Matching Items (4)
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- All Subjects: Biological Oceanography
- Creators: Hartnett, Hilairy
Description
Sea ice algae dominated by diatoms inhabit the brine channels of the Arctic sea ice and serve as the base of the Arctic marine food web in the spring. I studied sea ice diatoms in the bottom 10 cm of first year land-fast sea ice off the coast of Barrow, AK, in spring of 2011, 2012, and 2013. I investigated the variability in the biomass and the community composition of these sea-ice diatoms between bloom phases, as a function of overlying snow depth and over time. The dominant genera were the pennate diatoms Nitzschia, Navicula, Thalassiothrix, and Fragilariopsis with only a minor contribution by centric diatoms. While diatom biomass as estimated by organic carbon changed significantly between early, peak, and declining bloom phases (average of 1.6 mg C L-1, 5.7 mg C L-1, and 1.0 mg C L-1, respectively), the relative ratio of the dominant diatom groups did not change. However, after export, when the diatoms melt out of the ice into the underlying water, diatom biomass dropped by ~73% and the diatom community shifted to one dominated by centric diatoms. I also found that diatom biomass was ~77% lower under high snow cover (>20 cm) compared to low snow cover (<8 cm); however, the ratio of the diatom categories relative to particulate organic carbon (POC) was again unchanged. The diatom biomass was significantly different between the three sampling years (average of 2.4 mg C L-1 in 2011, 1.1 mg C L-1 in 2012, and 5.4 mg C L-1 in 2013, respectively) as was the contribution of all of the dominant genera to POC. I hypothesize the latter to be due to differences in the history of ice sheet formation each year. The temporal variability of these algal communities will influence their availability for pelagic or benthic consumers. Furthermore, in an Arctic that is changing rapidly with earlier sea ice and snowmelt, this time series study will constitute an important baseline for further studies on how the changing Arctic influences the algal community immured in sea ice.
ContributorsKinzler, Kyle (Author) / Neuer, Susanne (Thesis advisor) / Juhl, Andrew (Committee member) / Hartnett, Hilairy (Committee member) / Arizona State University (Publisher)
Created2014
Description
Biological soil crusts (BSCs) are critical components of arid and semiarid environments and provide the primary sources of bioavailable macronutrients and increase micronutrient availability to their surrounding ecosystems. BSCs are composed of a variety of microorganisms that perform a wide range of physiological processes requiring a multitude of bioessential micronutrients, such as iron, copper, and molybdenum. This work investigated the effects of BSC activity on soil solution concentrations of bioessential elements and examined the microbial production of organic chelators, called siderophores. I found that aluminum, vanadium, copper, zinc, and molybdenum were solubilized in the action of crusts, while nickel, zinc, arsenic, and zirconium were immobilized by crust activity. Potassium and manganese displayed behavior consistent with biological removal and mobilization, whereas phosphorus and iron solubility were dominated by abiotic processes. The addition of bioavailable nitrogen altered the effects of BSCs on soil element mobilization. In addition, I found that the biogeochemical activites of BSCs were limited by molybdenum, a fact that likely contributes to co-limitation by nitrogen. I confirmed the presence of siderophore producing microbes in BSCs. Siderophores are low-molecular weight organic compounds that are released by bacteria to increase element solubility and facilitate element uptake; siderophore production is likely the mechanism by which BSCs affect the patterns I observed in soil solution element concentrations. Siderophore producers were distributed across a range of bacterial groups and ecological niches within crusts, suggesting that siderophore production influences the availability of a variety of elements for use in many physiological processes. Four putative siderophore compounds were identified using electrospray ionization mass spectrometry; further attempts to characterize the compounds confirmed two true siderophores. Taken together, the results of my work provide information about micronutrient cycling within crusts that can be applied to BSC conservation and management. Fertilization with certain elements, particularly molybdenum, may prove to be a useful technique to promote BSC growth and development which would help prevent arid land degradation. Furthermore, understanding the effects of BSCs on soil element mobility could be used to develop useful biomarkers for the study of the existence and distribution of crust-like communities on ancient Earth, and perhaps other places, like Mars.
ContributorsNoonan, Kathryn Alexander (Author) / Hartnett, Hilairy (Thesis advisor) / Anbar, Ariel (Committee member) / Garcia-Pichel, Ferran (Committee member) / Shock, Everett (Committee member) / Sharp, Thomas (Committee member) / Arizona State University (Publisher)
Created2012
Description
The ocean sequesters more than 25% of the carbon released by anthropogenic action every year, and oligotrophic oceans, such as the Sargasso Sea, are responsible for about 50% of the global carbon export. Pico- and nano-phytoplankton (cells < 5 µm), mostly unicellular eukaryotes (protists) and cyanobacteria, dominate the primary production in the Sargasso Sea; however, little is known about their contribution to the export of carbon into the deep ocean via sinking particles. The overall goal of this study is to examine the link between growth and grazing rates of pico- and nano-phytoplankton and the carbon export in the Sargasso Sea. I investigate three aspects: 1) how microzooplankton grazing and physical forcing affect taxon-specific primary productivity in this region, 2) how these microbial trophic dynamics impact their contribution to the export of particulate matter, and 3) how much pico-phytoplankton, specifically the pico-cyanobacteria Synechococcus and Prochlorococcus, contribute to the carbon export. I collected seawater samples within the sunlit (euphotic) zone, and sinking particles at 150 m depth using particle traps in the Sargasso Sea during the winter and summer seasons of 2011 and 2012. I conducted dilution experiments to determine the growth and grazing rates of the pico- and nano-phytoplankton community, and used 454 pyrosequencing and quantitative Polymerase Chain Reaction to measure the relative and absolute contribution of these primary producers to the plankton community within the euphotic zone and in the sinking particles. I found that micrograzing controls taxon-specific primary production, and that microbial trophic dynamics impact directly the taxonomical composition of the sinking particles. For the first time, I was able to quantify clade-specific carbon export of pico-cyanobacteria and found that, despite their small size, these tiny primary producers are capable of sinking from the surface to the deeper oceans. However, their contribution to the carbon flux is often less than one tenth of their biomass contribution in the euphotic zone. Our study provides a comprehensive approach to better understand the role of pico- and nano-phytoplankton in the carbon cycle of oligotrophic oceans, and a baseline to study changes in the carbon export in future warmer oceans.
ContributorsDe Martini, Francesca (Author) / Neuer, Susanne (Thesis advisor) / Garcia-Pichel, Ferran (Committee member) / Hartnett, Hilairy (Committee member) / Lomas, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
The biological carbon pump in the ocean is initiated by the photosynthetic fixation of atmospheric carbon dioxide into particulate or dissolved organic carbon by phytoplankton. A fraction of this organic matter sinks to depth mainly in the form of microaggregates (5-60 μm) and visible macroaggregates. These aggregates are composed of cells, minerals, and other sources of organic carbon. Exopolymeric substances (EPS) are exudated by heterotrophic bacteria and phytoplankton and may form transparent exopolymeric particles (TEP) that act as a glue-like matrix for marine aggregates. Heterotrophic bacteria have been found to influence the aggregation of phytoplankton and in some cases result in an increase in TEP production, but it is unclear if marine heterotrophic bacteria can produce TEP and how they contribute to aggregation. Pseudoalteromonas carrageenovora, Vibrio thalassae, and Marinobacter adhaerens HP15 are heterotrophic marine bacteria that were found associated with sinking particles in an oligotrophic gyre station in the subtropical North Atlantic. These bacteria were grown in axenic cultures to determine growth, TEP production, and aggregation. They were also inoculated into roller tanks used to simulate open ocean conditions to determine their ability to form macroaggregates. Treatments with added kaolinite clay simulated aeolic dust input from the Sahara. M. adhaerens HP15 had the highest TEP concentration but the lowest cell-normalized TEP production at all growth stages compared to the other bacteria. Additionally, M. adhaerens HP15 also had the lowest microaggregate formation. The cell-normalized TEP production and microaggregate formation was not significantly different between P. carrageenovora and V. thalassae. All bacteria formed visible macroaggregates in roller tanks with clay addition and exhibited high sinking velocities (150-1200 m d-1) that are comparable to those of aggregates formed by large mineral ballasted phytoplankton. Microaggregates in the clay treatments declined during incubation, indicating that they aggregated to form the macroaggregates. The findings from this study show for the first time that heterotrophic bacteria can contribute to aggregation and the export of organic carbon to depth in the ocean.
ContributorsLivar, Britni (Author) / Neuer, Susanne (Thesis advisor) / Hartnett, Hilairy (Committee member) / Cadillo-Quiroz, Hinsby (Committee member) / Arizona State University (Publisher)
Created2022