Imaging eubacteria and archaean cell abundance and nitrification activity on marine snow particles collected from the South Pacific using microscopy and nanoSIMS

The abundance and nitrification activity by marine-snow associated eubacteria and archaea were measured on particles collected in the South Pacific. The particles were first collected from 24 hour floating sediment traps moored at 100 and 200 m and later amended and incubated with 13C-bicarbonate and 15NH4 for 48 hours. Enrichment for ¹³C and ¹⁵N in cells above natural abundance was quantified using a coupled halogenated in situ hybridization assay with nano-meter scale secondary ion mass spectrometry (HISH-SIMS). Approximately 82% ± 10 of the observed hybridized cells were Eubacterial, while 24% ± 5 were positively hybridized with the Archaean probe. There was high variability in ¹³C/¹²C and ¹⁵N/¹⁴N in both bacterial and archaeal cells. Complementary measurements of δ¹⁵N_NO3 at the conclusion of the experiment indicated no detectible nitrification activity associated with the particles. Thin sections of the particles imaged with Transmission Electron Microscopy (TEM) showed that a higher abundance of degrading phytoplanktonic cells, including numerous empty radiolarian and diatom frustules were associated with the deeper moored tap, while the shallow trap collected intact bacterial cells, including small picocyanobacteria. Single cell imaging and observations such as those presented here can provide subtle insights and measurements of cellular activity that are often diluted by bulk approaches and that are directly applicable to modeling N and C cycling.