Annual dynamics of eukaryotic and bacterial communities revealed by 18S and 16S rRNA metabarcoding in the coastal ecosystem of Sagami Bay, Japan

Abstract

Sagami Bay, Japan is influenced by both the warm Kuroshio Current and the cold Oyashio Current and rich nutrients are supplied from multiple river sources and the deep-sea, forming a dynamic ecosystem. The aim of the present study was to investigate eukaryotic and bacterial communities in the coastal waters of Sagami Bay, using 16S rRNA and 18S rRNA sequencing and to assess the seasonal and vertical dynamics in relation to physicochemical and biological conditions. Eukaryotic and bacterial communities showed synchronous seasonal and vertical changes along with environmental variability. Diversity of plankton community suspended in the surface was lower than those at the subsurface layers in both the eukaryotes and bacteria communities; however, community diversity showed different characteristics in the subsurface where the eukaryotic community at the deeper layer (100 m) was as low as the surface and highest in intermediate depth layers (10–50 m), while that of bacterial community was highest in the deeper layer (100 m). The annual variability of the coastal microbial communities was driven, not only by the seasonal changes of abiotic and biotic factors and short-term rapid changes by river water inflow and phytoplankton blooms, but also largely influenced by deep-seawater upwellings due to the unique seafloor topography.

Observation of a coccolithophore <i>Gephyrocapsa oceanica</i> bloom in the temperate coastal waters of Sagami Bay, Japan

“…On 19 April 2019, 19′-HF concentration was at the highest value recorded during the survey period (Fig. 9), and on the same day, within haptophytes, Prymnesiales abundance also comprised the highest number of genetic reads between 0-10 m depth followed by Phaeocystales and Coccolithales (Sogawa et al 2022). Although it should be noted in taxonomic discussions that G. oceanica belongs to the order Isochrysidales, these results suggest that haptophyte occurrence, including coccolithophores, may be more prevalent than previously assumed in Sagami Bay.…”

“…However, it should be noted that 19′-HF alone cannot confidently discriminate only coccolithophores since coccolithophores are only one kind of haptophyte. A recent study of prokaryotic and eukaryotic community structure using genetic analysis reported that coccolithophores were detected in October 2018, April, May, July, and August 2019, and February 2020 between 0-10 m depth at St. M, appearing six times out of 23 samplings (Sogawa et al 2022) burth, 1984burth, (Sogawa et al 2022). On 19 April 2019, 19′-HF concentration was at the highest value recorded during the survey period (Fig.…”