Influence of light on bacterivory of the smallest eukaryotes on cruise AMT20 (JC053)

This dataset consists of 672 individual measurements of ³⁵S methionine uptake of bacterial and protist populations during a pulse-chase labelling experiment collected during the Atlantic Meridional Transect programme cruise 20 (AMT20, JC053) between 14th October and 21st November 2010. Sampling was restricted to the South Atlantic gyre. Pre-dawn seawater samples were collected from 20 m depth in 20 L Niskin bottles attached to a standard conductivity-temperature-depth profiler. Experiments were set up for dark and light measurements in a dark room. Light incubation experiments were placed in a 6-L water tank illuminated by a warm white light-emitting diode (LED) array. Parallel dark incubations were done in a similar water tank but were isolated from light. All experiments were run at ambient temperatures controlled by refrigerated water bath. To 250 mL seawater sample 0.4 nM L-[³⁵S]methionine and 0.5 nM L-[4,5-³H]leucine were added. After 1.5 h incubation, nonradioactive methionine and leucine were added to final concentrations of 0.4 µM and 0.5 µM, respectively, to chase the radioactive amino acids. Samples were incubated for another 1.5 h to stabilize pulses in bacterioplankton cells before taking subsamples (120 mL fixed with 1% (w/v) paraformaldehyde at 3 h and 9 h) for the measurement of protist tracer uptake rates. Subsequently, different phytoplankton populations (i.e. bacterioplankton, small (approx. 2 µm) and large (approx. 3.1 µm) plastidic protists as well as aplastidic protists) were sorted according to light-scattering properties, relative concentration of SYBR Green I stain per particle and chlorophyll content using a FACSort flow cytometer (Becton Dickinson, UK). Radio-assaying of samples was carried out using an ultra-low-level liquid scintillation counter (1220 Quantulus, Wallac).