Metatranscriptome unveils the transcriptional responses of nitrogen-related genes in marine diatoms within the coastal waters of the Matsu archipelago

The availability of inorganic nitrogen has long been recognized as a limiting factor for primary production in the ocean. Nevertheless, various species or groups have evolved distinct strategies to uptake and assimilate nitrate and ammonium, enabling them to adapt to environmental changes. In order to understand the nitrogen uptake characteristics of diatoms and their transcriptional responses of nitrogen-related genes, 2 research cruises were carried out in the southern East China Sea in 2018 and 2019. Microphytoplankton cell counts revealed that diatoms were the major dominant group, comprising over 90% of microphytoplankton composition. Despite the high abundance of microphytoplankton in 2018, both the potential maximum uptake rates of ammonium and nitrate for microphytoplankton were extremely low. Metatranscriptomic analyses of microphytoplankton unveiled the nitrogen status of dominant diatoms, such as Chaetoceros and Coscinodiscus, based on the gene expression of nitrogen-related genes. In 2018, only nitrogen transporter genes and 3 genes at the beginning of the nitrate assimilation pathway were classified as N-limited induced genes. The low relative transcript percentage of these genes indicated repression of nitrate uptake and assimilation. The remaining nitrogen-related genes expressed more like constitutive genes, with transcripts maintaining low levels even in ammonium addition treatments and failing to be induced to higher transcript levels under N-free treatments. This result suggested that the elevated concentrations of nitrate in surface water might be residual nitrogen, as diatoms had already acquired sufficient nitrogen, leading to the cessation of nitrate and ammonium uptake. In contrast, most of nitrogen-related genes (15/20) were classified as N-limited induced genes in 2019. Among these genes, only one nitrate transporter gene was highly expressed in both genera. The remaining genes expressed as low as observed in ammonium-addition treatments. This result implied that diatoms engaged in competition for the limited nitrogen available in their environment. These findings showed the capability of metatranscriptomic approaches to elucidate the transcriptional responses of dominant diatoms, providing insight into how these diatoms regulate their nitrogen-related genes to adapt to environmental fluctuations within natural assemblages.

Primary Presenter: Lee-Kuo Kang, National Taiwan Ocean University (lkkang@mail.ntou.edu.tw)

Authors:

Chi-Yu Shih, National Taiwan Ocean University / Taiwan Ocean Genome Center  (d97340002@email.ntou.edu.tw)

Kai-Zhe Chang, National Taiwan Ocean University / Institute of Marine Environment and Ecology  (joey28220498@gmail.com)