Effects of Biochar on Microalgal Growth: Difference between Dissolved and Undissolved Fractions

Biochar (BC) exhibits potential risks to aquatic ecosystems following diffusion into water. Although microalgae have been employed as an efficient model species to investigate BC’s risks to aquatic systems, it remains unclear on the different biological responses of microalgae to pristine BC (PBC), and undissolved (water-extracted BC, WBC) and dissolved BC (DBC). Our results showed that inhibitory effects of different BC fractions on microalgae growth followed the order of PBC > WBC > DBC. Elevated pyrolysis temperature (i.e., 600 versus 400 °C) and water extraction treatment could change BC’s characterization and alleviate BC’s inhibitory effects. The higher inhibitory effect of PBC than WBC was likely due to the change of surface chemistry properties (i.e., persistent free radicals, C and O content, and O/C ratio), instead of shading effects, nutrient depletion, or the DBC extracted from PBC. Interestingly, the DBC could accelerate microalgal growth, probably due to the release of polysaccharides as well as other components of BC. This study provides useful implications for better applications of BC as more environmentally friendly materials.