Effect of elevated CO2 on marine bacterioplankton and biogeochemical processes

Abstract

It has been established that ocean acidification will adversely affect calcifying organisms but little is known about its effects on bacterioplankton and the biogeochemical processes which they catalyse. In this thesis, the impact these changes may have on microbialy driven processes is assessed through a mesocosm experiment conducted in a Norwegian fjord near Bergen in May 2006. Three mesocosms were bubbled with CO2(g) to simulate the predicted future conditions of rising atmospheric CO2 concentrations (~760ppm, pH ~7.8), while another three were treated as controls and bubbled with ambient air to represent present day conditions (~380ppm, pH ~8.15). The mesocosms were amended with nitrate and phosphate [16:1] to stimulate a phytoplankton bloom and scientific measurements and analyses were conducted over a 23 day period. At the peak of the phytoplankton bloom chlorophyll-a concentration was ~34% higher under ambient CO2 conditions compared to the high CO2. This was reflected in the flow cytometry results which showed a significant decrease of coccolithophorid and picoeukaryote cell numbers in the high CO2 treatment. Analysis of 16S rRNA gene clone libraries supported by qPCR data revealed that elevated CO2 resulted in a sharp decline in Roseobacter-like bacteria from the Alphaproteobacteria which are significant consumers of the algal osmolyte dimethylsulphoniopropionate (DMSP). Stable isotope probing using 13C labelled sodium bicarbonate revealed that the assimilation of dissolved inorganic carbon by Roseobacter-like bacteria is more prevalent than previously thought making them major contributors to global CO2 fixation. Furthermore, metagenomic analysis of the ambient and high CO2 libraries revealed a significant decrease in genes coding for DMSP demethylase in the high CO2 metagenome. This gene is responsible for the catabolism of DMSP resulting in the eventual release of methanethiol, a source of reduced sulfur for marine bacteria. However, DMSP degradation may proceed down an alternate route known as the lyase pathway resulting in the release of the climatically active gas dimethylsulfide (DMS). In conclusion, the findings of this study strongly suggest that the subsequent decline in Roseobacter species will shift the balance in the degradation of DMSP in favour of the alternate lyase pathway resulting in increased production of DMS and a decrease in the concentration of methanethiol. It is believed that the consequent loss of fixed sulfur will affect ocean productivity and that global climate patterns may change due to the scattering of solar radiation by increased atmospheric sulfur.