Introduction: Algae are a significant bioactive compounds source, with variation in biological functions including antioxidant, anti-inflammatory and antiviral. Both microalgae and macroalgae biomass are a rich source of high value nutraceuticals and other bio products of commercial value, making algae worth investigating into. Objectives: To analyze bioactive phytochemicals and their antioxidants properties of extracts from algae from lake Taihu, Microcystis aeruginosa, Spirogyra and Seaweed extracted with ultrasound assisted method in comparison to the traditional extraction method, and a subsequent use of the residue in producing biochar. Materials and Methods: The different algae species viz Microcystis aeruginosa, Spirogyra and Seaweed were subjected to ultrasonic and thermostatted water bath extraction with a 70% ethanol and 30% water (v/v) as solvent. The extracts were characterized using gas chromatography-mass spectrometry. The total phenol and flavonoid and antioxidant activities were assessed in vitro viz hydroxyl radical (•OH) scavenging activity, ferric- reducing antioxidant property, total antioxidant capacity and 1, 1‑diphenyl‑2‑picrylhydrazyl (DPPH) activity. Results: The chemical analysis indicated the existence of bioactive compounds such as alkaloids, phenols, tannins, flavonoids, terpenoids, steroids, proteins, carbohydrates, phytosterols, quinones, saponins, coumarins and glycosides. In vitro screening of the extracts also showed good antioxidant capacity (hydroxyl radical (•OH) scavenging activity, ferricreducing antioxidant property, total antioxidant capacity and 1, 1‑diphenyl‑2‑picrylhydrazyl (DPPH) activity), the extracted compounds are of biological and pharmacological importance. The ultrasound assisted extraction method significantly increased yield with 27.0% in comparison to conventional method with 18.6%. Conclusion: The study showed the different algae species were enriched with phytochemicals and that the ultrasound assisted method was an effective extraction method environmentally benignant. A complete utilization of algae biomass was achieved after biochar was made from the residue.
