Molecular diagnosis of Trichodesmiumnutrient limitation in the oligotrophic North Atlantic

The filamentous cyanobacterium Trichodesmium is responsible for almost half of marine nitrogen (N₂) fixation, contributing to primary production and strongly influencing ocean biogeochemical cycles. Iron (Fe) and phosphorous (P) supply have both been found to limit the growth and distribution of N₂fixers in open ocean regions. In this study, we used a natural longitudinal gradient in Fe and PO₄to understand the molecular response of Trichodesmiumto nutrient availability in the subtropical North Atlantic. nifH sequencing revealed genotype T.thiebautiito be ubiquitous and abundant throughout the transect, but lower in abundance at the two easternmost stations, where T. spiralisdominated. A metatranscriptomic RNA sequencing analysis showed strong differences in gene expression across geographical regions, including the upregulation of known markers of Fe stress in stations where the measured Fe was low. In contrast, the expression of P stress bio-markers did not show a significant trend with PO₄supply, suggesting that other P sources may be available to meet Trichodesmium’s total P demand in situ. Nitrogen fixation related genes were upregulated in the middle of the transect where nifH abundance and N₂ fixation rates were the highest, while the expression of photosynthesis-related genes had the opposite trend. This type of study will help understand the functional dynamics of Trichodesmiumto adapt and survive in the oligotrophic North Atlantic, as well as to help predict their N and C fixation capabilities under future climate change.