Depth-dependent transcriptomic response of diatoms during spring bloom in the western subarctic Pacific Ocean

Shigekatsu Suzuki,Takafumi Kataoka,Tsuyoshi Watanabe,Masanobu Kawachi,Akira Kuwata,Haruyo Yamaguchi

doi:10.1038/s41598-019-51150-8

Abstract

Diatoms play important roles in primary production and carbon transportation in various environments. Large-scale diatom bloom occurs worldwide; however, metabolic responses of diatoms to environmental conditions have been little studied. Here, we targeted the Oyashio region of the western subarctic Pacific where diatoms bloom every spring and investigated metabolic response of major diatoms to bloom formation by comparing metatranscriptomes between two depths corresponding to different bloom phases. Thalassiosira nordenskioeldii and Chaetoceros debilis are two commonly occurring species at the study site. The gene expression profile was drastically different between the surface (late decline phase of the bloom; 10 m depth) and the subsurface chlorophyll maximum (SCM, initial decline phase of the bloom; 30 m depth); in particular, both species had high expression of genes for nitrate uptake at the surface, but for ammonia uptake at the SCM. Our culture experiments using T. nordenskioeldii imitating the environmental conditions showed that gene expression for nitrate and ammonia transporters was induced by nitrate addition and active cell division, respectively. These results indicate that the requirement for different nitrogen compounds is a major determinant of diatom species responses during bloom maturing.

Highlights

Diatoms contribute up to 20–40% of oceanic primary productivity forming large-scale blooms mainly in coastal and upwelling regions[1,2,3]
We compared two diatom communities obtained from the sea surface and subsurface chlorophyll maximum (SCM, initial decline phase of the bloom; 30 m), targeting T. nordenskioeldii and C. debilis, which commonly occur during spring bloom in this region[21]
Temperature and salinity at the SCM (5.2 °C and 33.0) were lower than at the surface (8.7 °C and 33.5), indicating that the SCM and surface were affected by the water columns Coastal Oyashio Water (COW) and modified Kuroshio Water (MKW), respectively[27]

Summary

Introduction

Diatoms contribute up to 20–40% of oceanic primary productivity forming large-scale blooms mainly in coastal and upwelling regions[1,2,3]. The western subarctic Pacific is a preferable field to study cell responses of bloom-forming species because large-scale diatom blooms occur annually in spring in this region[5,20,21]. The hydrography of this region in spring is complex, with dominant water columns: Oyashio Water (OW), Coastal Oyashio Water (COW), and modified Kuroshio Water (MKW)[22]. We performed metatranscriptome analyses of diatom communities during spring blooms in the western North Pacific to reveal bloom-specific cell responses, focusing on differences in depth. Gene expression analysis of nutrient-controlled cultures was conducted to confirm the metatranscriptomic analysis

Methods

Results

Conclusion