In the Margins: Reconsidering the Range and Contribution of Diazotrophs in Nearshore Environments

Dinitrogen (N2) fixation enables primary production and, consequently, carbon dioxide drawdown in nitrogen (N) limited marine systems, exerting a powerful influence over the coupled carbon and N cycles. Our understanding of the environmental factors regulating its distribution and magnitude are largely based on the range and sensitivity of one genus, Trichodesmium. However, recent work suggests that the niche preferences of distinct diazotrophic (N2 fixing) clades differ due to their metabolic and ecological diversity, hampering efforts to close the N budget and model N2 fixation accurately. Here, I explore the range of N2 fixation across physico-chemical gradients (e.g., light, nutrients, oxygen) in nearshore environments of significance in global biogeochemical cycling: the major pelagic oxygen deficient zones (ODZs) in the Eastern Tropical South (ETSP) and North (ETNP) Pacific Ocean, and the broad continental shelf of the Western North Atlantic Ocean (WNA). The ODZs are hypothesized to play an important role in N cycle homeostasis by generating conditions thought to promote diazotrophy; recent work suggests that broad continental shelf environments may contribute substantially to new reactive N inputs globally. N2 fixation rates were measured using a robust 15N tracer method that accounts for the slow dissolution of N2 gas. To explore niche partitioning and better characterize spatial heterogeneity on the WNA shelf, I built an empirical model of N2 fixation and investigated diazotroph identity using amplicon sequencing and qPCR. In the ETSP, N2 fixation was only detected in a subset of low-oxygen samples. N2 fixation within the ETNP ODZ was patchy and driven by organic carbon availability; however, significant rates were observed at coastal stations near the Gulf of California. Frontal mixing on the WNA shelf resulted in exceptionally high rates of N2 fixation, associated with high UCYN-A activity. My findings suggest that (1) diazotrophy is more energetically favorable (relative to dissolved inorganic N) in low-oxygen waters but may be carbon-limited, and (2) continental inputs and dynamic conditions at coastal margins can favor significant N inputs via diazotrophy

On Two Models of the Light Pulse Delay in a Saturable Absorber

A comparative analysis of two approaches to description of the light modulation pulse delay in a saturable absorber is presented. According to the simplest model, the delay of the optical pulse is a result of distortion of its shape due to absorption self-modulation in the nonlinear medium. The second model of the effect, proposed at the beginning of our century, connects the pulse delay with the so-called "slow light" resulting from the group velocity reduction under conditions of the coherent population oscillations. It is shown that all the known experimental data on the light pulse delay in saturable absorbers can be comprehensively described in the framework of the simplest model of saturable absorber and do not require invoking the effect of coherent population oscillations with spectral hole-burning and anomalous modifications of the light group velocity. It is concluded that the effect of group velocity reduction under conditions of coherent population oscillations has not received so far any experimental confirmation, and the assertions about real observation of the "slow light" based on this mechanism are groundless.Comment: Regretfully, the journal version of the paper (in Optics and Spectroscopy) appeared to be strongly corrupted due to ignorant editing. In particular, "coherent population oscillations" (CPO) was replaced by "population coherent oscillations" (PCO), "bleaching" - by "clearing", and "bleachable absorber " - by "clearable absorber". Here we present original version of the pape

Toward Resolving Disparate Accounts of the Extent and Magnitude of Nitrogen Fixation in the Eastern Tropical South Pacific Oxygen Deficient Zone

Examination of dinitrogen (N2) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N2 fixation rates were measured using a robust 15N tracer method (bubble removal) that accounts for the slow dissolution of N2 gas and calculated using a conservative approach. N2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (\u3c 20 μmol O2 kg−1) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non-cyanobacterial diazotrophs. Nevertheless, this study indicates that N2 fixation is neither widespread nor quantitatively important throughout this region

Analysis of the Ceramic Sherds from Area C at the Ware Acres Site (41GG31), Gregg County, Texas

The Ware Acres site (41GG31) was discovered by Buddy Calvin Jones in 1951 on an alluvial terrace of Grace Creek, a southern-flowing tributary to the Sabine River in the southwestern part of the city of Longview, Texas. The site is best known for Jones’ discovery and excavation of an eighteenthcentury Caddo burial with an abundance of European trade goods (Jones 1968:21-24). However, Jones also investigated other parts of the site, which contained extensive Caddo habitation deposits, especially one area at the southern part of the site that had Late Caddo Titus phase midden deposits and remnants of house structures. A large assemblage of ceramic sherds were collected from this area, and although Jones (1968:17) indicated that “a complete analysis of them will be given in a later report,” this was never done. This article presents an analysis of these ceramic sherds, primarily to put the ceramic assemblages from from this important East Texas site on record. The stylistic attributes and known ceramic types in the Ware Acres assemblage are also compared to the ceramic assemblage from the Pine Tree Mound site (Fields and Gadus 2012), as the Ware Acres site may be a component of the Pine Tree Mound Titus phase community found in the middle reaches of the Sabine River basin