Date of Award

Fall 2025

Project Type

Thesis

Program or Major

Microbiology

Degree Name

Master of Science

First Advisor

Anna O'Brien

Second Advisor

Serita Frey

Third Advisor

Wilfred Wollheim

Abstract

The increasing reliance on inefficient synthetic fertilizers, which readily leach from soils and contribute to downstream pollution, underscores the urgent need for more sustainable nutrient sources, such as green manures (GMs). Floating aquatic plants like duckweeds are particularly promising candidates due to their natural abundance, rapid growth rates, and ability to efficiently uptake nutrients. However, the presence of toxin-producing cyanobacteria in duckweed microbiomes raises concerns about the potential for toxin accumulation, like microcystins (MCs), both in the GM biomass and in the edible crops to which it is applied. To evaluate the suitability and safety of duckweed-based GMs, we conducted two complementary experiments – a laboratory Microcosm Experiment and a Greenhouse Experiment – focusing on how duckweed source and microbiome composition, including the presence of toxin-producing cyanobacteria, influence GM quality and crop safety.

The Microcosm Experiment, which manipulated the microbiomes (including a spike of Microcystis aeruginosa, a known MC-producer) associated with different duckweed genotypes in well plates, revealed that duckweed growth, nutrient uptake, and toxicity are strongly influenced by microbiome composition. These findings highlight the significant role of microbial communities in shaping host plant function. Specifically, we observed that microbiome manipulation can alter the nutrient uptake efficiency of duckweeds, suggesting that microbial symbionts, like cyanobacteria, contribute to the agronomic potential of duckweed GMs. Additionally, microbiome composition differed between source ponds, indicating that local environmental conditions may shape microbial communities in ways that affect GM quality. While duckweeds exposed to the toxin-producing cyanobacterium M. aeruginosa resulted in GMs with accumulated MCs, we did not observe any visible negative effects on germinants, and lettuce seeds were still able to germinate in the presence of toxic GM, suggesting that if any negative effects on crop health occur, they may not manifest until later developmental stages.

Because New Hampshire farmers are unlikely to manipulate plant microbiomes, the Greenhouse Experiment tested how natural duckweed populations and their associated microbiomes harvested from agricultural ponds perform as GM for lettuce cultivation. This experiment revealed substantial variability in microbiome composition among duckweeds, despite the close geographic proximity of the source ponds, and variability in the species composition of the duckweeds themselves. Natural variation across sites contributed to various changes in quality, such as variation in GM carbon-to-nitrogen ratios and MC concentrations. These factors likely influenced crop outcomes, including lettuce biomass and tissue toxicity. Compared to the Microcosm Experiment, microbiome diversity was notably higher in the Greenhouse Experiment, with greater cyanobacterial diversity supported in the unmanipulated duckweed field microbiomes. Overall, this experiment demonstrated that duckweed and microbiome diversity in natural systems is substantial. As a result, predicting the microbial composition of duckweeds sourced from runoff ponds, and thus the safety and efficacy of GMs derived from them, may be inherently challenging.

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