Browsing by Subject "oxygen"

The Biological Carbon Pump (BCP) is a natural mechanism in the ocean that exports carbon in the deep ocean and is estimated to transfer between 5 and 12 Pg C from the surface to the deep ocean annually. While the underlying mechanisms of this process – primary producers create organic carbon from CO2 through photosynthesis, some of this organic carbon is recycled in the surface ocean, while some of it is exported to depth via physical or biological processes – have been identified for decades, this process remains difficult to quantify and predict. We estimate the carbon export potential from the surface ocean by estimating net community production (NCP) from continuously measured in situ O2/Ar ratios. In this dissertation, I aimed to assess the coherence of many methods of measuring NCP and determine factors, both physical and biological, that drive changes in NCP. Together, these goals allowed me to offer suggestions to improve modeling efforts to estimate the BCP from autonomous or remote sensing observations. To explore these topics, I used many different methods. In Chapter 2, I showed that measurements of NCP collected from different methods were consistent around Ocean Station Papa in the North Pacific after accounting for spatial heterogeneity. I compared estimates of NCP from shipboard O2/Ar measurements; O2, NO3-, particulate organic carbon (POC), and dissolved inorganic carbon (DIC) measurements from autonomous platforms, and shipboard incubations based on changes in Chl a and NO3-. I used a generalized additive mixed model to compare the datasets when spatial and temporal differences in the measurements were considered. In Chapter 3, I explored drivers of NCP by comparing how NCP related to various in situ biomarkers and biogeochemical rates measurements. I used moving Pearson’s correlations to assess how continuous measurements of biomarkers such as Chl a, POC, phytoplankton carbon, temperature, and community particle size distribution correlated to changes in continuous NCP. In addition, I showed that NCP was likely driven by changes in production, rather than respiration, in both the North Pacific and North Atlantic by comparing NCP with incubation-based estimates of gross primary production (GPP), net primary production (NPP), and microbial community respiration (mCR). Finally, I modeled NCP from the available biomarker data and determined that POC is a better proxy for estimating NCP than Chl a, in both locations. Finally, in Chapter 4, I examined how changes in the microbial community (from 16S and 18S amplicon sequencing) paired with changes in NCP in the North Pacific. I showed that at coarse taxonomic groupings, such as Phylum, Class, or plankton functional type, had no correlation to changes in NCP, while individual amplicon sequencing variants (ASVs) had strong correlations to changes in the surface ocean organic carbon balance. This indicates a need for increased granularity in microbial community composition estimates to effectively model NCP or carbon export from surface ocean microbial communities. Altogether, my research increases confidence in global NCP estimates from various platforms, presents potential improvements to biogeochemical modelling efforts, and suggests that respiration does not drive changes in NCP in the ocean.

The Arctic Ocean and Western Antarctic Peninsula (WAP) are the fastest warming regions on the planet and are undergoing rapid climate and ecosystem changes. Until we can fully resolve the coupling between biological and physical processes we cannot predict how warming will influence carbon cycling and ecosystem function and structure in these sensitive and climactically important regions. My dissertation centers on the use of high-resolution measurements of surface dissolved gases, primarily O2 and Ar, as tracers or physical and biological functioning that we measure underway using an optode and Equilibrator Inlet Mass Spectrometry (EIMS). Total O2 measurements are common throughout the historical and autonomous record but are influenced by biological (net metabolic balance) and physical (temperature, salinity, pressure changes, ice melt/freeze, mixing, bubbles and diffusive gas exchange) processes. We use Ar, an inert gas with similar solubility properties to O2, to devolve distinct records of biological (O2/Ar) and physical (Ar) oxygen. These high-resolution measurements that expose intersystem coupling and submesoscale variability were central to studies in the Arctic Ocean, WAP and open Southern Ocean that make up this dissertation.

Key findings of this work include the documentation of under ice and ice-edge blooms and basin scale net sea ice freeze/melt processes in the Arctic Ocean. In the WAP O2 and pCO2 are both biologically driven and net community production (NCP) variability is controlled by Fe and light availability tied to glacial and sea ice meltwater input. Further, we present a feasibility study that shows the ability to use modeled Ar to derive NCP from total O2 records. This approach has the potential to unlock critical carbon flux estimates from historical and autonomous O2 measurements in the global oceans.

Radiation therapy is a frequently used treatment method for malignant tumors despite the heterogeneous response in tumors with a hypoxic microenvironment. Specific features in this microenvironment like poorly formed and inefficient vasculature contribute to chronic and cycling hypoxia. Notably, hypoxic tumor cells are three times more radioresistant than normoxic cells, which make hypoxia a key contributor to poor treatment outcome. There have been many previous attempts to re-oxygenate tumors either through increasing the supply or decreasing the demand of oxygen. However, no study has yet been performed to investigate the combined effect of increasing the oxygen supply and decrease the oxygen demand in vivo.

There are two main purposes of this study, which are tested in two individual rounds: 1) assessing the combined effect of oxygen micro-bubbles and papaverine in alleviating tumor hypoxia in murine sarcoma model and 2) assessing the combined effect of oxygen micro bubble and papaverine as radiosensitizers. Using nu-nu mice with subcutaneous sarcoma tumors, the change resulting from oxygen micro-bubbles and/or papaverine was evaluated by changes in hemoglobin saturation from baseline and control groups. By further monitoring tumor growth and percent hemoglobin saturation after administration of papaverine and oxygen micro-bubbles followed by a single fraction of 15 Gy of radiation, we also tested the effects of the combination of papaverine and oxygen micro-bubbles in tumor control and oxygenation.

The result of the non-irradiated study showed no significant improvement of the combination of oxygen micro-bubbles and papaverine group in the percent hemoglobin saturation level compared with other groups. Notably, percent hemoglobin saturation changes are rather heterogenous within each group. However, this unexpected result may be due to certain practical and theoretic limitations. The follow-up immunohistochemistry study may provide more information of the overall oxygenation of the tumors. For the irradiated study, the percent hemoglobin saturation measurement of the oxygen micro-bubbles and papaverine group is the only one showed improved level the day after the treatment compared with the day before treatment. The combination of oxygen micro-bubbles and papaverine did not show increased tumor control after radiotherapy compared with other groups. However, it should be noted that there are some practical and theoretical limitations in the study that may have contributed in this which is discussed in detail in the discussion chapter. Further studies might be needed to investigate the reasons for this unexpected result.

The carotid body (CB) is a major arterial chemoreceptor containing glomus cells that are activated by changes in arterial blood contents including oxygen. Despite significant advancement in the characterization of their physiological properties, our understanding on the underlying molecular machinery and signaling pathway in CB glomus cells is still limited.

To overcome these limitations, in chapter 1, I demonstrated the first transcriptome profile of CB glomus cells using single cell sequencing technology, which allowed us to uncover a set of abundantly expressed genes, including novel glomus cell-specific transcripts. These results revealed involvement of G protein-coupled receptor (GPCR) signaling pathway, various types of ion channels, as well as atypical mitochondrial subunits in CB function. I also identified ligands for the mostly highly expressed GPCR (Olfr78) in CB glomus cells and examined this receptor’s role in CB mediated hypoxic ventilatory response.

Current knowledge of CB suggest glomus cells rely on unusual mitochondria for their sensitivity to hypoxia. I previously identified the atypical mitochondrial subunit Ndufa4l2 as a highly over-represented gene in CB glomus cells. In chapter 2, to investigate the functional significance of Ndufa4l2 in CB function, I phenotyped both Ndufa4l2 knockout mice and mice with conditional Ndufa4l2 deletion in CB glomus cells. I found that Ndufa4l2 is essential to the establishment of regular breathing after birth. Ablating Ndufa4l2 in postnatal CB glomus cells resulted in defective CB sensitivity to hypoxia as well as CB mediated hypoxic ventilatory response. Together, our data showed that Ndufa4l2 is critical to respiratory control and the oxygen sensitivity of CB glomus cells.