Benjamin S. Twining

Nanominerals, Mineral Nanoparticles, and Earth Systems

Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.

Quantitative 3D elemental microtomography of Cyclotella meneghiniana at 400-nm resolution

X-ray fluorescence tomography promises to map elemental distributions in unstained and unfixed biological specimens in three dimensions at high resolution and sensitivity, offering unparalleled insight in medical, biological, and environmental sciences. X-ray fluorescence tomography of biological specimens has been viewed as impractical—and perhaps even impossible for routine application—due to the large time required for scanning tomography and significant radiation dose delivered to the specimen during the imaging process. Here, we demonstrate submicron resolution X-ray fluorescence tomography of a whole unstained biological specimen, quantifying three-dimensional distributions of the elements Si, P, S, Cl, K, Ca, Mn, Fe, Cu, and Zn in the freshwater diatom Cyclotella meneghiniana with 400-nm resolution, improving the spatial resolution by over an order of magnitude. The resulting maps faithfully reproduce cellular structure revealing unexpected patterns that may elucidate the role of metals in diatom biology and of diatoms in global element cycles. With anticipated improvements in data acquisition and detector sensitivity, such measurements could become routine in the near future.

Microbial control of diatom bloom dynamics in the open ocean

[1] Diatom blooms play a central role in supporting foodwebs and sequestering biogenic carbon to depth. Oceanic conditions set bloom initiation, whereas both environmental and ecological factors determine bloom magnitude and longevity. Our study reveals another fundamental determinant of bloom dynamics. A diatom spring bloom in offshore New Zealand waters was likely terminated by iron limitation, even though diatoms consumed <1/3 of the mixed-layer dissolved iron inventory. Thus, bloom duration and magnitude were primarily set by competition for dissolved iron between microbes and small phytoplankton versus diatoms. Significantly, such a microbial mode of control probably relies both upon out-competing diatoms for iron (i.e., K-strategy), and having high iron requirements (i.e., r-strategy). Such resource competition for iron has implications for carbon biogeochemistry, as, blooming diatoms fixed three-fold more carbon per unit iron than resident non-blooming microbes. Microbial sequestration of iron has major ramifications for determining the biogeochemical imprint of oceanic diatom blooms. Citation: Boyd, P. W., et al. (2012), Microbial control of diatom bloom dynamics in the open ocean, Geophys. Res. Lett., 39, L18601, doi:10.1029/2012GL053448.

The Unique Biogeochemical Signature of the Marine Diazotroph Trichodesmium

The elemental composition of phytoplankton can depart from canonical Redfield values under conditions of nutrient limitation or production (e.g., N fixation). Similarly, the trace metal metallome of phytoplankton may be expected to vary as a function of both ambient nutrient concentrations and the biochemical processes of the cell. Diazotrophs such as the colonial cyanobacteria Trichodesmium are likely to have unique metal signatures due to their cell physiology. We present metal (Fe, V, Zn, Ni, Mo, Mn, Cu, Cd) quotas for Trichodesmium collected from the Sargasso Sea which highlight the unique metallome of this organism. The element concentrations of bulk colonies and trichomes sections were analyzed by ICP-MS and synchrotron x-ray fluorescence, respectively. The cells were characterized by low P contents but enrichment in V, Fe, Mo, Ni, and Zn in comparison to other phytoplankton. Vanadium was the most abundant metal in Trichodesmium, and the V quota was up to fourfold higher than the corresponding Fe quota. The stoichiometry of 600C:101N:1P (mol mol−1) reflects P-limiting conditions. Iron and V were enriched in contiguous cells of 10 and 50% of Trichodesmium trichomes, respectively. The distribution of Ni differed from other elements, with the highest concentration in the transverse walls between attached cells. We hypothesize that the enrichments of V, Fe, Mo, and Ni are linked to the biochemical requirements for N fixation either directly through enrichment in the N-fixing enzyme nitrogenase or indirectly by the expression of enzymes responsible for the removal of reactive oxygen species. Unintentional uptake of V via P pathways may also be occurring. Overall, the cellular content of trace metals and macronutrients differs significantly from the (extended) Redfield ratio. The Trichodesmium metallome is an example of how physiology and environmental conditions can cause significant deviations from the idealized stoichiometry.

Quantification of phosphorus in single cells using synchrotron X-ray fluorescence

Phosphorus abundance was quantified in individual phytoplankton cells by synchrotron X-ray fluorescence and compared with bulk spectrophotometric measurements to confirm accuracy of quantification. Figures of merit for P quantification on three different types of transmission electron microscopy grids are compared to assess possible interferences.

Exploring ocean biogeochemistry by single-cell microprobe analysis of protist elemental composition.

ABSTRACT. The biogeochemical cycles of many elements in the ocean are linked by their simultaneous incorporation into protists. In order to understand these elemental interactions and their implications for global biogeochemical cycles, accurate measures of cellular element stoichiometries are needed. Bulk analysis of size‐fractionated particulate material obscures the unique biogeochemical roles of different functional groups such as diatoms, calcifying protists, and diazotrophs. Elemental analysis of individual protist cells can be performed using electron, proton, and synchrotron X‐ray microprobes. Here we review the capabilities and limitations of each approach and the application of these advanced techniques to cells collected from natural communities. Particular attention is paid to recent studies of plankton biogeochemistry in low‐iron waters of the Southern Ocean. Single‐cell analyses have revealed significant inter‐taxa differences in phosphorus, iron, and nickel quotas. Differences in the response of autotrophs and heterotrophs to iron fertilization were also observed. Two‐dimensional sub‐cellular mapping indicates the importance of iron to photosynthetic machinery and of zinc to nuclear organelles. Observed changes in diatom silicification and cytoplasm content following iron fertilization modify our understanding of the relationship between iron availability and silicification. These examples demonstrate the advantages of studying ocean biogeochemistry at the level of individual cells.

Factors regulating the Great Calcite Belt in the Southern Ocean and its biogeochemical significance

The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the subtropical, Agulhas, and Subantarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO2 (pCO2) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO2 source. There was no calcium carbonate (CaCO3) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234Th-based flux estimates, and remotely sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.

LOCALIZATION OF IRON WITHIN CENTRIC DIATOMS OF THE GENUS THALASSIOSIRA1

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X‐ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low‐Fe cells. Cellular Fe quotas of this strain increased 2.9‐fold, the spatial extent of the features increased 4.6‐fold, and the Fe content of the features increased 14‐fold under high‐Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate‐bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5‐fold in T. pseudonana grown at high Fe, but storage features contained only 2‐fold more Fe and did not increase in size compared to low‐Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures.

Why are biotic iron pools uniform across high‐ and low‐iron pelagic ecosystems?

Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here we compare mixed-layer biotic iron inventories from a low-iron (~0.06 nmol L−1) subantarctic (FeCycle study) and a seasonally high-iron (~0.6 nmol L−1) subtropical (FeCycle II study) site. Both studies were quasi-Lagrangian, and had multi-day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100 ± 30 pmol L−1) for low- and high-iron waters, despite a tenfold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80 ± 24 pmol L−1, largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed twofold to threefold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site-specific differences in iron recycling efficiencies (which provide 20–50% and 90% of total iron supply in high- and low-iron waters, respectively) help offset the differences in new iron inputs between low- and high-iron sites. Future parameterization of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.

Microscale patchiness leads to large and important intraspecific internal nutrient heterogeneity in phytoplankton

Phytoplankton stoichiometry or nutrient content has been shown to vary in a number of dimensions (species, condition, time, space), but the heterogeneity within a species at a given time and location, and the underlying mechanisms and importance have not been explored. There are a number of mechanisms that can create intraspecific heterogeneity, and theory suggests it can affect the population growth rate. We studied heterogeneity in P content of the freshwater diatom Cyclotella meneghiniana in the Charles River in Boston. Single-cell observations using synchrotron-based X-ray fluorescence show that the nutrient status varies from P-starved to P-replete. We simulate individual cells using an agent-based model that accounts for a number of mechanisms that can create heterogeneity, including surface area–based uptake, mortality differentiation, stochastic biological variability in states and behavior, macroscale mixing, and microscale nutrient patch encounter. By performing a number of simulations with various mechanisms turned on/off and comparing to data, we conclude that the heterogeneity is mostly due to microscale patchiness (85%). We explore the importance of accounting for heterogeneity in models by performing a simulation with the growth rate based on the population-average internal nutrient, as is done in conventional population-level models. This shows that ignoring heterogeneity increases the population growth rate by a factor of 1.47. To account for different heterogeneity in the laboratory and field, population-level ecosystem models should reduce maximum growth rates. The magnitude of this correction depends on local conditions, and in our case, it is a factor of 0.72.