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25
warmer temperatures fluctuated (Figs. 5b, S5). The match-ing
18S rRNA sequence recovered for asv4 belonged to the
genus Minutocellus (Fig. S7C, Table S6). Because Arco-cellulus
and Minutocellus are very closely related [63], we
used the plastid 16S rRNA sequence identity to assign its
taxonomy.
Testing of differential abundance using DESeq2 [61]
compared all treatments to the present-day, constant control
treatment. Some statistically significant shifts in abundance
were measured in every experiment (Table S7); however, in
general most of these shifts between treatments did not
involve dominant ASVs (>10% for bacteria and >5% for
eukaryotic phytoplankton). Those that did were the increase
in ASVs matching the genus Rugeria (asv31) in the
spring future-fluctuating treatment (Fig. 5a), and the shift
from a Leptosylindrus-dominated to a Chaetoceros-domi-nated
(future-constant) and Arcocellulus-dominated (future-fluctuating)
phytoplankton community in the summer
(Fig. 5b).
Discussion
Enrichments showed positive growth rates even at tem-peratures
higher than those ever experienced at the coastal
California site where they were collected (>25°). In addi-tion,
the bulk community growth rates were relatively stable
across temperatures and did not show the typical negative
skewness of phytoplankton isolate thermal response curves
[11]. This broad optimum range across temperatures is
likely the result of thermal functional redundancy in these
natural communities. In other words, different members
likely have different limits defining their optimum and
stressful temperatures, thus providing redundancy and
robustness to bulk growth rates, even with a shifting
underlying community. Careful examination of the com-munities
in our comparison of warming and fluctuating
conditions showed that incubations largely maintained the
same dominant taxa until they were exposed transiently to
unusually high temperatures exceeding the summer
experiment upper temperature limit.
In our experiments, no consistent difference was
observed in our POC- and BSi-determined growth rates
between constant and fluctuating treatments. Thermal fluc-tuations
and nutrient inputs are often linked through vertical
mixing and advection events in the coastal regime where
SPOT is located, so the phytoplankton we enriched for may
already be well-adapted to fluctuating temperatures. At
future mean temperatures in both the summer and fall (26
and 20 °C respectively), we did observe significant and near
significant differences between constant and fluctuating
conditions in the ratio of diatom frustule mass (BSi) relative
to the POC present. Plotting these ratios during the final
dilution series (3 days) shows that BSi accumulated faster
relative to POC in the future-constant treatment in the
summer and fall experiments, than in future-fluctuating
treatments (Fig. S9B, C). This difference was particularly
pronounced in the summer experiment, when the slope of a
line fitted to the data was 6.5 times higher in the future-constant
vs. the future-fluctuating treatment (Fig. S9B).
Fig. 5 Heatmaps of amplicon
sequence variants (ASVs) that
comprised: a >10% of total
reads within a sample for
bacteria, and >5% of the total
reads for b diatoms and c
picoeukaryotic phytoplankton.
Triangles represent treatments
where a given ASV was
differentially abundant
compared to the control
(present-constant treatment
outlined with dashed lines)
J. D. Kling et al.
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 30 |
| Full text | 25 warmer temperatures fluctuated (Figs. 5b, S5). The match-ing 18S rRNA sequence recovered for asv4 belonged to the genus Minutocellus (Fig. S7C, Table S6). Because Arco-cellulus and Minutocellus are very closely related [63], we used the plastid 16S rRNA sequence identity to assign its taxonomy. Testing of differential abundance using DESeq2 [61] compared all treatments to the present-day, constant control treatment. Some statistically significant shifts in abundance were measured in every experiment (Table S7); however, in general most of these shifts between treatments did not involve dominant ASVs (>10% for bacteria and >5% for eukaryotic phytoplankton). Those that did were the increase in ASVs matching the genus Rugeria (asv31) in the spring future-fluctuating treatment (Fig. 5a), and the shift from a Leptosylindrus-dominated to a Chaetoceros-domi-nated (future-constant) and Arcocellulus-dominated (future-fluctuating) phytoplankton community in the summer (Fig. 5b). Discussion Enrichments showed positive growth rates even at tem-peratures higher than those ever experienced at the coastal California site where they were collected (>25°). In addi-tion, the bulk community growth rates were relatively stable across temperatures and did not show the typical negative skewness of phytoplankton isolate thermal response curves [11]. This broad optimum range across temperatures is likely the result of thermal functional redundancy in these natural communities. In other words, different members likely have different limits defining their optimum and stressful temperatures, thus providing redundancy and robustness to bulk growth rates, even with a shifting underlying community. Careful examination of the com-munities in our comparison of warming and fluctuating conditions showed that incubations largely maintained the same dominant taxa until they were exposed transiently to unusually high temperatures exceeding the summer experiment upper temperature limit. In our experiments, no consistent difference was observed in our POC- and BSi-determined growth rates between constant and fluctuating treatments. Thermal fluc-tuations and nutrient inputs are often linked through vertical mixing and advection events in the coastal regime where SPOT is located, so the phytoplankton we enriched for may already be well-adapted to fluctuating temperatures. At future mean temperatures in both the summer and fall (26 and 20 °C respectively), we did observe significant and near significant differences between constant and fluctuating conditions in the ratio of diatom frustule mass (BSi) relative to the POC present. Plotting these ratios during the final dilution series (3 days) shows that BSi accumulated faster relative to POC in the future-constant treatment in the summer and fall experiments, than in future-fluctuating treatments (Fig. S9B, C). This difference was particularly pronounced in the summer experiment, when the slope of a line fitted to the data was 6.5 times higher in the future-constant vs. the future-fluctuating treatment (Fig. S9B). Fig. 5 Heatmaps of amplicon sequence variants (ASVs) that comprised: a >10% of total reads within a sample for bacteria, and >5% of the total reads for b diatoms and c picoeukaryotic phytoplankton. Triangles represent treatments where a given ASV was differentially abundant compared to the control (present-constant treatment outlined with dashed lines) J. D. Kling et al. |
