The Effects of Ocean Acidity and Elevated Temperature on Bacterioplankton Community Structure and Metabolism

Since the beginning of the industrial revolution, anthropogenic activities have rapidly increased the concentration of CO₂ in the atmosphere, contributing to global climate change. One major consequence of the change in global climate is the rise of temperatures in a variety of habitats including the oceans. And by the end of the 21st century, mean sea surface temperatures are expected to increase 4 ˚C, while atmospheric CO₂ concentrations are predicted to triple causing seawater to become acidic. Consequently, marine ecosystems are especially vulnerable to these changes to the environment as organisms are under the compounding stress of ocean acidification and warming.

Bacterioplankton play a vital role in the marine carbon cycle and the oceans’ ability to sequester CO₂. In this paper, the authors utilized pCO₂ perturbation experiments to investigate the effects of ocean acidity and elevated temperature on bacterioplankton community structure and metabolism. Water samples were collected from the central Salish Sea (San Juan Channel, San Juan Islands, WA) utilizing a conductivity, temperature and depth (CTD) rosette from a research vessel and the seawater intake system at Shannon Point Marine Center. Three representative environmental bacterioplankton communities were collected from two locations (north and south) along the San Juan Channel and SPMC’s seawater intake system.

Terminal-restriction fragment length polymorphism (T-RFLP) of small subunit ribosomal (SSU) genes revealed that bacterioplankton incubated in lower pH conditions exhibited a reduction of species richness, evenness, and overall diversity, relative to those incubated in ambient pH conditions. Non-metric multidimensional scaling (MDS) of T-RFLP data resulted in clustering by pH suggesting that pH influenced the structure of these communities. Shifts in the dominant members of bacterioplankton communities incubated under different pH were observed in both T-RFLP and SSU clone library analyses. Both ambient and low pH communities were dominated by Gammaproteobacteria and Alphaproteobacteria, although abundance of Alphaproteobacteria increased in communities incubated at lower pH. This was expressed by the gamma to alpha ratio dropping from ~9 to 4, respectively. In general, the representative taxa from these two classes were distinctly different between the treatments, with a few taxa found to be persistent in both treatments. Changes in the structure of bacterioplankton communities coincided with significant changes to their overall metabolism. Bacterial production rates decreased, while bacterial respiration increased under lower pH conditions.

In short, this study highlights the ability of bacterioplankton communities to respond to ocean acidification both structurally and metabolically, which may have significant implications for their ecological function in the marine carbon cycle and the ocean’s response to global climate change.

Article by Nam Siu, et al, from USA.

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