Our mechanistic understanding of the processes controlling the ocean's biological pump is limited, in part, by our lack of observational data at appropriate timescales. The "optical sediment trap" (OST) technique utilizes a transmissometer on a quasi-Lagrangian platform to collect sedimenting particles. This method could help fill the observational gap by providing autonomous measurements of particulate carbon (PC) flux in the upper mesopelagic ocean at high spatiotemporal resolution. Here, we used a combination of field measurements and laboratory experiments to test hydrodynamic and zooplankton-swimmer effects on the OST method, and we quantitatively calibrated this method against PC flux measured directly in same-platform, neutrally buoyant sediment traps (NBSTs) during 5 monthly cruises at the Bermuda Atlantic Time-series Study (BATS) site. We found a well-correlated, positive relationship (R 2 =0.66, n=15) between the OST proxy, and the PC flux measured directly using NBSTs. Laboratory tests showed that scattering of light from multiple particles between the source and detector was unlikely to affect OST proxy results. We found that the carbon-specific attenuance of sinking particles was larger than literature values for smaller, suspended particles in the ocean, and consistent with variable carbon: size relationships reported in the literature for sinking particles. We also found evidence for variability in PC flux at high spatiotemporal resolution. Our results are consistent with the literature on particle carbon content and optical properties in the ocean, and support more widespread use of the OST proxy, with proper site-specific and platform-specific calibration, to better understand variability in the ocean biological pump.

The complex interplay of biological and physical mechanisms comprising the ocean's biological carbon pump has not been well characterized to date, due to the difficulty of observing these mechanisms in situ at adequate spatial and temporal resolution. An annual time series is presented of direct measurements of export production and particle properties collected using optical sediment trap‐equipped profiling floats cycling every 1.5–2.5 days. The observations indicate strong variability in particle export and bio‐optical properties, influenced by the spring bloom, mesoscale eddy activity, and the mixed layer pump. Temporal and vertical decoupling of fluxes at depths ranging from 150 to 1,000 m was also observed, and remineralization length scales were more variable than predicted by temperature‐ and oxygen‐based models. Net primary production was computed from float observations using a modification of the Carbon‐based Productivity Model and used to estimate export and export (e‐) ratios, which were compared to predictions of literature export models. Mechanistic models explicitly incorporating ecosystem processes and their depth dependence may perform better at reproducing regional observations collected at high temporal resolution.

Synthesis of asparaginyl-tRNA (Asn-tRNA(Asn)) in bacteria can be formed either by directly ligating Asn to tRNA(Asn) using an asparaginyl-tRNA synthetase (AsnRS) or by synthesizing Asn on the tRNA. In the latter two-step indirect pathway, a non-discriminating aspartyl-tRNA synthetase (ND-AspRS) attaches Asp to tRNA(Asn) and the amidotransferase GatCAB transamidates the Asp to Asn on the tRNA. GatCAB can be similarly used for Gln-tRNA(Gln) formation. Most bacteria are predicted to use only one route for Asn-tRNA(Asn) formation. Given that Bacillus halodurans and Bacillus subtilis encode AsnRS for Asn-tRNA(Asn) formation and Asn synthetases to synthesize Asn and GatCAB for Gln-tRNA(Gln) synthesis, their AspRS enzymes were thought to be specific for tRNA(Asp). However, we demonstrate that the AspRSs are non-discriminating and can be used with GatCAB to synthesize Asn. The results explain why B. subtilis with its Asn synthetase genes knocked out is still an Asn prototroph. Our phylogenetic analysis suggests that this may be common among Firmicutes and 30% of all bacteria. In addition, the phylogeny revealed that discrimination toward tRNA(Asp) by AspRS has evolved independently multiple times. The retention of the indirect pathway in B. subtilis and B. halodurans likely reflects the ancient link between Asn biosynthesis and its use in translation that enabled Asn to be added to the genetic code.

New production (NP) and net community production (NCP) measurements are often used as estimates of carbon export potential from the mixed layer of the ocean, an important process in the regulation of global climate. Diverse methods can be used to measure NP and NCP, from research vessels, autonomous platforms, and remote sensing, each with its own set of benefits and uncertainties. The various methods are rarely applied simultaneously in a single location, limiting our ability for direct comparisons of the resulting measurements. In this study, we evaluated NP and NCP from thirteen independent datasets collected via in situ, in vitro, and satellite-based methods near Ocean Station Papa during the 2018 Northeast Pacific field campaign of the NASA project EXport Processes in the Ocean from RemoTe Sensing (EXPORTS). Altogether, the datasets indicate that carbon export potential was relatively low (median daily averages between −5.1 and 12.6 mmol C m−2 d−1), with most measurements indicating slight net autotrophy in the region. This result is consistent with NCP estimates based on satellite measurements of sea surface temperature and chlorophyll a. We explored possible causes of discrepancies among methods, including differences in assumptions about stoichiometry, vertical integration, total volume sampled, and the spatiotemporal extent considered. Results of a generalized additive mixed model indicate that the spatial variation across platforms can explain much of the difference among methods. Once spatial variation and temporal autocorrelation are considered, a variety of methods can provide consistent estimates of NP and NCP, leveraging the strengths of each approach.