Interactions between marine phytoplankton and bacteria drive ocean chemical cycling at the global scale and underpin the transfer of nutrients and energy throughout the marine foodweb. However, despite their global-scale significance, it has remained impossible to measure and visualize these interactions at the scale where they occur - the single cell level. In addition, the relative contribution of different bacterial taxa to marine nutrient cycling is still unknown.
Here we isolated different bacterial strains from some of the most ecologically relevant phytoplankton taxa (Talassiosira pseudonana, Synechococcus spp.). The phytoplankton cells were subsequently grown in batch for two months with ¹⁵N-labelled sodium nitrate as the only nitrogen source, ensuring complete cellular replacement of ¹⁴N with ¹⁵N. ¹⁵N-labeled phytoplankton cells were subsequently transferred to a fresh, unlabelled medium and isolated bacterial strains pre-labelled with 13C were inoculated to their respective 15N-labelled phytoplankton culture and samples were collected through a time.
The samples were analysed with the cutting edge of single-cell imaging techniques nano-scale secondary-ion mass spectrometry (NanoSIMS). This approach enables the first accurate quantification of the uptake rates of phytoplankton-derived nitrogen compounds by different bacterial taxa.