Date of Award

Spring 2021

Project Type

Thesis

Program or Major

Oceanography

Degree Name

Master of Science

First Advisor

Kai Ziervogel

Second Advisor

Robert Letscher

Third Advisor

Beth Orcutt

Abstract

Microorganisms contribute to the cycling of organic matter and inorganic nutrients through extracellular enzymatic depolymerization of high molecular weight compounds, providing essential nutrients to the aquatic environment. Since microorganisms adapt relatively quickly to their environment, their metabolic rates reflect biological responses to changing environmental conditions, such as those predicted as a result of climate change. Yet, before it is possible to predict how microorganisms will respond to future change, it is important to understand how current environmental conditions influence microbial activity. To capture a range of environmental conditions, the response of microbial enzymatic activities was studied seasonally at three Eastern US coastal sites in the Gulf of Maine, and comparatively with a one-time sampling at an offshore deep ocean site in the northeast Pacific Ocean. The study hypothesizes that patters in enzymatic activities will be influenced by environmental parameters such as sea temperature, salinity, chlorophyll-a (chl-a), oxygen, and bacterial cell abundance. To evaluate this hypothesis, variations of environmental parameters were compared to the rates and patterns of β-glucosidase (BG), N-Acetyl-β-D-Glucosaminidase (NAG), xylosidase (XYL), leucine aminopeptidase (LAP), and alkaline phosphatase (AP), enzymes which catalyze the hydrolysis of carbohydrates, peptides, and phosphorous compounds, respectively. The Gulf of Maine is a highly productive and rapidly warming body of water, but unlike what has been observed in warmer settings, water temperature was not a strong influencing factor on seasonal patterns of enzyme activities there. The lack of correlation might be due to these sites in situ temperature being further from the temperature optima of the enzymes. Moderately strong relationships were found between chl-a and AP activities at two of the three coastal sites, and at the deep ocean site, enzymatic activities correlated to temperature, oxygen, and bacterial cell abundance. Enzymatic activities were closely coupled with oxygen concentration, highlighting the influence of oxygen minimum zones on microbial activity. AP activity was decoupled from bacterial cell abundance, yet high activity was maintained throughout the column indicating activity by cell-free AP. Patterns in enzymatic activity at all four sites were not attributed to any one environmental factor, but rather, a combination of environmental influences that varied temporally and spatially. Results from this study further our understanding of the current relationships between enzymatic activities and environmental parameters, insights necessary for accurately predicting ecosystem responses to climate change.

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