Return from the depths: examination of diatom resting cysts from the North Atlantic Bloom Experiment

Mass sinking of cells following spring diatom blooms is known to be a major source of carbon flux to the ocean's interior. The timing of flux events is thought to correlate with nutrient depletion of surface waters, inducing physiological stress and robbing silicified diatoms of their ability to maintain buoyancy. Here, we examined sediment trap material collected during the 2008 North Atlantic Bloom Experiment to determine the physiological status of a sinking spring diatom bloom and evaluate its potential to "seed" future blooms. Sediment trap material was collected from 24hr deployments of floating sediment traps set at 300, 600 and 750m. Diatom cells were examined immediately after trap recovery and the assemblage was dominated by resting cysts of the diatom genus Chaetoceros. Resting cysts were strongly fluorescent under excitation at 470-490nm indicating that accessory pigments were intact and that cells were photosynthetically competent. Variable fluorescence measurements of bulk sediment trap material were high indicating the presence of live and potentially active cells. To test the hypothesis that cysts were viable and could re-seed a future bloom, sediment trap material from each depth was inoculated into nutrient-amended seawater and incubated at 9°C and 50 μmol photons m2 s1 on a 12:12 light:dark cycle. Within 38hrs, the heavily silicified valves of Chaetoceros sp. cysts were cast off and replaced by vegetative valves indicating that excystment had occurred. Initial excystment was followed by rapid cell division, with growth rates of Chaetoceros sp. reaching 1 doubling day1. This phenomenon was observed in samples collected on May 15. Sediment trap materials collected from deployments on May 11 and 19 were not dominated by resting cysts. Diatom resting stages, such as cysts, are known to form within 6 hrs and can sink up to 35 times faster than vegetative cells, potentially explaining the brief pulse of cysts we observed and highlighting the ephemeral nature of important C flux events in the open ocean.