Diel Patterns in the Composition and Activity of Planktonic Microbes in a Subtropical Bay

Planktonic microbes play a crucial role in the matter transformation and energy transfer of marine ecosystems. Marine microbial communities are profoundly influenced by various environmental factors, and their variations are typically investigated in a time-dependent manner. However, studies conducted on a monthly or annual basis do not offer sufficient temporal resolution to reveal changes that can occur on an hourly basis. We conducted a high-resolution time-series study using amplicon sequencing on seawater samples taken at 2-h intervals over a 3-d period from the subtropical Daya Bay to investigate changes in the microbial community composition and activity. The results showed that 46.8% of the microbial rRNA sequences exhibited circadian rhythms, including phytoplankton (e.g., Bacillariophyta, Cryptophyta, and Dinophyceae), heterotrophic bacteria (e.g., Actinomarinaceae, Flavobacteriaceae, and SAR11_clade), and zooplankton (e.g., Bestiolina and Phyllopharyngea). The genera of Bacillariophyta (e.g., Chaetoceros and Pseudo-nitzschia) exhibited higher activity than those of Chlorophyta (e.g., Micromonas). A similar trend was observed for zooplankton, where Choanozoa (e.g., Copepoda) showed a strong circadian rhythm in abundance whereas Ciliophora (e.g., Pithites) were more active. Light intensity and tides were identified as the driving factors behind the periodic succession and activity of the heterotrophic bacteria and phytoplankton with the exception of picocyanobacteria, which were mainly influenced by fluctuations in temperature, nutrients, and salinity. These high-resolution time-dependent observations showed that, despite the dynamic coastal environment, the periodic changes in the microbial community and activity were distinct and phylogenetically conserved. The findings may help in the development of ecosystem models for predicting microbial abundance and activity in rapidly changing coastal and estuary environments.