Jiaxing Liu,Huangchen Zhang,Xiang Ding,Linbin Zhou,Zhixin Ke,Jiajun Li,Yehui Tan
Diazotrophic organisms are vulnerable to physical dynamic processes. However, the interactions and mechanisms of multiple dynamic processes on nitrogen fixation remain unclear. This study aimed to investigate one of these interactions; we conducted field investigations and bioassay experiments in the northeastern South China Sea, which faces the Kuroshio current and coastal upwelling. Using 15 N tracer and molecular biology methods, we found that the eastern Guangdong upwelling, having high phosphorus and a lower N:P ratio, had higher nitrogen fixation rates as well as a higher abundance of diazotrophic cyanobacteria Trichodesmium spp. and UCYN-A compared with nonupwelling area. Additionally, a higher abundance of diazotrophic cyanobacteria occurred near the bottom of the euphotic zone along the shelf break that was affected by the Kuroshio current. Field incubation experiments confirmed that phosphorus increased the nitrogen fixation rates and stimulated the growth of cyanobacteria in the area affected by upwelling. Taken together, our results indicated that warmer surface water with a high N:P ratio likely mixed with upwelled phosphorus-rich water containing diazotrophic cyanobacteria, which were transported by the intensified Kuroshio current under the influence of the 2015/16 El Niño . This mixing then triggered a high abundance of diazotrophic cyanobacteria. These findings provide new insights into how the interaction of multiple dynamic processes can regulate nitrogen fixation.
Microalgal biofilm can remove nutrients and refractory pollutants from livestock wastewater, transforming them into biomass or fuel. However, these applications are hampered by biofilm shedding. Coculture enhances effects, and its influence on the adhesion of attached microalgal biofilms requires exploration. In this study, two unicellular algae, Euglena (E) and Chlorella pyrenoidosa (C). and two filamentous algae, Klebsodium sp. (K) and Phormidium sp. (P) , were paired two by two, aiming to investigate the effect of mixing on the adhesion of microalgal biofilm. The results showed that suitable microalgal combinations significantly enhanced the performance of the co-cultured microalgal biofilm. Biomass enhancement of 35 % and 21.9 % was observed in the PC group compared to the individual P and C groups, respectively. The content of tightly bound extracellular polymeric substances (TB-EPS) and its protein and polysaccharide content significantly correlating with the net biomass increase in all treatments, p < 0.05 . Furthermore, measurement of zeta potentials and water contact angles indicated that alterations in protein secondary structures contributed to a lower intermolecular repulsion and greater hydrophobicity of the TB-EPS, which together with the stability provided by the higher β-glucan content supported the adhesion of the co-cultured microalgal biofilm.
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