Ph.D. Defense Seminar of Melanie Cohn

Bacterial Metabolism Mediates Biogeochemical Cycling in the Open and Coastal Ocean 

Microbes play a critical role in marine ecosystems, driving organic matter alterations and regulating global biogeochemical cycles. I seek to gain insight into organic matter composition, lability, and fate in the ocean through the lens of bacterial activity and function. In Chapter 1, I focus on microbial respiration, a sizable carbon loss process that has been poorly defined due to methodological challenges. Using novel oxygen optode-based approaches to measure low rates, I quantified size-fractionated respiration (operationally parsing phytoplankton from bacterioplankton) in contrasting ocean environments. Trends revealed dominant contributions by small cells in the low-productivity Pacific (Ocean Station Papa) and higher, dynamic rates in the bloom-driven Atlantic (Porcupine Abyssal Plain). This work contributed to carbon cycle models for the EXport Processes in the Ocean through RemoTe Sensing (EXPORTS) program. In Chapter 2, I similarly explore biogeochemical cycles in a complex estuarine environment influenced by diverse inputs and dynamic processes. Using semi-controlled microcosms to simplify the system, I focus on microbial metabolism and light processes that shape organic matter cycling. I analyzed microbial and organic matter stocks and composition seasonally across three light treatments: photosynthetically active radiation, ultraviolet radiation, and darkness, revealing season and light distinctions. Bacterial and phytoplankton abundance was greater in summer than spring. Photosynthetically active radiation drove net organic carbon production and microbial activity, while ultraviolet and dark conditions led to net carbon consumption and reduced activity. In Chapter 3, I target organic matter characteristics that elude traditional analytical methods by analyzing model bacterial growth responses. Using sterilized microcosm seawater, I conducted growth assays to assess the availability (a function of concentration and reactivity) of organic matter. In dark treatments, organic matter was more effectively utilized compared to photosynthetically active radiation treatments. Model community assays showed that higher species richness enhanced resource partitioning and processing, with this effect being most pronounced in summer. The model bacteria revealed how the available organic matter fraction translated to growth, serving as an in-situ sensor of changes in lability under different conditions. This work emphasizes microbial processes as both key regulators and indicators of organic matter stock, lability, and transformations.