2017 SCOPE Particle Enrichment Rates

2017 SCOPE Particle Enrichment Rates Samples were collected during the HOT 286, HOT 291, HOT 292, and KOK 1703 cruises in the North Pacific Ocean. Rates of heterotrophic bacterial production were estimated based on the incorporation of 3H-leucine into protein. These rate measurements were conducted on experiments where whole seawater was amended with sinking particles. Particles were collected using a large diameter (1 m), surface-tethered, 50 μm nylon mesh net trap. The net trap was hung at 175 m for 24 h. Collected particles were concentrated in the net's cod-end and subsampled using a Folsom plankton splitter; these sample splits were subsequently screened through a 335-μm mesh to remove large zooplankton. The screened splits were then further divided into ten 100 mL aliquots using a McLane Wet Sample Divider and triplicate splits were pooled. 300 mL of the pooled splits were added to triplicate 10-liter carboys containing seawater collected from the same depth as the net trap deployment (175 m). Unamended seawater controls (no particle amendments) consisted of triplicate 10-liter carboys filled with seawater collected from 175 m from the same cast as the seawater used for the particle-amended treatments. Rates of bacterial production from the particle-amended experiments were estimated based on the incorporation of 3H-leucine into protein into filter size fractionated plankton samples. From each carboy (triplicate particle-amended and triplicate controls), six individual 40 mL polycarbonate centrifuge tubes were filled and amended with 20 nM 3H-leucine. Three of these 40 mL incubation tubes from each carboy served as time zero blanks and were immediately parallel filtered onto 25 mm diameter polycarbonate filters of varying pore sizes (0.2 μm, 2 μm, and 20 μm, respectively) and subsequently processed as described below. The remaining triplicate samples from each carboy were incubated in the dark at in situ temperatures for 4 h. Incubations were terminated by parallel filtration (one 40 mL tube per filter per carboy) onto the same filter pore sizes used for the time zero blanks. Filters were rinsed three times with 5 mL of cold 5% trichloroacetic acid, followed by three rinses (5 mL each) with cold 80% ethanol. Filters were removed from the vacuum filtration manifold and stored frozen in glass scintillation vials. In the shore-based laboratory, 10 mL of scintillation cocktail was added to each scintillation vial containing the filters and radioactivity on each filter was measured by liquid scintillation counting. Dark C-fixation was estimated based on 14C-bicarbonate assimilation into filter size fractionated plankton biomass. Each of the particle-amended and control carboys were subsampled into six 40 mL polycarbonate centrifuge tubes which were amended with 14C-bicarbonate. Three tubes from each carboy were parallel filtered onto 0.2 μm, 2 μm, and 20 μm, respectively. These samples served as time zero blanks. The remaining triplicate tubes were incubated for 24 h at in situ temperatures in the dark. At the end of the incubation period, a 25 μL subsample of the 14C-amended seawater was removed from each tube and added to a 20 mL scintillation vial containing 500 μL ß-phenylethylamine; these samples enabled quantification of the total amount of radioactivity added to each sample. The remaining sample volume was immediately parallel filtered onto the same filter types previously described (one tube per filter per carboy). Filters were removed from the filtration manifold and stored frozen in 20 mL glass scintillation vials, acidified with 1 mL of 2 mol L−1 hydrochloric acid and allowed to vent, uncapped, for 24 h in a fume hood. After acidification, 10 mL of scintillation cocktail was added and radioactivity was quantified by liquid scintillation counting. Rates of dark C-fixation were calculated, following subtraction of the time zero blanks, using HOT program measurements of dissolved inorganic C concentrations at 175 m in March and April of 2017 (2060 μmol C L−1). The timestamp is in UTC.