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32
Introduction
Phytoplankton photosynthesis in the ocean is responsible for 50% of the global
conversion of inorganic CO2 to organic biomass (Field et al. 1998). This process provides energy
for higher trophic levels (Smith and Hollibaugh 1993) and shuttles atmospheric CO2 into deep
ocean waters or marine sediments, storing carbon on geological time scales (Lyle 1988). These
fundamental ecosystem services make marine phytoplankton essential to study in the context
of global climate change, particularly given their ability to offset atmospheric CO2 emissions
(Hutchins et al. 2019). Current projections suggest a rise in mean sea surface temperature (SST)
of 4 °C by 2100 (Pachauri et al. 2014) which could have large impacts on the physiology and
composition of these microbial photosynthetic communities, and their ability to sequester
carbon (Hutchins and Fu 2017).
Some studies predict a decrease in net marine primary productivity (Behrenfeld et al.
2006). However, making predictions about warming effects on phytoplankton can be difficult
because of the still relatively underexplored biological diversity of these microorganisms, and
their regionally-specific responses. For instance, in some regimes rising SST may lead to an
increase in stratification, which can decrease the supply of available nutrients for
photosynthetic growth (Capotondi et al. 2012). This could be particularly problematic for
phytoplankton in the already nutrient-poor subtropical gyres; however, in high latitudes where
mixed layers are deep and nutrients are often relatively abundant, warming-induced
stratification may relieve light limitation, and so contribute to growth (Riebesell et al. 2009).
Phytoplankton communities are also incredibly diverse, containing numerous coexisting
photosynthetic bacterial and protistan lineages. Despite nearly a century of research, new and
Object Description
| Title | Thermal diversity within marine phytoplankton communities |
| Author | Kling, Joshua David |
| Author email | Joshuakl@usc.edu;Joshuakl@berkeley.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Biology (Marine Biology and Biological Oceanography) |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2020-08-11 |
| Date submitted | 2020-08-11 |
| Date approved | 2020-08-11 |
| Restricted until | 2020-08-11 |
| Date published | 2020-08-11 |
| Advisor (committee chair) | Hutchins, David |
| Advisor (committee member) |
Levine, Naomi Heidelberg, John Ehrenreich, Ian |
| Abstract | Marine photosynthetic carbon fixation in the sunlit upper reaches of the ocean is almost entirely carried out by chlorophyll-containing, single-celled microorganisms, and is responsible for half of the net primary production on the planet. Because of this connection to the marine carbon cycle, it is essential to assess the responses of marine phytoplankton to global change. However, this work is challenged by the dazzling diversity of both eukaryotic and prokaryotic lineages which coexist in complex phytoplankton assemblages. My dissertation contributes to this effort by investigating how the diversity of phytoplankton influences their resilience to rising temperatures. In my first study, I used natural California coastal communities collected across three seasons to show that the phytoplankton assemblage as a whole was able to maintain growth well above typical temperature ranges. However, either steady or fluctuating temperatures exceeding the maximum threshold recorded in a decade-long observational dataset caused drastic rearrangements in the phytoplankton community, including the appearance of novel dominant species. My dissertation work also highlights that there are still unrecognized but environmentally-important taxa with bizarre and unexpected life histories and thermal responses, even in the most well-studied environments. In my second study, I characterized a recently isolated nanoplanktonic diatom from the Narragansett Bay Time Series that occupies a distinct low-light, low-temperature niche. This isolate demonstrated an unusual sensitivity to light, whereby its ability to respond to what should be favorable increases in temperature is constrained by light intensity. Six years of amplicon sequencing data from the time series site suggest that this diatom is a temperate wintertime/early spring specialist, and will likely not fare well in a warmer and more stratified future ocean. In addition to expanding knowledge of functional diversity at the species level, my work also examines the potential of intra-specific diversity to house hidden adaptations to rising temperatures. Natural microbial populations are composed of distinct individual strains, whose relative abilities to contribute to the success of the whole population in a changing environment have not been well-studied. In my third study, I compared the thermal responses of 11 strains of the marine unicellular cyanobacterium Synechococcus simultaneously isolated from a single estuarine water sample to explore this cryptic intra-specific diversity. Surprisingly, these nearly genetically-identical strains showed distinct low and high temperature phenotypes. This study indicates that strain-level variation could be a key yet understudied element in the responses of phytoplankton to global change. Together, these studies highlight that the diversity of marine phytoplankton at the species and individual level includes both functional variability and redundancy relative to temperature. We can expect community composition to change over time in a warming ocean, reflecting the increasing abundance of preadapted groups or individual strains; however, wherever there are winners there are also losers. Besides providing new insights into the contribution of diversity to climate resilience, this dissertation also highlights the need to expand our knowledge of functional thermal traits, especially for typically under-studied pico- and nanoplankton which are often only known from sequence data. |
| Keyword | thermal response; phytoplankton; community ecology |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m |
| Contributing entity | University of Southern California |
| Rights | Kling, Joshua David |
| Physical access | The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given. |
| Repository name | University of Southern California Digital Library |
| Repository address | USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-KlingJoshu-8915.pdf |
| Archival file | Volume13/etd-KlingJoshu-8915.pdf |
Description
| Title | Page 37 |
| Full text | 32 Introduction Phytoplankton photosynthesis in the ocean is responsible for 50% of the global conversion of inorganic CO2 to organic biomass (Field et al. 1998). This process provides energy for higher trophic levels (Smith and Hollibaugh 1993) and shuttles atmospheric CO2 into deep ocean waters or marine sediments, storing carbon on geological time scales (Lyle 1988). These fundamental ecosystem services make marine phytoplankton essential to study in the context of global climate change, particularly given their ability to offset atmospheric CO2 emissions (Hutchins et al. 2019). Current projections suggest a rise in mean sea surface temperature (SST) of 4 °C by 2100 (Pachauri et al. 2014) which could have large impacts on the physiology and composition of these microbial photosynthetic communities, and their ability to sequester carbon (Hutchins and Fu 2017). Some studies predict a decrease in net marine primary productivity (Behrenfeld et al. 2006). However, making predictions about warming effects on phytoplankton can be difficult because of the still relatively underexplored biological diversity of these microorganisms, and their regionally-specific responses. For instance, in some regimes rising SST may lead to an increase in stratification, which can decrease the supply of available nutrients for photosynthetic growth (Capotondi et al. 2012). This could be particularly problematic for phytoplankton in the already nutrient-poor subtropical gyres; however, in high latitudes where mixed layers are deep and nutrients are often relatively abundant, warming-induced stratification may relieve light limitation, and so contribute to growth (Riebesell et al. 2009). Phytoplankton communities are also incredibly diverse, containing numerous coexisting photosynthetic bacterial and protistan lineages. Despite nearly a century of research, new and |
