Effects of ocean acidification on Antarctic microbial communities

Antarctic waters are amongst the most vulnerable in the world to ocean acidification due to their cold temperatures, naturally low levels of calcium carbonate and upwelling that brings deep CO2-rich waters to the surface. A meta-analysis demonstrated groups of Antarctic marine biota in waters south of 60!S have a range of tolerances to ocean acidification. Invertebrates and phytoplankton showed negative effects above 500 μatm and 1000 μatm CO2 respectively, while bacteria appear tolerant to elevated CO2. Phytoplankton studied as part of a natural microbial community were found to be more
sensitive than those studied as a single species in culture. This highlights the importance of community and ecosystem level studies, which incorporate the interaction and competition among species and trophic levels, to accurately assess the effects of ocean acidification on the Antarctic ecosystem.

Antarctic marine microbes (comprising phytoplankton, protozoa and bacteria) drive ocean productivity, nutrient cycling and mediate trophodynamics and the biological pump. While they appear vulnerable to changes in ocean chemistry, little is known about the nature and magnitude of their responses to ocean acidification, especially for natural communities. To address this lack of information, a six level, dose-response ocean acidification experiment was conducted in Prydz Bay, East Antarctica, using 650 L incubation tanks (minicosms). The minicosms were filled with Antarctic nearshore water and adjusted to a gradient of carbon dioxide (CO2) from 343 to 1641 μatm. Microscopy
and phylogenetic marker gene sequence analysis found the microbial community
composition altered at CO2 levels above approximately 1000 μatm. The CO2-
induced responses of microeukaryotes (>20 μm) and nanoeukaryotes (2 to 20 μm) were taxon-specific. For diatoms the response of taxa was related to cell size with micro-sized diatoms (>20 μm) increasing in abundance with moderate CO2 (506 to 634 μatm), while above this level their abundance declined. In contrast, nano-size diatoms (<20 μm) tolerated elevated CO2. Like large diatoms, Phaeocystis antarctica increased in abundance between 343 to 634 μatm CO2 but fell at higher levels. 18S and 16S rDNA sequencing showed that picoeukaryotic and prokaryotic composition was unaffected by CO2, despite having higher abundances at CO2 levels !634 μatm. This was likely due to the lower abundance of heterotrophic nanoflagellates at CO2 levels exceeding 953 μatm, which reduced the top-down control of their pico- and nanoplanktonic prey. As a result of the differences in the tolerance of individual taxa/size categories, CO2 caused a
significant change in the microbial community structure to one dominated by nano-sized diatoms, picoeukaryotes and prokaryotes.

Based on the CO2-induced changes in the microbial community, modelling was performed to investigate the future effects of different levels of elevated CO2 on the structure and function of microbial communities in Antarctic coastal systems. These models indicate CO2 levels predicted toward the end of the century under a “business as usual scenario” elicit changes in microbial composition, significantly altering trophodynamic pathways, reducing energy transfer to higher trophic levels and favouring respiration of carbon within the microbial loop. Such responses would alter elemental cycles, jeopardise the productivity that underpins the wealth and diversity of life for which Antarctica is renowned. In addition, it would reduce carbon sequestration in coastal Antarctic waters thereby having a positive feedback on global climate change.

Hancock A. M., 2019. Effects of ocean acidification on Antarctic microbial communities. PhD thesis, University of Tasmania. Thesis (restricted access).

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