Impact of climate change variables on nutrient cycling by marine microorganisms in the Southern California Bight and Ross Sea, Antarctica

Ocean environments are being impacted by climate warming, elevated carbon dioxide (CO2) levels, and shifting nutrient sources and sinks. It is essential to quantify the sensitivity of microorganisms to these effects of global change because they form the base of the marine food web and are an integral component of nutrient cycling on the planet. Their role in photosynthesis, nutrient uptake, and transfer of organic matter into higher trophic levels or to the deep ocean via the biological pump render microorganisms key in ecosystem structure and function and in regulating the global climate. The goal of this dissertation research was to determine how changing environmental conditions impact microbial communities and the rates at which they take up nutrients. Research for this dissertation took place in the Southern California Bight and in the Ross Sea, Antarctica, where fully factorial designs were used to investigate the response of microorganisms to multiple global change parameters. Nutrient uptake rates were measured using 13C and 15N stable isotopes for carbon and nitrogen substrates and 33P radioisotopes for phosphorus substrates. In the Southern California Bight, a microbial assemblage was collected and incubated in an ‘ecostat’ continuous culture system, where elevated temperature, CO2, and the dominant nitrogen substrate (nitrate or urea) in the diluent were manipulated. During this experiment uptake rates of dissolved inorganic carbon (DIC), nitrate (NO3-), and urea were determined for two microbial size classes (0.7-5.0 μm and >5.0 μm). Urea uptake rates were greater than NO3-, and uptake rates of urea and DIC for both size fractions increased at elevated temperature, while uptake rates of NO3- by smaller microorganisms increased when CO2 levels were high. In the Ross Sea, the impact of elevated temperature, CO2, and iron addition on DIC and NO3- uptake rates by two size classes (0.7-5.0 μm and >5.0 μm ) of a late-season microbial community were investigated using a semi-continuous and continuous ‘ecostat’ culturing approach. Temperature impacted the microbial community the most, significantly increasing NO3- and DIC uptake rates by larger microorganisms. The effects of iron addition were more apparent when temperature was also elevated, and CO2 did not impact rates. Bioassay experiments were also conducted in the Ross Sea to determine how increasing and decreasing the N:P supply ratio in combination with other parameters (temperature and iron) impact uptake rates of DIC, NO3-, and amino acids. Results from these experiments show that changes to the dissolved N:P supply ratio have the potential to alter nutrient uptake rates over short time scales, but that temperature elevation and iron addition have a larger impact. Additional experiments were completed on diatoms (Fragilariopsis cylindrus and Pseudo-nitzschia subcurvata) and Phaeocystis antarctica, three important phytoplankton species collected from the Ross Sea, to assess how temperature elevation and iron addition impact uptake rates of a number of inorganic and organic carbon, nitrogen, and phosphorus substrates. These culture studies generally show that when temperature is increased, diatoms are able to take up nutrients more rapidly than Phaeocystis antarctica.
Results from this dissertation show that nutrient cycles and phytoplankton communities in the Southern California Bight and the Ross Sea, Antarctica will likely be different in the future. Although all variables tested were found to exert some influence on microbial nutrient cycling, temperature elevation generally had the largest effect, increasing biomass and uptake rates, structuring the composition of the microbial community, and altering stoichiometry. This research did not include top down effects and it is limited spatially and temporally, however, it demonstrates the importance of studying different nutrient substrates and looking at multiple interactive stressors to gain a more comprehensive view of potential change.

Spackeen J. L., 2017. Impact of climate change variables on nutrient cycling by marine microorganisms in the Southern California bight and Ross Sea, Antarctica. PhD thesis, College of William and Mary in Virginia, 237 p. Thesis (restricted access).

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