Seasonal Variations of Grazing and Viral Lysis on Prokaryotic Mortality in the Coastal Waters of Qingdao

The temperate coastal waters are characterized by significant interannual variations of environmental factors, as well as seasonal succession patterns of protists, viruses, and prokaryotic communities. This highlights the importance of investigating seasonal changes in prokaryotic mortality caused by protistan grazing and viral lysis in this region, as well as the impact of environmental factors on these mortality rates. In this study, we applied a modified dilution experiment to estimate prokaryotic mortality resulting from grazing and viral lysis in the coastal waters of Qingdao from December 2021 to November 2022. Our results indicated that grazing was the primary factor driving prokaryotic mortality. Notably, viral lysis resulted in negative mortality rates and exhibited a strong negative correlation with grazing mortality (r=−0.91, p<0.001). Additionally, prokaryotic mortality followed a distinct seasonal pattern, with higher rates in summer and autumn and lower rates in winter and spring. Spearman correlation analysis revealed that the seasonal variations in mortality rates were related to the changes of dissolved organic carbon (DOC), inorganic nutrients, and prokaryotic abundance. Finally, the high prokaryotic mortality rate during summer resulted in substantial release of organic carbon and nutrients in the water, which may stimulate phytoplankton growth and contribute to the frequent occurrences of eutrophication and algal blooms in coastal waters during this season. Our study highlights the intricate relationships among prokaryotes, protists, viruses, as well as environmental factors.