Date of Award

7-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean Engineering and Marine Sciences

First Advisor

Andrew G. Palmer

Second Advisor

Alan Leonard

Third Advisor

Toby Daly-Engel

Fourth Advisor

Eric Guisbert

Abstract

As life has evolved on Earth, gravity has been a constant around which biological processes have developed, rather than a pressure that has been adapted to. As humanity ventures into space, we remove this constant and can observe a variety of changes in biological systems, such as bone loss in humans, stunted plant growth, or increased virulence in pathogens. The growing body of data shows that life adapts to spaceflight and microgravity in unique ways that, due to our limited understanding, can be difficult to predict. A particularly under-studied area is that of host-microbe interactions. Despite efforts to limit their introduction, microbes have been found nearly everywhere on the International Space Station (ISS). This is because virtually all macroscopic life relies on a diverse microbiome to support it, such as in humans, we rely on gut microbes for healthy digestion and nutrient uptake. Analogously, plants rely on a diverse microbiome in their tissues and around their roots, for nutrient uptake, pathogen mediation, and even phytohormone regulation.

All materials launched to the ISS are required to be sterilized or sanitized as thoroughly as possible. This includes the materials for hydroponic plant growth systems, even the seeds to be grown. Despite these efforts, microbial analysis of plants grown in the Vegetable Production System (Veggie) have revealed an active microbiome. This begs the question, how is spaceflight affecting the interactions in these holobiont communities?

The spaceflight environment causes a variety of issues for plant growth; abiotic stressors such as waterlogging due to unique fluid dynamics in microgravity, the lack of convective currents causing the buildup of oxygen and ethylene, which leave plants vulnerable to biotic stressors such as opportunistic fungi causing plant loss. The plants that survive these stressors are returned to Earth for analysis, allowing access to this population of microbes made unique by their spaceflight history. By studying these samples, I aim to improve our understanding of the microbe-plant interactions occurring in the microgravity environment and identify bacteria that could improve plant growth.

In order to better understand this population of microbes, which has been made unique by its presence in space, I have devised an experimental pipeline to elucidate their plant growth promoting (PGP) potential. By screening these microbes for plant growth promoting properties such as indole production, 1-aminocyclopropane-1-carboxylic acid deaminase activity, siderophore production, and phosphate solubilization, I have identified candidate isolates that may have contributed to the health and quality of the plants harvested.

However, many bacteria experience altered phenotypic expression. Simulated microgravity (SM) can be used to determine how these behaviors may have been altered in spaceflight, giving insight to advantages these microbes may have had over others. Most studies on bacteria use fluid suspended cultures in low fluid-shear modeled SM. However, I am interested in the interface between these isolates and their host plants, where symbionts inhabit the surface and interior of host tissues. As such, I have designed experiments to use solid media to test these phenotypes. In testing this initial group of bacteria, I have noted that these bacteria do experience changes in simulated microgravity, though they are unique to each other, with each species responding to SM differently, implicating highly dynamic interactions when in conjunction with hosts in SF. This work is the first effort, to my knowledge, to investigate these plant-growth promoting phenotypes under simulated microgravity on surfaces that mimic the host-microbial interface as it occurs naturally.

Finally, these isolates must be tested as inoculants to determine definitively whether they are friend, foe, or neutral to host plants, as many of these phenotypes are shared between bacterial pathogens and mutualists. This may also give insight into the opportunistic infections afflicting stressed plants grown on the space station. Of the bacteria investigated here, I have observed both beneficial and harmful effects on inoculated plants dependent on the species. This indicates that opportunistic phytopathogens are among this population, yet the good health of the originating host indicates that the beneficial members are able to maintain host health.

In these studies, I have identified numerous microbes capable of PGP activities. SM investigations into samples most likely to be PGP have yielded promising results for their ability to retain these phenotypes in microgravity. In most cases, phenotype expression remained unchanged. In cases where the phenotype expression was changed, I have observed an upregulation of expression in all but one case where the indole production of one strain was downregulated. Inoculation experiments have revealed that these ISS derived isolates contain not only PGP strains, as is the case with Curtobacterium flaccumfaciens and Pantoea agglomerans, but also contain potentially problematic microbes, as the strain of Burkholderia pyrrocinia was observed to inhibit plant growth.

The information gained from these studies will improve the collective understanding of host-microbe interactions in spaceflight and provide insight to how these interactions may be leveraged to benefit humanity’s continued exploration of space. As humanity continues to expand deeper into space, exploration and settlement efforts will become increasingly dependent on robust and sustainable bioregenerative life support systems (BLSS). A key feature to these BLSS will be plant growth systems for air revitalization, waste management, and most importantly, food production. Beneficial microbes whose behaviors in spaceflight can support host resistance to the stressors faced in spaceflight will be key in ensuring the systems continuous production in support of the crew.

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