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

A Mn-54 Radiotracer Study of Mn Isotope Solid–Liquid Exchange during Reductive Transformation of Vernadite (δ-MnO₂) by Aqueous Mn(II)

We employed Mn-54 radiotracers to characterize the extent and dynamics of Mn atom exchange between aqueous Mn­(II) and vernadite (δ-Mn­(IV)­O₂) at pH 7.5 under anoxic conditions. Exchange of Mn atoms between the solid and liquid phase is rapid, reaching dynamic equilibrium in 2–4 days. We propose that during the initial stages of reaction, Mn atom exchange occurs through consecutive comproportionation-disproportionation reactions where interfacial electron transfer from adsorbed Mn­(II) to lattice Mn­(IV) generates labile Mn­(III) cations that rapidly disproportionate to reform aqueous Mn­(II) and solid-phase Mn­(IV). Following nucleation of Mn­(III)­OOH phases, additional exchange likely occurs through electron transfer from aqueous Mn­(II) to solid-phase Mn­(III). Our results provide evidence for the fast and extensive production of transient Mn­(III) species at the vernadite surface upon contact of this substrate with dissolved Mn­(II). We further show that HEPES buffer is a reductant of lattice Mn­(IV) in the vernadite structure in our experiments. The methods and results presented here introduce application of Mn-54 tracers as a facile tool to further investigate the formation kinetics of labile Mn­(III) surface species and their impacts on Mn-oxide structure and reactivity over a range of environmentally relevant geochemical conditions

Interbasin isotopic correspondence between upper‐ocean bulk DON and subsurface nitrate and its implications for marine nitrogen cycling

Measurements to date have shown that both bulk and high molecular weight marine dissolved organic nitrogen (DON) have a 15N/14N that is substantially higher than the 15N/14N of suspended particulate organic nitrogen (PNsusp) found in the same surface waters (with δ15N of ∼4 to 5‰ and ∼−1 to 1‰, respectively). Moreover, the concentration and 15N/14N of DON are much less dynamic than those of PNsusp. These observations raise questions regarding the role of DON in the upper ocean nitrogen (N) cycle. In this study, the concentration and 15N/14N of nitrate and DON was measured in the upper 300 m of the oligotrophic North Atlantic and North Pacific Oceans. Comparing these two regions, the average DON concentration in the upper 100 m is similar, between 4.5 and 5.0 μM, but the average δ15N of DON is significantly different, 3.9‰ versus air in the North Atlantic and 4.7‰ in the North Pacific. This difference parallels a similar isotopic difference between shallow nitrate in these two regions; at 200 m in the North Atlantic, the δ15N of nitrate is 2.6‰, while it is 4.0‰ in the North Pacific. This isotopic correlation between surface DON and subsurface nitrate indicates that DON is actively participating in the upper ocean N cycle of each region. We describe a conceptual model that explains the elevation of the 15N/14N of DON relative to surface ocean PNsusp as well as the interbasin difference in the 15N/14N of DON. In this model, DON is produced from PNsusp without isotopic fractionation but DON is removed by fractionating processes. The ammonium and simple organic N compounds released by DON decomposition reactions are reassimilated by algae into the PNsusp pool, as an integral part of the ammonium‐centered cycle that lowers the 15N/14N of PNsusp relative to the nitrate supply from below. This interpretation is consistent with the understanding of the chemical controls on isotope fractionation and is analogous to the previously posed explanation for the 15N/14N elevation of herbivorous zooplankton. In addition, it explains a lack of correlation between in situ N2 fixation rates and DON concentration and 15N/14N on short time scales

Mesoscale variability of the summer bloom over the northern Ross Sea shelf: A tale of two banks

Multi-year satellite records indicate an asymmetric spatial pattern in the summer bloom in the Northern Ross Sea, with the largest blooms over the shallows of Pennell Bank compared to Mawson Bank. In 2010–2011, high-resolution spatiotemporal in situ sampling focused on these two banks to better understand factors contributing to this pattern. Dissolved and particulate Fe profiles suggested similar surface water depletion of dissolved Fe on both banks. The surface sediments and velocity observations indicate a more energetic water column over Mawson Bank. Consequently, the surface mixed layer over Pennell Bank was more homogeneous and shallower. Over Mawson Bank we observed a thicker more homogeneous bottom boundary layer resulting from stronger tidal and sub-tidal currents. These stronger currents scour the seafloor resulting in sediments less likely to release additional sedimentary iron. Estimates of the quantum yield of photosynthesis and the initial slope of the photosynthesis-irradiance response were lower over Mawson Bank, indicating higher iron stress over Mawson Bank. Overall, the apparent additional sedimentary source of iron to, and longer surface residence time over Pennell Bank, as well as the reduced fluxes from the more isolated bottom mixed layer over Mawson Bank, sustain the observed asymmetric pattern across both banks

Transcriptional Orchestration of the Global Cellular Response of a Model Pennate Diatom to Diel Light Cycling under Iron Limitation.

Environmental fluctuations affect distribution, growth and abundance of diatoms in nature, with iron (Fe) availability playing a central role. Studies on the response of diatoms to low Fe have either utilized continuous (24 hr) illumination or sampled a single time of day, missing any temporal dynamics. We profiled the physiology, metabolite composition, and global transcripts of the pennate diatom Phaeodactylum tricornutum during steady-state growth at low, intermediate, and high levels of dissolved Fe over light:dark cycles, to better understand fundamental aspects of genetic control of physiological acclimation to growth under Fe-limitation. We greatly expand the catalog of genes involved in the low Fe response, highlighting the importance of intracellular trafficking in Fe-limited diatoms. P. tricornutum exhibited transcriptomic hallmarks of slowed growth leading to prolonged periods of cell division/silica deposition, which could impact biogeochemical carbon sequestration in Fe-limited regions. Light harvesting and ribosome biogenesis transcripts were generally reduced under low Fe while transcript levels for genes putatively involved in the acquisition and recycling of Fe were increased. We also noted shifts in expression towards increased synthesis and catabolism of branched chain amino acids in P. tricornutum grown at low Fe whereas expression of genes involved in central core metabolism were relatively unaffected, indicating that essential cellular function is protected. Beyond the response of P. tricornutum to low Fe, we observed major coordinated shifts in transcript control of primary and intermediate metabolism over light:dark cycles which contribute to a new view of the significance of distinctive diatom pathways, such as mitochondrial glycolysis and the ornithine-urea cycle. This study provides new insight into transcriptional modulation of diatom physiology and metabolism across light:dark cycles in response to Fe availability, providing mechanistic understanding for the ability of diatoms to remain metabolically poised to respond quickly to Fe input and revealing strategies underlying their ecological success