPE were compared to those obtained using a synthetic mineral growth medium only (BG11, autotrophic control) or the same mineral medium added with sucrose, glucose, and fructose, i.e. the main sugars found in the PPE. This peach-derived product was chosen for tests due to its lower sucrose content than that measured, for example, on puree extracts; sucrose, indeed, is a disaccharide known to be poorly assimilable by microalgae if compared to glucose and fructose. Growth, morpho-physiological aspects, and photosynthetic pigments content of microalgae were investigated during cultivation. Exhausted culture media from each cultivation were analyzed both through fast and simple °Brix and pH measurements and through more complete NMR spectroscopy analysis to evaluate the specific sugars consumption. Although this study is still in its early stage, microalgae grown in the presence of PPE and synthetic sugars showed greater biomass production and cell density than in the BG11 medium; enhanced growth was associated with good photosynthetic efficiency. No differences in photosynthetic pigments content were recorded. At the end of cultivations, sugars in the cultures set up with PPE were removed from the medium, indicating the suitability of this substrate as a carbon-rich medium for microalgal cultivation. Further investigations are planned to assess the presence of biobased compounds (mainly proteins) and bioactivity with antimicrobial/fungicide potential within the microalgal biomass or spent growth medium.