Coupling of cylindrospermopsin and pho-harboring Verrucomicrobia supports the formation of Raphidiopsis blooms in low-phosphorus waters.

Cylindrospermopsin (CYN) can induce phytoplankton community to secrete alkaline phosphatase (ALP), which is one of the important strategies for the bloom-forming cyanobacterium Raphidiopsis to thrive in extremely low-phosphorus (P) waters. However, how bacterioplankton community, another major contributor to ALPs in waters, couples to Raphidiopsis through CYN, and the role of this coupling in supporting the dominance of Raphidiopsis in nature remain largely unknown. Here, we conducted microcosm experiments to address this knowledge gap, using a combination of differential filtration-based and metagenomics-based methods to identify the sources of ALPs. We found that, compared with algal-derived ALPs, bacteria-derived ALPs exhibited a more pronounced and sensitive response to CYN. This response to CYN was enhanced under low-P conditions. Interestingly, we found that Verrucomicrobia made the largest contribution to the total abundance of pho genes, which encode ALPs. Having high gene abundance of the CYN-sensing PI3K-AKT signaling pathway, Verrucomicrobia's proportion increased with higher concentrations of CYN under low-P conditions, thereby explaining the observed increase in pho gene abundance. Compared with other cyanobacterial genera, Raphidiopsis had a higher abundance of the pst gene. This suggests that Raphidiopsis exhibited a greater capacity to uptake the inorganic P generated by ALPs secreted by other organisms. Overall, our results reveal the mechanism of CYN-induced ALP secretion and its impact on planktonic P-cycling, and provide valuable insights into the role of CYN in supporting the formation of Raphidiopsis blooms.

Simultaneous removal of cyanobacterial blooms and production of clean water by coupling flocculation with a rotary drum filter.

A mechanical harvesting technology based on coupling flocculation with a rotary drum filter (RDF, 35-µm) was applied to remove cyanobacterial blooms and produce clean water in Lake Caohai, a sub-lake of Lake Dianchi (Kunming, China). After treatment with a shipboard RDF and cationic polyacrylamide (CPAM, 0.5-2 mg/L) flocculation, > 95% of cyanobacterial biomass was removed. The chlorophyll-a (Chl-a) concentration and turbidity in the effluent were < 8 µg/L and < 3 NTU, respectively. Nutrient concentrations were also markedly reduced, with a permanganate index (PI) of < 2 mg/L and total phosphorus concentration of < 20 µg/L. The total nitrogen concentration was reduced from 2.75 to 1.65 mg/L, and most of the residual nitrogen was nitrate. Although powerful for the removal of suspended particles and an enhanced water transparency, the combined technology showed no significant reduction in inorganic nutrients and only a slight reduction in dissolved organic matter (DOM). The concentrations of protein and polysaccharide were significantly reduced, while that of humic matter did not change during the process. After flushing with the effluent of the RDF, a 20,000-m3 enclosure of lake water became clear when the volume of the effluent was 1.8 times that of the water enclosure. The electrical energy per order (EE/O) was calculated to be 0.053kWh/m3, which is lower than that of transferring water from more than 10 km away (0.058 kWh/m3). Thus, a shipboard RDF coupled with CPAM flocculation is a promising approach to remove harmful cyanobacterial blooms and improve the water environment of eutrophic lakes.