Date of Graduation

8-2022

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Savin, Mary C.

Committee Member

Haggard, Brian E.

Second Committee Member

Ashworth, Amanda J.

Third Committee Member

Zhang, Wen

Keywords

algae; allelopathy; cyanobacteria; harmful algal bloom; inhibition; Microcystis aeruginosa

Abstract

Harmful algal blooms are increasing in size, duration, and intensity around the globe. For several decades, cereal straws have been recognized as a viable algal control method, though the mechanisms by which cereal straws inhibit algae remain a topic of research. The objectives of this study were to evaluate the effectiveness of decomposing cereal straw extract, particularly rice (Oryza sativa) and barley (Hordeum vulgare) straw extract, to inhibit the green alga Raphidocelis subcapitata or cyanobacteria Microcystis aeruginosa in bioassays and to determine if the effectiveness of decomposing rice straw extract is altered by the presence of natural organic matter or microbial communities in pond water. In rice and barley straw extracts of the same concentration, total phenolic and flavonoid concentrations were also similar; however, rice straw was more effective than barley straw at inhibiting the growth of both R. subcapitata and M. aeruginosa. Combined with a lack of significant changes in H₂O₂ between cereal straw treatments, results support the hypothesis that algal inhibition cannot be attributed to H₂O₂ alone but depends on the release of specific phenolic and polyphenolic compounds from decomposing cereal straw. In media-based assays, both M. aeruginosa and R. subcapitata were inhibited by 5.0 g/L rice straw extract (P<0.05). In pond water bioassays, 10 g/L rice straw extract showed the greatest inhibitory effect in cultures with and without bacteria. M. aeruginosa showed signs of recovery 15 days following treatment with 2.5 g/L in media and 5.0 g/L rice straw extract in pond water. For reliable long-term suppression, greater concentrations of rice straw are necessary, possibly due to the instability of allelochemicals. In pond water, variability in responses between the controls (with and without bacteria) and 5.0 g/L treatments with bacteria versus without bacteria show that the microbial community within the context of abiotic conditions of natural waters (pH, nutrient concentrations, sediment, and temperature, etc.) is important in determining bloom behavior and treatment response. Though rice straw was an effective method for the control of M. aeruginosa and R. subcapitata, complex interactions between the algae, the microbial community, and abiotic factors found in natural waters make the determination the of the inhibitory mechanism a challenge.

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