Contrasting drivers of bacterial metabolism in the euphotic and mesopelagic zones of tropical oligotrophic oceans

Two cruises were conducted: the first in the Western Pacific (WP) aboard the R/V Kexue from 2 December 2021 to 3 January 2022, and the second in the South China Sea (SCS) aboard the R/V Tan Kah Kee from 28 November to 23 December 2022. A total of 221 samples were collected from 26 sites, spanning latitudes from 8°N to 22°N and longitudes from 111.5°E to 132.5°E, at discrete water depths ranging from the surface (5 m) to 1000 m in the mesopelagic zone. The data includes the bottom depth, depth, temperature, salinity, nutrient concentrations (nitrate plus nitrite: NOx, phosphate: PO₄³⁻, silicate: Si(OH)₄), chlorophyll a (Chl-a), bacterial abundance, bacterial production (BP), bacterial respiration (BR), bacterial carbon demand (BCD), net primary production (NPP), and dissolved organic carbon (DOC). Seawater temperature, and salinity were measured using a conductivity‐temperature‐depth (CTD) probe (SBE 911, Sea‐Bird Electronics) attached to a rosette sampler. The nutrient concentrations analyzed using a nutrient analyzer (QUATTRO, Seal, Germany) (Du et al., 2013). The Chl-a was measured by high‐performance liquid chromatography (HPLC) (Xiao et al., 2018a; Xiao et al., 2018b). Bacterial abundance was later assessed in the laboratory using a flow cytometer (Accuri C6) (Jiao et al., 2014). Rates of BP were determined through the uptake of ³H-leucine following the JGOFS protocol (Knap et al., 1996). BR was estimated by measuring the reduction of the tetrazolium salt INT (2-(p-iodophenyl)-3(p-nitrophenyl)-5-phenyltetrazolium chloride) to INT-formazan (INT-F) by Electron Transport System (ETS) dehydrogenase enzymes (Martínez‐García et al., 2009). BCD was calculated as the sum of BP and BR (García-Martín et al., 2019). Weekly NPP, a metric carbon export potential from the photic layer (Emerson, 2014), was estimated using Vertically Generalized Productivity Model (VGPM) products, available from the Ocean Productivity database (http://orca.science.oregonstate.edu/index.php/). DOC was analyzed according to high-temperature catalytic oxidation using a Shimadzu TOC-VCPH (Li et al., 2018).