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

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.

Unstable Relationship Between Braarudosphaera bigelowii (= Chrysochromulina parkeae) and Its Nitrogen-Fixing Endosymbiont

“…Moreover, the RNA was extracted from cells in nitrogen-rich/-poor media and under a light–dark phase ( Supplementary Table 3 ). For some algae and plants, the presence of nitrate induces the expression of genes encoding nitrate transporters ( Clarkson and Lüttge, 1991 ; Navarro et al, 1996 ; Crawford and Glass, 1998 ; Daniel-Vedele et al, 1998 ; Suzuki et al, 2019 ); however, we could not detect the transcript in our RNA-seq analyses with nitrate. Together with the essential function of these genes in nitrogen metabolism, it is possible that these genes are absent or pseudogenized in the genome.…”

“…Marine phytoplankton consume various nitrogen compounds, such as nitrate, nitrite, ammonia, and cyanate, and often produce large-scale blooms triggered by the abundance of specific nitrogen compounds ( Suzuki et al, 2019 ). In oligotrophic ocean environments, phytoplankton growth is primarily limited by the availability of nitrogen compounds.…”

Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton

“…The higher nitrate than ammonium uptake rates in spring and early summer are consistent with previous studies based on isotope labeling techniques (Shiozaki et al., 2016; Tripathy et al., 2018; Varela et al., 2013). In the northwestern North Pacific, the diatoms that have high expression of genes for nitrate uptake bloom every spring (Suzuki et al., 2019). The simulated nitrate is gradually enriched in 15 N from 6.5 to 7.6‰ owing to the phytoplankton assimilation, whereas the ammonium is depleted in 15 N from 14.7 to 7.7‰ due to decomposition and/or excretion.…”

“…In the northwestern North Pacific, the diatoms that have high expression of genes for nitrate uptake bloom every spring (Suzuki et al, 2019). The simulated nitrate is gradually enriched in 15 N from 6.5 to 7.6‰ owing to the phytoplankton assimilation, whereas the ammonium is depleted in 15 N from 14.7 to 7.7‰ due to decomposition and/or excretion.…”