Dissolved organic nitrogen and phosphorus derived from different cyanobacteria regulate distinctly different nitrate reduction pathways in sediments.

The differentiation of the nitrate reduction pathway is of great significance in the ecosystem, as it determines the occurrence form of ecosystem N. In order to explore the impact and mechanism of different algal dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) on differentiation of nitrate (NO-N) reduction pathway, small-scale enclosure experiment was conducted to analyze the DON and DOP composition, nutrient level, microbial community composition and NO-N reduction pathway in ponds with Microcystis and Dolichospermum blooms. The main DON produced by Microcystis included lipids and proteins as well as carbohydrate which were readily degradable, whereas the DOP produced by Dolichospermum predominantly consists of readily degradable forms such as carbohydrate and protein. The active DON from Microcystis induced excess NO-N and organic carbon (OC), favoring DNRA evidenced by high DNRA rate and functional genes abundance as well as relative microbial community. This finally resulted in elevated nitrogen (N) levels in the water. Conversely, Dolichospermum blooms promoted polyphosphate-denitrifying bacteria growth through decomposable DOP, enhancing denitrification over dissimilatory nitrate reduction to ammonium (DNRA) and resulting in higher phosphorus (P) levels. In addition, the mixed growth of Microcystis and Dolichospermum supported the denitrification and P accumulation as well as the mutualism between algae and bacteria. This study highlights the role of DOP and DON in shaping microbial communities and providing OC that govern NO-N reduction processes.