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- All Subjects: Sinking particles
- Creators: Neuer, Susanne
Description
The efficiency of the ocean’s biological carbon pump is mediated by fast-sinking particles that quickly settle out of the euphotic zone. These particles are conventionally associated with micro- (> 20 µm) sized diatoms and coccolithophorids, thought to efficiently transport carbon to depth owing to their dense mineral structures, while pico- (< 2 µm) and nanophytoplankton (2-20 µm) are considered to contribute negligibly due to their small size and low sinking speed. Despite burgeoning evidence of their export, the mechanisms behind it remain poorly understood. The objective of this dissertation is to acquire a mechanistic understanding of the contribution of pico- and nanophytoplankton to particle fluxes. I tested the hypotheses that pico- and nanophytoplankton may be exported via the following pathways: 1) physical aggregation due to the production of sticky Transparent Exopolymeric Particles (TEP), mediated by interactions with heterotrophic bacteria, 2) attachment to lithogenic minerals, and 3) repackaging by zooplankton. I found that despite the traditional view of being too small to sink, pico- and nanophytoplankton form aggregates rich in TEP, allowing cells to scavenge lithogenic minerals and thus increase their effective size and density. I discovered that interactions with heterotrophic bacteria were significant in mediating the process of aggregation by influencing the production and/or the composition of the phytoplankton-derived TEP. Bacteria differentially influenced aggregation and TEP production; some species enhanced aggregation without affecting TEP production, and vice-versa. Finally, by determining the microbial composition of sinking particles in an open-ocean site, I found pico- and nanophytoplankton significantly associated with particles sourced from crustaceous zooplankton, suggesting that their export is largely mediated by mesozooplankton.
Overall, I show that the hypothesized mechanisms of pico- and nanophytoplankton export are not mutually exclusive, but instead occur subsequently. Given the right conditions for their aggregation in the natural environment, such as interactions with aggregation-enhancing heterotrophic bacteria and/or the presence of lithogenic minerals, their cells and aggregates can escape remineralization within the euphotic zone, and thus be susceptible to grazing by mesozooplankton export within fecal pellets. The results of this dissertation provide a mechanistic framework for the contribution of pico- and nanophytoplankton to ocean particle fluxes.
ContributorsCruz, Bianca Nahir (Author) / Neuer, Susanne (Thesis advisor) / Lomas, Michael W (Committee member) / Passow, Uta (Committee member) / Cadillo-Quiroz, Hinsby (Committee member) / Arizona State University (Publisher)
Created2021
Description
Sinking particles are important conduits of organic carbon from the euphotic zone to the deep ocean and microhabitats for diverse microbial communities, but little is known about what determines their origin and community composition. Events in the northwestern Sargasso Sea, such as winter convective mixing, summer stratification, and mesoscale (10–100 km) eddies, characteristic features of this region, affect the vertical and temporal composition and abundance of pelagic and particle-attached microorganisms. To assess the connections of the microbial communities between the euphotic zone and sinking particles, I carried out indicator and differential abundance analyses of prokaryotes and photoautotrophs based on the V4-V5 amplicons of the 16S rDNA from samples collected in the Sargasso Sea during the spring and summer of 2012. I found that gammaproteobacteria such as Pseudoalteromonas sp. and Vibrio sp., common particle-associated bacteria often linked with zooplankton, dominated the sequence libraries of the sinking particles. The analysis also revealed that members of Flavobacteria, particularly the fish pathogen Tenacibaculum sp., as well as Chloropicon sp. and Chloroparvula sp., among the smallest known green algae, were indicators taxa of sinking particles. The cryptophyte Teleaulax and the diatom Chaetoceros were overrepresented in the particle communities during both seasons. Interestingly, I also found that the large centric diatom, Rhizosolenia sp., generally rare in the oligotrophic Sargasso Sea, dominated photoautotrophic communities of sinking particles collected in the center of an anticyclonic eddy with unusual upwelling due to eddy-wind interactions. I hypothesize that the steady contribution by picophytoplankton to particle flux is punctuated by pulses of production and flux of larger-sized phytoplankton in response to episodic eddy upwelling events and can lead to higher export of particulate organic matter during the summer.
ContributorsFontánez Ortiz, Marc Alec (Author) / Neuer, Susanne (Thesis advisor) / Zhu, Qiyun (Committee member) / Trembath-Reichert, Elizabeth (Committee member) / Arizona State University (Publisher)
Created2022