Nitrogen cycling and bacterial production

I. Bacterial abundance and production in Nueces and Guadalupe Estuaries, Texas -- II. Ammonium regeneration and utilization in Nueces Estuary, Texas -- III. Comparison of benthic ammonium flux with ammonium regeneration in the water column of Nueces and Guadalupe Estuaries, Texas -- IV. Denitrification in Nueces and Guadalupe Estuaries, Texas -- V. Ammonium regeneration and utilization in Guadalupe Estuary, Texas.Marine Scienc

Decomposition of Senescent Blades of the Seagrass \u3cem\u3eHalodule wrightii\u3c/em\u3e in a Subtropical Lagoon

Senescent blades from the seagrass Halodule wrightii Aschers were suspended in the water column of Laguna Madre (Texas, USA) for a period of 419 d, representing the longest seagrass decomposition study to date. The initial stage of decomposition was characterized by a rapid loss of organic matter (36 % in 24 d) attributed to leaching. A total of 76 % of the organic matter from seagrass tissues was lost by the end of the decomposition period. Of the major bulk constituents measured, neutral sugars were most abundant and accounted for 23 % of the ash-free dry wt of the initial senescent material. A complete and early loss of the cyclitol, myo-inositol, a reduction in glucose yields, and a relative enrichment in mannose were the most dynamic features of the neutral sugar fraction. An overall stability series: mannose \u3e fucose \u3e arabinose ≈ rhamnose ≈ galactose ≈ xylose \u3e glucose, reflects patterns of selective degradation of the polymers from which these sugars are derived. Soluble and ester-bound phenolic acids were lost from tissues at relatively high rates. Susceptibility of individual phenolic acids to removal from bulk tissue upon treatment with weak base correlated well to initial losses in the field. Early changes in total CuO oxidation phenol yields from H. wrightii were caused primarily by the relatively rapid loss of soluble and ester-bound phenolic acids. Therefore, we took a new approach to quantify lignin-derived phenols by subtracting soluble and ester-bound phenols from total CuO oxidation yields. When these losses were taken into account, lignin-derived phenol yields were similar from freshly senescent and highly degraded detritus. Cutin, although initially selectively preserved relative to bulk tissue, was also found at near initial yields in the highly degraded detritus. Overall, the distribution of polymeric constituents (cellulose, hemicellulose, lignin and cutin) was similar in freshly senescent and highly degraded tissues suggesting that the compounds which comprise the ultrastructure of H. wrightii are degraded at similar rates. Photobleached H. wrightii blades were characterized by a much higher neutral sugar content, slmilar levels of cutin acids and a nearly complete absence of phenols compared to freshly senescent tissue. Photobleaching appears to be an important degradative mechanism which yields a polysaccharide-rich detritus that is devoid of the lignin signature characteristic of vascular plant tissues

Sources, Distributions, and Dynamics of Dissolved Organic Matter in the Canada and Makarov Basins

A comprehensive survey of dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) was conducted in the Canada and Makarov Basins and adjacent seas during 2010–2012 to investigate the dynamics of dissolved organic matter (DOM) in the Arctic Ocean. Sources and distributions of DOM in polar surface waters were very heterogeneous and closely linked to hydrological conditions. Canada Basin surface waters had relatively low DOC concentrations (69 ± 6 μmol L−1), CDOM absorption (a325: 0.32 ± 0.07 m−1) and CDOM-derived lignin phenols (3 ± 0.4 nmol L−1), and high spectral slope values (S275–295: 31.7 ± 2.3 μm−1), indicating minor terrigenous inputs and evidence of photochemical alteration in the Beaufort Gyre. By contrast, surface waters of the Makarov Basin had elevated DOC (108 ± 9 μmol L−1) and lignin phenol concentrations (15 ± 3 nmol L−1), high a325 values (1.36 ± 0.18 m−1), and low S275–295 values (22.8 ± 0.8 μm−1), indicating pronounced Siberian river inputs associated with the Transpolar Drift and minor photochemical alteration. Observations near the Mendeleev Plain suggested limited interactions of the Transpolar Drift with Canada Basin waters, a scenario favoring export of Arctic DOM to the North Atlantic. The influence of sea-ice melt on DOM was region-dependent, resulting in an increase (Beaufort Sea), a decrease (Bering-Chukchi Seas), and negligible change (deep basins) in surface DOC concentrations and a325 values. Halocline structures differed between basins, but the Canada Basin upper halocline and Makarov Basin halocline were comparable in their average DOC (65–70 μmol L−1) and lignin phenol concentrations (3–4 nmol L−1) and S275–295 values (22.9–23.7 μm−1). Deep-water DOC concentrations decreased by 6–8 μmol L−1 with increasing depth, water mass age, nutrient concentrations, and apparent oxygen utilization. Maximal estimates of DOC degradation rates (0.036–0.039 μmol L−1 yr−1) in the deep Arctic were lower than those in other ocean basins, possibly due to low water temperatures. DOC concentrations in bottom waters (\u3e2500 m; 46 ± 2 μmol L−1) of the Canada and Makarov Basins were slightly lower than those reported for deep waters of the Eurasian Basin and Nordic Seas. Elevated a325 values (by 10–20%) were observed near the seafloor, indicating biological activity in Arctic basin sediments

Molecular Properties Are a Primary Control on the Microbial Utilization of Dissolved Organic Matter in the Ocean

The global ocean sequesters a large amount of reduced carbon in dissolved organic molecules that can persist for centuries to millennia. The persistence of dissolved organic carbon (DOC) in the deep ocean has been attributed to inherently refractory molecules and to low concentrations of molecules, but the relative roles of molecular properties and molecular concentrations remain uncertain. We investigate both of these possibilities using bioassay experiments with unfiltered seawater collected from five depths (50–1500 m) at the Bermuda Atlantic TimeSeries Study site. The microbial utilization of compositionally distinct forms of seawater DOC at in situ and elevated concentrations was determined. Microbial utilization of in situ organic carbon ranged from 6% to 7% in surface waters to 0% in deep water after 180 d. Additions of surface plankton-derived DOC (~18 μmol L−1 ), which was enriched in amino acids and carbohydrates, revealed substantial (50–75%) removal of the added DOC at all depths within 7 d. In sharp contrast, additions of C-18 isolated deep-sea DOC (~20 μmol L−1 ) showed insignificant or minimal utilization at all depths after 7 or 180 d, even when primed with labile substrates. These experiments demonstrate microbial communities from varying depths and environments in the ocean could rapidly utilize elevated concentrations of plankton-derived DOC, whereas these same microbes failed to utilize elevated concentrations of C-18 DOC. These results indicate molecular properties are the primary control on the microbial utilization of DOC in the ocean. Our findings imply a dynamic DOC reservoir with a flexible capacity for carbon sequestration in the global ocean

Biological hot spots and the accumulation of marine dissolved organic matter in a highly productive ocean margin

Concentrations of dissolved organic carbon (DOC) and major biochemicals (amino acids and carbohydrates) were measured during five cruises (2009–2010) to the Louisiana margin in the northern Gulf of Mexico. Concentrations of amino acids and carbohydrates were elevated at mid-salinities and were indicative of plankton production of dissolved organic matter (DOM) in surface waters. Hot spots of two compositionally distinct types of labile DOM were identified based on the relative abundances of amino acids and carbohydrates. Amino acid-rich hot spots occurred sporadically in regions of high phytoplankton biomass and were mostly observed between dusk and dawn, reflecting a grazing source. In contrast, carbohydrate-rich hot spots were more widespread and were often found in nutrient-poor waters, indicating the production of carbon-rich DOM associated with nutrient limitation. Major biochemical indicators and bioassay experiments indicated labile DOM comprised a relatively small fraction of the DOC. Most DOM was degraded and had a semi-labile nature. Substantial accumulations of marine (plankton-derived) DOC were observed in surface waters, particularly at mid-salinities during the summer. Microbial alteration of marine DOC and nutrient limitation of microbial utilization of carbon-rich DOM appeared largely responsible for the accumulation of DOC. The reservoir of accumulated marine DOC in the shelf surface mixed layer ranged from 0.11 Tg C to 0.23 Tg C, with the lowest and highest values occurring during winter and summer. Substantial cross-shelf export of semi-labile marine DOM occurred during the summer and provided a major carbon and energy subsidy to microbial food webs in offshore waters.We are grateful to Steven E. Lohrenz and Wei-Jun Cai for providing the opportunity to participate in the GulfCarbon cruises. We appreciate the sampling assistance by Leanne Powers and the crews of the R/V Cape Hatteras and the R/V Hugh Sharp. We thank the anonymous reviewers for their comments and suggestions. This research was funded by a grant from the U.S. National Science Foundation (0850653 to RB) and by the 111 Project of China (B13030 to SKL). (0850653 - U.S. National Science Foundation; B13030 - 111 Project of China

Gatekeeping after Gilbert: How Lawyers Should Address the Court\u27s New Emphasis

In the world of modern trials, expert witnesses are the coin of the realm. Lawyers know that most of the time, experts are case-breakers. Their demeanor, knowledge, and presentation ability are key qualities. Accordingly, their persuasive effect on modern lay jurors makes it incumbent on judges to ensure that an expert\u27s opinions are appropriately directed. That means not allowing an economist to testify about the medical dynamics of bone disease, for example