Daniel C. Ohnemus

How well can we quantify dust deposition to the ocean

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2–5, suggesting that the model may overestimate dust deposition in our study area. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Patterns and Regulation of Silicon Accumulation in Synechococcus spp

Six clones of the marine cyanobacterium Synechococcus, representing four major clades, were all found to contain significant amounts of silicon in culture. Growth rate was unaffected by silicic acid, Si(OH)4, concentration between 1 and 120 μM suggesting that Synechococcus lacks an obligate need for silicon (Si). Strains contained two major pools of Si: an aqueous soluble and an aqueous insoluble pool. Soluble pool sizes correspond to estimated intracellular dissolved Si concentrations of 2–24 mM, which would be thermodynamically unstable implying the binding of intracellular soluble Si to organic ligands. The Si content of all clones was inversely related to growth rate and increased with higher [Si(OH)4] in the growth medium. Accumulation rates showed a unique bilinear response to increasing [Si(OH)4] from 1 to 500 μM with the rate of Si acquisition increasing abruptly between 80 and 100 μM Si(OH)4. Although these linear responses imply some form of diffusion‐mediated transport, Si uptake rates at low Si (~1 μM Si) were inhibited by orthophosphate, suggesting a role of phosphate transporters in Si acquisition. Theoretical calculations imply that observed Si acquisition rates are too rapid to be supported by lipid‐solubility diffusion of Si through the plasmalemma; however, facilitated diffusion involving membrane protein channels may suffice. The data are used to construct a working model of the mechanisms governing the Si content and rate of Si acquisition in Synechococcus.

Picoplankton contribution to biogenic silica stocks and production rates in the Sargasso Sea

Picocyanobacteria in the Sargasso Sea accumulate significant amounts of Si, a finding which questions how we interpret previous regional measurements of biogenic silica (bSi) production and the role of diatoms in the open ocean. The picoplankton (<3-µm cells) contributed a measurable, and at times significant, proportion of both the total bSi standing stock and its rate of production. The 100-m integrated bSi stock and bSi production rate in the <3 µm size fraction averaged 14% and 16%, respectively, of the total. At some stations, specific rates of bSi production in the <3-µm cells were up to three-fold higher than for larger cells. But among all stations and depths, the two size fractions had statistically indistinguishable specific-production rates (~0.35 d-1). The estimated contributions of Synechococcus alone to the 100-m integrated bSi stock and bSi production in cells <3 µm were 15% and 55%, respectively, suggesting that half of bSi production in this small size fraction could be sustained by Synechococcus, but a majority of the bSi was not associated with living Synechococcus. Our results suggest picoplankton have a small but persistent regional contribution to bSi stocks, which is masked by a dynamic bSi pool driven by larger cells. While a significant fraction of bSi production is attributable to picoplankton, their contributions are likely to have been included in previous analyses, making prior regional budgets still relevant. However, our understanding of the factors controlling regional bSi production and our interpretations of particulate matter elemental ratios (e.g. Si:C) may require revision.

Instrumentation for fluorescence-based fiber optic biosensors.

This chapter summarizes the construction principles, operation, and calibration of (single-fiber) fluorescence-based fiber optic sensors. These sensors transduce recognition of a chemical analyte by a transducer such as a protein molecule as a change in fluorescence wavelength or lifetime that can be measured remotely through a length of fiber optic. Examples are given of determination of metal ions in aqueous solution by fluorescence ratio and lifetime. Included are descriptions of instruments, alignment procedures, identification of noise sources, use of calibration standards, factors in the use of long fibers for sensing, issues in field and shipboard operation, and probe preparation.

Global Spatial and Temporal Variation of Cd:P in Euphotic Zone Particulates

Author(s): Bourne, HL; Bishop, JKB; Lam, PJ; Ohnemus, DC | Abstract: ©2018. American Geophysical Union. All Rights Reserved. Concentrations of Cd and P were determined in particle samples collected using the multiple unit large volume in situ filtration system (MULVFS) from 50 profiles at 34 different locations throughout the Atlantic, Pacific, and Southern Oceans since 1991. Consistent methodology has been used. This data set of Cd:P in size fractionated particles gives insight into the processes that lead to differences in regional Cd:P particle values as well as how the formation and remineralization of these particles lead to dissolved deep water ratios that increase from the North Atlantic to the North Pacific. With large spatial and temporal variation, this data set allows us to study the effects of an El Nino, upwelling, large-scale in situ Fe fertilization, low-oxygen conditions, and seasonal variation on the Cd:P in particles. Overall, Cd:P tends to be higher (~1–2 mmol/mol) in particles gathered in biologically dynamic waters and is much lower (typically ~0.1 mmol/mol) in oligotrophic regions. Using multiple linear regression analysis, we investigate how euphotic zone parameters important to photosynthesis including nitrate, phosphate, silicate, temperature, and euphotic zone depth affect the Cd:P ratio in particles. Using the results of the analysis, we create global seasonal maps of predicted particulate Cd:P distributions. We find that three factors—local dissolved nitrate, silicate concentrations, and euphotic zone depth—can predict 59% of the variation in particulate Cd:P. We verified our projections using in situ filtration samples collected during GEOTRACES expeditions GA03 (North Atlantic) and GP16 (South Pacific).