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Exploring the Diversity, Host-Microbe Effects, and Functional Properties of Diatom-Associated Bacteria in the Oligotrophic Ocean.

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Title:Exploring the Diversity, Host-Microbe Effects, and Functional Properties of Diatom-Associated Bacteria in the Oligotrophic Ocean.
Authors:Baker, Lydia J.
Contributors:Oceanography (department)
Keywords:bacterioplankton diversity
marine diatoms
algal-bacterial interaction
Date Issued:May 2017
Publisher:University of Hawaiʻi at Mānoa
Abstract:The ecology of diatoms may be better explained by conceptualizing them as composite organisms consisting of
the host cell and its bacterial associates. Attached bacteria have been shown to impact diatom growth, prolong
blooms, and trigger the production of algal toxins in relatively eutrophic systems. This research is among the
first to explore diatom-bacterial interactions in an oligotrophic system. In previous work, we found that
bacterial assemblages attached to individual host cells varied substantially even among closely related hosts.
The assemblages could be separated into three distinct groups, irrespective of host cell identity. Instead, the
distinct groups were best explained by the diversity of the bacteria in each group. For example, in one group, a
single bacterial genus (Arthrobacter) occurred to the near-exclusion of any other bacteria on the same host
cell, whereas a second group was much more diverse. The presence of markedly different bacteria
assemblages led to the present body of work, which examines whether such groups represent persistent
diatom-bacterial associations, and how these associations might be affected by abiotic (e.g. nutrients) or biotic
(e.g. host-bacteria or bacteria-bacteria interactions) factors. The effect of abiotic stressors was tested in a
multi-factor experiment designed to examine changes in the phylogenetic composition of bacteria attached to
a Chaetoceros diatom. Ribosomal DNA was used to monitor changes in the composition of host-associated
bacteria in response to changes in the host's growth induced by manipulations of nutrient concentration and
viral infection. Marinobacter and Alteromonas phylotypes dominated the bacterial consortia attached to the
Chaetoceros in all treatments, regardless of host growth stage. Nutrient concentration and host growth stage
were found to have a statistically significant effect on the phylogenetic composition of the attached bacteria.
Additionally, interactions between attached bacteria were found to significantly affect the composition of hostassociated
bacteria. These results led to an exploration of the effects of perturbations of the relative
proportions of bacteria on a xenic diatom host. Multiple strains of Alteromonas and Marinobacter were
isolated from a Chaetoceros culture descended from a single isolated Chaetoceros cell. Individual bacterial
strains were added to three different xenic diatom hosts (the origin host Chaetoceros sp. KBDT20, a naïve
Chaetoceros host, and a naïve Amphiprora host), to evaluate whether perturbations in their bacterial consortia
could affect host growth, carrying capacity, and decline. Additionally, inoculations were repeated in vitaminrich
and vitamin-poor media to evaluate if vitamin concentrations modify the effect of bacterial inoculations on
host trajectory. For the naïve Chaetoceros host, manipulating the co-cultured bacteria had lasting effects on
host trajectory, demonstrating a lack of resilience or resistance of the host-bacteria association to
perturbations of the bacterial component. In contrast, manipulating the co-cultured bacteria had minimal
effects on the origin Chaetoceros host or the more distantly related Amphiprora host, implying that hostbacteria
associations may vary greatly in their resistance or resilience to perturbation. None of the bacterial
strains had a constantly negative or positive impact on all three tested hosts, but the most common significant
outcome was a negative effect on the host. Additionally, different strains of the same bacterial genus had
different magnitudes of impact on the host. The concentration of vitamins affected the impact of bacterial
inoculations. Mutualistic effects were only observed in vitamin-replete media, whereas effects that suggest
parasitism or competition were observed in vitamin-deplete media. Finally, a metagenomic analysis focused on
the bacteria associated with field-collected, single diatom cells to gain a better understanding of the functional
capabilities contributed by bacteria to a diatom-bacterial association in nature. Six diatom cells derived from
two of the three distinct groups mentioned above were selected for analysis; three cells were from a group
with more diverse bacterial associates, and three from a group dominated by a single bacterial genus. Although
the groups were phylogenetically distinct, the general functional capacities of the bacterial consortia did not
significantly differ between groups; however, individual genes were significantly enriched in the less diverse
group. Although many of the genes previously suggested to be indicative of diatom-bacterial interactions were
absent, functional genes associated with enhanced metabolic and colonization capacities were present in these
diatom microbiomes. As a whole, diatom-associated bacteria were genetically distinct from all other samples
used for comparison, including free-living and microplastic-associated bacteria from the oligotrophic North
Pacific Ocean. This suggests that the Thalassiosira-associated bacteria in our samples are in some ways
functionally distinct from free-living and plastic-associated bacteria from the oligotrophic open ocean, even if
these functions were not previously considered indicative of symbiotic interactions. Overall, the findings of this
research suggest that diatom-bacterial interactions are dependent on host growth state as well as both microand
macronutrients, and advocates for further study of this interaction in oligotrophic systems and xenic
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Oceanography

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