Dual stable isotope tracing the source and composition of biogenic substance in microalgae-bacteria symbiosis: What do the bacteria contribute to the microalgae bloom?

Microalgae blooms are often regarded as harmful, but during the process of microalgae blooms, there are positive ecological values that the biochemical interactions of symbiosis of microalgae and bacteria could inform new strategies for microalgae blooms management and water treatment. This study used 13 C and 15 N stable isotope labeling and elemental analysis to elucidate carbon and nitrogen metabolism and migration between microalgae and bacteria. Anabaena flos-aquae (AF) was selected as the target microalgae species to establish a symbiotic system with Brevundimonas lenta ( B. lenta ), Pseudomonas plecoglossicida ( P. plecoglossicida ) and mixed bacteria obtained from the Zhuxi River (ZX). The 14-day co-culture using sequential batch methods revealed that the synergy of AF with B. lenta and ZX enhanced carbon utilization efficiency in microalgae, with contribution rates of 9.69 % and 29.69 %, respectively, while also boosting nitrogen utilization by 17.72 % and 46.39 %, respectively. High-biodiversity bacterial communities in symbiotic systems improved carbon and nitrogen utilization while reducing CO₂ emissions. Stable isotope analysis showed that symbiosis increased lighter isotope percentages in microalgae, with bacteria shifting from lighter to heavier isotopes. This study provides theoretical and experimental insights into managing microalgae blooms and water treatment by analyzing biogenic substance transport in microalgae-bacteria interactions.