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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2020)

Vanishingly low concentrations of iron in the sunlit surface ocean are one

of the greatest challenges to marine phytoplankton which rely on this element

to preform photosynthesis. The impacts of iron limitation are observed across

the world’s oceans and are imprinted in the genomes of photoautotrophic

organisms. In chapter one, I present observations of native phytoplankton

assemblages in the California Current ecosystem and characterize their

molecular strategies for persisting in low and/or variable iron environments. I

contrast the strategies of tiny cyanobacteria and picoeukaryotic phytoplankton

with those of diatoms, which are considered keystone species in iron limited

regions. These new insights into gene expression by the primary producing

inhabitants of California current ecosystem reveal metabolic tactics which help

determine gradients in community structure across the region.

In chapter two I subject the pelagophyte Pelagomonas calceolata to

physiological, transcriptomic and proteomic scrutiny under a range of iron and

light conditions. P. calceolata is globally distributed, tolerates low iron

remarkably well, and is one of the most numerically abundant eukaryote

species on the planet. This study is the first to sequence either transcriptomes or

proteomes from P. calceolata, and revealed are novel utilization of motility and

mixotrophy in response to impaired photosynthetic capabilities. Low iron/low

light adaptations in this organism have consequences for marine ecosystems

and biogeochemical cycles.

Chapter three investigates specific iron acquisition proteins in the model

pennate diatom Phaeodactylum tricornutum. I used reverse genetics

techniques to demonstrate that a siderophore assimilation pathway exists in

diatoms, separate from other iron acquisition mechanisms. I characterize the

substrate specificity of this system and its role in the marine environment. The

proteins involved arrived in P. tricornutum as a result of different evolutionary

histories. I show a prokaryotic receptor protein and a eukaryotic reductase

working together to acquire organically complexed iron, survey their

occurrence across diatom genomes, discuss the implication of this system for

microbial interactions and refine our mechanistic understanding of iron

acquisition in important marine primary producers.

    Cover page: Characterizing the influence of iron on the cellular processes of marine phytoplankton
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2024)
    Iron is an essential nutrient for all microorganisms of the marine environment. Iron limitation of primary production has been well documented across a significant portion of the global surface ocean, but much less is known regarding the potential for iron limitation of the marine heterotrophic microbial community. In this work, we characterize the transcriptomic response of the heterotrophic bacterial community to iron additions in the California Current System, an eastern boundary upwelling system, to detect in situ iron stress of heterotrophic bacteria. Changes in gene expression in response to iron availability by heterotrophic bacteria were detected under conditions of high productivity when carbon limitation was relieved but when iron availability remained low. The ratio of particulate organic carbon to dissolved iron emerged as a biogeochemical proxy for iron limitation of heterotrophic bacteria in this system. Iron stress was characterized by high expression levels of iron transport pathways and decreased expression of iron-containing enzymes involved in carbon metabolism, where a majority of the heterotrophic bacterial iron requirement resides. Expression of iron stress biomarkers, as identified in the iron-addition experiments, was also detected insitu. These results suggest iron availability will impact the processing of organic matter by heterotrophic bacteria with potential consequences for the marine biological carbon pump.
      Cover page: Iron limitation of heterotrophic bacteria in the California Current System tracks relative availability of organic carbon and iron.Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2024)
      Biological nitrogen fixation provides fixed nitrogen for microbes living in the oligotrophic open ocean. UCYN-A2, the previously known symbiont of Braarudosphaera bigelowii, now believed to be an early-stage B. bigelowii organelle that exchanges fixed nitrogen for fixed carbon, is globally distributed. Indirect evidence suggested that B. bigelowii might be a mixotrophic (phagotrophic) phototrophic flagellate. The goal of this study was to determine if B. bigelowii can graze on bacteria using several independent approaches. The results showed that B. bigelowii grazed on co-occurring bacteria at a rate of 5-7 cells/h/B. bigelowii and that the overall grazing rate was significantly higher at nighttime than at daytime. Bacterial abundance changes, assessed with 16S rRNA gene amplicon sequencing analysis, may have indicated preferential grazing by B. bigelowii on specific bacterial genotypes. In addition, Lysotracker™ staining of B. bigelowii suggested digestive activity inside B. bigelowii. Carbon and nitrogen fixation measurements revealed that the carbon demand of B. bigelowii could not be fulfilled by photosynthesis alone, implying supplementation by heterotrophy. These independent lines of evidence together revealed that B. bigelowii engages in phagotrophy, which, beyond serving as a supplementary source of carbon and energy, may also facilitate the indirect assimilation of inorganic nutrients.
        Cover page: Phagotrophy in the nitrogen-fixing haptophyte Braarudosphaera bigelowii.
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2023)
        Coastal upwelling regions are among the most productive marine ecosystems but may be threatened by amplified ocean acidification. Increased acidification is hypothesized to reduce iron bioavailability for phytoplankton thereby expanding iron limitation and impacting primary production. Here we show from community to molecular levels that phytoplankton in an upwelling region respond to short-term acidification exposure with iron uptake pathways and strategies that reduce cellular iron demand. A combined physiological and multi-omics approach was applied to trace metal clean incubations that introduced 1200 ppm CO2 for up to four days. Although variable, molecular-level responses indicate a prioritization of iron uptake pathways that are less hindered by acidification and reductions in iron utilization. Growth, nutrient uptake, and community compositions remained largely unaffected suggesting that these mechanisms may confer short-term resistance to acidification; however, we speculate that cellular iron demand is only temporarily satisfied, and longer-term acidification exposure without increased iron inputs may result in increased iron stress.
          Cover page: Short-term acidification promotes diverse iron acquisition and conservation mechanisms in upwelling-associated phytoplanktonCreative Commons 'BY' version 4.0 license
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