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27
temperatures. Even when communities were grown under
conditions that periodically exposed them to temperatures
close to this maximum (17–25 °C), the dominant primary
producers did not change. Only when consistently culturing at
high temperatures (26 °C) that exceeded historic maximum
temperature (25.3 °C as measured over the past 10 years) did
the community of primary producers shift. Further, periodi-cally
exposing the community to 30 °C (4.7 °C above the 10-
year maximum) pushed the Chaetoceros simplex beyond their
thermal maximum, and resulted in enriched abundance of
Arcocellulus mammifer regardless of the brevity of exposure
and the shift back to lower temperatures (22 °C) the
following day.
Thermal impacts such as shifts in dominant taxa or
declining phytoplankton abundance could happen at even
lower temperatures in situ. For instance, some grazers are
less susceptible to thermal stress than eukaryotic phototrophs
[70, 71]. Because we removed these from our system, we are
unable to assess the interaction of temperature and grazing
pressure in shaping community composition. Nutrient levels
could also be confounding, as recent work suggests an
interactive effect between nutrient concentrations and tem-peratures,
with less thermal resilience under oligotrophic
conditions [72], although limiting nutrients can increase
thermal tolerance in some marine diazotrophs [73]. Our
nutrient concentrations were kept replete through frequent
dilutions with nutrient-amended seawater, masking any
potential temperature/nutrient availability interactions in our
future treatments. It is possible that in situ grazing pressures
and nutrient limitation could interact strongly with warming
to allow impacts on phytoplankton communities with
smaller temperature perturbations.
These data suggest that temperatures that match or
slightly exceed historic high temperatures even briefly (on
a timescale of weeks) can cause major shifts in dominant
phytoplankton, even under nutrient-replete conditions.
This is in addition to the impacts of long-term elevated
mean temperatures such as those that have been predicted
by ocean/atmosphere models [6]. Our observations sug-gest
these new communities are stable, and still capable of
maintaining their role in marine ecosystems. This is also
consistent with observations from other marine ecosys-tems,
where short-term heat waves have had ecological
consequences over and above those of more modest,
longer-term warming. A temperature anomaly off Aus-tralia’s
west coast in 2010/2011 increased temperatures
2.5 °C above seasonal norms, and for a brief period
(~1 week) exceeded the normal seasonal temperatures by
5 °C [74]. The effects of these anomalous conditions
seemingly irreversibly shifted the ecosystem from a kelp-dominated
community with an abundance of temperate
fish to a benthic, turf algal assemblage with tropical fish
species that were not present before the heatwave. Recent
studies that experimentally manipulated thermal regimes
with the marine copepod Tigriopus californicus suggested
how this process might happen, hypothesizing that prior
exposure to sub-lethal warm temperatures made indivi-duals
more vulnerable to short-term extreme heat events
[75]. This makes sense given the typical shape of micro-bial
thermal curves and the unequal impacts that thermal
fluctuations have on growth rates at higher temperature,
where they can often decrease optimal and lethal thermal
limits.
As marine microbial ecosystems continue to experience
warming, it is likely that these scenarios combining warming
with fluctuating, short-term heat waves could become more
common. This work suggests that encounters with unprece-dented
high-temperatures could lead to broad shifts in
dominant phytoplankton taxa. By simulating the bloom-forming
conditions that periodically occur in this coastal
regime following upwelling, we observed that the onset of
high temperatures not previously experienced in situ may
serve to delineate a threshold where warming affects the
composition of the microbial community. This threshold is
likely modulated by other co-stressors such as nutrient
availability and grazing, by the duration of high-temperature
exposure, and potentially by the range of temperatures in a
given regions of the ocean. Here we offer a testable hypoth-esis
that we believe can act as a starting point for testing the
limits of present-day community structure and function in the
context of a warming ocean.
Data Availability
All scripts used for quality control, analysis of sequence
data, and figure preparation can be found at: https://doi.org/
10.6084/m9.figshare.7603790.v2. Sequence data have been
uploaded to NCBI under the Bioproject ID PRJNA512541.
SRA accession numbers and associated metadata are found
in Table S8. Data from San Pedro Ocean Time-series
(SPOT) monthly sampling can be found at https://dornsife.
usc.edu/spot/data/, and daily temperature data is from the
National Data Buoy Center (https://www.ndbc.noaa.gov/),
station 46222.
Acknowledgements Thanks to Troy Gunderson, Elaina Graham,
Babak Hassanzadeh and the USC Wrigley Institute for Environmental
Studies for help with logistics and analyses. Funding was provided by
National Science Foundation awards OCE 1538525 and OCE 1638804
to FF and DAH.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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 32 |
| Full text | 27 temperatures. Even when communities were grown under conditions that periodically exposed them to temperatures close to this maximum (17–25 °C), the dominant primary producers did not change. Only when consistently culturing at high temperatures (26 °C) that exceeded historic maximum temperature (25.3 °C as measured over the past 10 years) did the community of primary producers shift. Further, periodi-cally exposing the community to 30 °C (4.7 °C above the 10- year maximum) pushed the Chaetoceros simplex beyond their thermal maximum, and resulted in enriched abundance of Arcocellulus mammifer regardless of the brevity of exposure and the shift back to lower temperatures (22 °C) the following day. Thermal impacts such as shifts in dominant taxa or declining phytoplankton abundance could happen at even lower temperatures in situ. For instance, some grazers are less susceptible to thermal stress than eukaryotic phototrophs [70, 71]. Because we removed these from our system, we are unable to assess the interaction of temperature and grazing pressure in shaping community composition. Nutrient levels could also be confounding, as recent work suggests an interactive effect between nutrient concentrations and tem-peratures, with less thermal resilience under oligotrophic conditions [72], although limiting nutrients can increase thermal tolerance in some marine diazotrophs [73]. Our nutrient concentrations were kept replete through frequent dilutions with nutrient-amended seawater, masking any potential temperature/nutrient availability interactions in our future treatments. It is possible that in situ grazing pressures and nutrient limitation could interact strongly with warming to allow impacts on phytoplankton communities with smaller temperature perturbations. These data suggest that temperatures that match or slightly exceed historic high temperatures even briefly (on a timescale of weeks) can cause major shifts in dominant phytoplankton, even under nutrient-replete conditions. This is in addition to the impacts of long-term elevated mean temperatures such as those that have been predicted by ocean/atmosphere models [6]. Our observations sug-gest these new communities are stable, and still capable of maintaining their role in marine ecosystems. This is also consistent with observations from other marine ecosys-tems, where short-term heat waves have had ecological consequences over and above those of more modest, longer-term warming. A temperature anomaly off Aus-tralia’s west coast in 2010/2011 increased temperatures 2.5 °C above seasonal norms, and for a brief period (~1 week) exceeded the normal seasonal temperatures by 5 °C [74]. The effects of these anomalous conditions seemingly irreversibly shifted the ecosystem from a kelp-dominated community with an abundance of temperate fish to a benthic, turf algal assemblage with tropical fish species that were not present before the heatwave. Recent studies that experimentally manipulated thermal regimes with the marine copepod Tigriopus californicus suggested how this process might happen, hypothesizing that prior exposure to sub-lethal warm temperatures made indivi-duals more vulnerable to short-term extreme heat events [75]. This makes sense given the typical shape of micro-bial thermal curves and the unequal impacts that thermal fluctuations have on growth rates at higher temperature, where they can often decrease optimal and lethal thermal limits. As marine microbial ecosystems continue to experience warming, it is likely that these scenarios combining warming with fluctuating, short-term heat waves could become more common. This work suggests that encounters with unprece-dented high-temperatures could lead to broad shifts in dominant phytoplankton taxa. By simulating the bloom-forming conditions that periodically occur in this coastal regime following upwelling, we observed that the onset of high temperatures not previously experienced in situ may serve to delineate a threshold where warming affects the composition of the microbial community. This threshold is likely modulated by other co-stressors such as nutrient availability and grazing, by the duration of high-temperature exposure, and potentially by the range of temperatures in a given regions of the ocean. Here we offer a testable hypoth-esis that we believe can act as a starting point for testing the limits of present-day community structure and function in the context of a warming ocean. Data Availability All scripts used for quality control, analysis of sequence data, and figure preparation can be found at: https://doi.org/ 10.6084/m9.figshare.7603790.v2. Sequence data have been uploaded to NCBI under the Bioproject ID PRJNA512541. SRA accession numbers and associated metadata are found in Table S8. Data from San Pedro Ocean Time-series (SPOT) monthly sampling can be found at https://dornsife. usc.edu/spot/data/, and daily temperature data is from the National Data Buoy Center (https://www.ndbc.noaa.gov/), station 46222. Acknowledgements Thanks to Troy Gunderson, Elaina Graham, Babak Hassanzadeh and the USC Wrigley Institute for Environmental Studies for help with logistics and analyses. Funding was provided by National Science Foundation awards OCE 1538525 and OCE 1638804 to FF and DAH. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest. J. D. Kling et al. |
