Single cell dynamics and nitrogen transformations in the chain forming diatom Chaetoceros affinis.

Colony formation in phytoplankton is often considered a disadvantage during nutrient limitation in aquatic systems. Using stable isotopic tracers combined with secondary ion mass spectrometry (SIMS), we unravel cell-specific activities of a chain-forming diatom and interactions with attached bacteria. The uptake of 13 C-bicarbonate and 15 N-nitrate or 15 N-ammonium was studied in Chaetoceros affinis during the stationary growth phase. Low cell-to-cell variance of 13 C-bicarbonate and 15 N-nitrate assimilation within diatom chains prevailed during the early stationary phase. Up to 5% of freshly assimilated 13 C and 15 N was detected in attached bacteria within 12 h and supported bacterial C- and N-growth rates up to 0.026 h -1 . During the mid-stationary phase, diatom chain-length decreased and 13 C and 15 N-nitrate assimilation was significantly higher in solitary cells as compared to that in chain cells. During the late stationary phase, nitrate assimilation ceased and ammonium assimilation balanced C fixation. At this stage, we observed highly active cells neighboring inactive cells within the same chain. In N-limited regimes, bacterial remineralization of N and the short diffusion distance between neighbors in chains may support surviving cells. This combination of "microbial gardening" and nutrient transfer within diatom chains represents a strategy which challenges current paradigms of nutrient fluxes in plankton communities.