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26
Our experimental design was intended to simulate cli-mate
change impacts on primary producers within a coastal
zone. Outside of a brief spring bloom, SPOT is typically
oligotrophic, with new primary production often relying on
episodic and ephemeral Ekman upwelling [36]. The low
chlorophyll, picoeukaryote-dominated initial community
shifted to a high chlorophyll, largely diatom-dominated
community in our experiment, which is what we would
expect following a typical transitory upwelling or mixing
event at the SPOT time series station. Because of the strong
influence of the experimental nutrient additions, initial
communities were excluded from any statistical testing.
The communities we observed post nutrient addition
were distinct to each season. The differences in the out-comes
of fall and spring incubations, despite similar tem-peratures,
are likely due to differences in the communities
when the samples were collected. The microbial community
at SPOT when sampled monthly over multiple years is most
dissimilar to the microbial community 6 months before or
after the sample is taken [38]. Our fall and spring samples
were taken exactly 6 months apart, so it is not surprising
that after the incubations we got different results. Of course,
as also suggested by our results, these distinct spring and
fall community structures may be largely a function of their
differing recent thermal histories.
Bacterial communities in all three experiments were
consistent with those often associated with diatom blooms.
For instance, ASVs from genera such as Pseudophaeo-bacter
(asv16), Phaeobacter (asv18), and Rugeria (asv31,
Alphaproteobacteria) as well as Alteromonas (asv3, Gam-maproteobacteria)
and Lewinella (asv32 and asv26, Bac-teroidetes)
all made up 10% or more of the amplicons in any
one sample. These genera are copiotrophic heterotrophs,
and so are frequently reported in diatom cultures and in situ
blooms [65]. Interestingly, a Bacteroidetes ASV matching
Kordia jejudonensis (asv11) was only relatively abundant in
the spring present-fluctuating treatment, in which diatom
levels were the lowest within that experiment. This genus
contains species that produce allelopathic compounds
known to be lethal to phytoplankton, which could explain
the relatively low diatom counts [66].
Phytoplankton species that emerged following nutrient
additions also were typical of eutrophic periods in the Cali-fornia
Current. Pseudo-nitzschia, Minidiscus, Leptocylindrus,
and Chaetoceros are common bloom-forming diatom genera
in the California Current System [67, 68]. The dominant
diatom from the future-fluctuating treatment in the summer,
Arcocellulus mammifer, is not mentioned in the literature at
this site. Instead, blooms of this species have been recorded in
aquaculture ponds in the tropical South Pacific [69]. The
apparent thermophilic niche of this species is consistent with
the fact that it was only observed in our treatment inter-mittently
exposed to extreme high temperatures.
Leptocylindrus convexus sequences were only recovered in
relative abundance at present-constant and present-future
temperatures. In the future-constant treatment from the same
experiment (mean temperature=26 °C), this organism
seemed to be supplanted by the chain forming diatom
Chaetoceros simplex. Chaetoceros spp. are heavily silicified,
potentially explaining the significantly higher BSi:C ratio in
the future-constant treatment. This species was only dominant
in the future-constant treatment, however, and in the future-fluctuating
treatment the dominant phytoplankter changed
again to the diatom Arcocellulus mammifer. This shift was
also seen in declining diatom-specific growth rates and BSi
concentrations relative to the future-constant treatment.
In our experiment the phytoplankton bloom from each
seasonal enrichment was distinct from those collected during
the other seasons. Past work at SPOT has showed that
microbial assemblages from a given month are most similar to
other months from the same season, even across years (378).
Further, we compared the abundance of the dominant phy-toplankton
taxa from our spring (ASV9 Pseudo-nitzschia sp.)
and summer (asv1 Leptocylindrus convexus, asv8 Chaeto-ceros
simplex, and asv4 Arcocellulus mammifer) incubation
experiments with previously published data from Needham
and Fuhrman (2016; 49), who followed the response to a
spring-time nutrient pulse over the course of 6 months
(Fig. S10). All three dominant ASVs from our spring incu-bations
were detected in nearly every in situ sample from this
prior study. In March when temperatures were low and
available nitrogen high (>4 μM), amplicons matching ASV9
(>99% similarity across the entire length of the sequence)
became more abundant, while amplicons matching ASV1 or
ASV8 remained barely detectable. In mid-May in the Need-ham
and Fuhrman (2016; 49) study as temperatures began to
rise, a modest increase in available nitrogen (~1 μM) stimu-lated
chlorophyll a production and resulted in a small increase
in amplicons matching asv1 and asv8 that were dominant in
our summer experiment, while those matching ASV9
remained low. In addition, asv4 that was dominant in the
summer future-fluctuating treatment, where it experienced
much higher temperatures than those observed at SPOT,
remained the same throughout this dataset. The consistency
between our experimental community structure and in situ
observations suggests that it is the seasonal thermal environ-ment
that dictates which species are able to respond to
ephemeral nutrient inputs, and that our experiments accurately
simulated these seasonal patterns.
With these experiments, we repeatedly observed that
incubation temperatures (whether fluctuating or constant) that
fell within present-day norms stimulated dense phytoplankton
blooms that were largely taxonomically indistinguishable
across all treatments. Similarities in composition also suggest
a functional redundancy that maintained biogeochemical and
bulk biochemical processes within the envelope of historic
Transient exposure to novel high temperatures reshapes coastal phytoplankton communities
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 31 |
| Full text | 26 Our experimental design was intended to simulate cli-mate change impacts on primary producers within a coastal zone. Outside of a brief spring bloom, SPOT is typically oligotrophic, with new primary production often relying on episodic and ephemeral Ekman upwelling [36]. The low chlorophyll, picoeukaryote-dominated initial community shifted to a high chlorophyll, largely diatom-dominated community in our experiment, which is what we would expect following a typical transitory upwelling or mixing event at the SPOT time series station. Because of the strong influence of the experimental nutrient additions, initial communities were excluded from any statistical testing. The communities we observed post nutrient addition were distinct to each season. The differences in the out-comes of fall and spring incubations, despite similar tem-peratures, are likely due to differences in the communities when the samples were collected. The microbial community at SPOT when sampled monthly over multiple years is most dissimilar to the microbial community 6 months before or after the sample is taken [38]. Our fall and spring samples were taken exactly 6 months apart, so it is not surprising that after the incubations we got different results. Of course, as also suggested by our results, these distinct spring and fall community structures may be largely a function of their differing recent thermal histories. Bacterial communities in all three experiments were consistent with those often associated with diatom blooms. For instance, ASVs from genera such as Pseudophaeo-bacter (asv16), Phaeobacter (asv18), and Rugeria (asv31, Alphaproteobacteria) as well as Alteromonas (asv3, Gam-maproteobacteria) and Lewinella (asv32 and asv26, Bac-teroidetes) all made up 10% or more of the amplicons in any one sample. These genera are copiotrophic heterotrophs, and so are frequently reported in diatom cultures and in situ blooms [65]. Interestingly, a Bacteroidetes ASV matching Kordia jejudonensis (asv11) was only relatively abundant in the spring present-fluctuating treatment, in which diatom levels were the lowest within that experiment. This genus contains species that produce allelopathic compounds known to be lethal to phytoplankton, which could explain the relatively low diatom counts [66]. Phytoplankton species that emerged following nutrient additions also were typical of eutrophic periods in the Cali-fornia Current. Pseudo-nitzschia, Minidiscus, Leptocylindrus, and Chaetoceros are common bloom-forming diatom genera in the California Current System [67, 68]. The dominant diatom from the future-fluctuating treatment in the summer, Arcocellulus mammifer, is not mentioned in the literature at this site. Instead, blooms of this species have been recorded in aquaculture ponds in the tropical South Pacific [69]. The apparent thermophilic niche of this species is consistent with the fact that it was only observed in our treatment inter-mittently exposed to extreme high temperatures. Leptocylindrus convexus sequences were only recovered in relative abundance at present-constant and present-future temperatures. In the future-constant treatment from the same experiment (mean temperature=26 °C), this organism seemed to be supplanted by the chain forming diatom Chaetoceros simplex. Chaetoceros spp. are heavily silicified, potentially explaining the significantly higher BSi:C ratio in the future-constant treatment. This species was only dominant in the future-constant treatment, however, and in the future-fluctuating treatment the dominant phytoplankter changed again to the diatom Arcocellulus mammifer. This shift was also seen in declining diatom-specific growth rates and BSi concentrations relative to the future-constant treatment. In our experiment the phytoplankton bloom from each seasonal enrichment was distinct from those collected during the other seasons. Past work at SPOT has showed that microbial assemblages from a given month are most similar to other months from the same season, even across years (378). Further, we compared the abundance of the dominant phy-toplankton taxa from our spring (ASV9 Pseudo-nitzschia sp.) and summer (asv1 Leptocylindrus convexus, asv8 Chaeto-ceros simplex, and asv4 Arcocellulus mammifer) incubation experiments with previously published data from Needham and Fuhrman (2016; 49), who followed the response to a spring-time nutrient pulse over the course of 6 months (Fig. S10). All three dominant ASVs from our spring incu-bations were detected in nearly every in situ sample from this prior study. In March when temperatures were low and available nitrogen high (>4 μM), amplicons matching ASV9 (>99% similarity across the entire length of the sequence) became more abundant, while amplicons matching ASV1 or ASV8 remained barely detectable. In mid-May in the Need-ham and Fuhrman (2016; 49) study as temperatures began to rise, a modest increase in available nitrogen (~1 μM) stimu-lated chlorophyll a production and resulted in a small increase in amplicons matching asv1 and asv8 that were dominant in our summer experiment, while those matching ASV9 remained low. In addition, asv4 that was dominant in the summer future-fluctuating treatment, where it experienced much higher temperatures than those observed at SPOT, remained the same throughout this dataset. The consistency between our experimental community structure and in situ observations suggests that it is the seasonal thermal environ-ment that dictates which species are able to respond to ephemeral nutrient inputs, and that our experiments accurately simulated these seasonal patterns. With these experiments, we repeatedly observed that incubation temperatures (whether fluctuating or constant) that fell within present-day norms stimulated dense phytoplankton blooms that were largely taxonomically indistinguishable across all treatments. Similarities in composition also suggest a functional redundancy that maintained biogeochemical and bulk biochemical processes within the envelope of historic Transient exposure to novel high temperatures reshapes coastal phytoplankton communities |
