Chair: Ken Caldeira

Alyce Hancock1,2, Andrew Davidson2, Rick van den Enden2, Kai Schulz3 and Andrew McMinn1

1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, 7001, Australia

2Australian Antarctic Division, Kingston, Tasmania, 7050, Australia

3Southern Cross University, East Lismore, NSW, 2480, Australia

Background:

Marine microbes support the vast wealth of Antarctic life, underpinning fisheries productivity and contributing significantly to the mediation of global climate.  Their small size, lack of protection and rapid growth mean marine microbes are vulnerable to changes in ocean chemistry and likely to be sensitive indicators of CO2-induced stress. Antarctic waters are amongst the most vulnerable to ocean acidification however the effects of increased CO2 on Antarctic marine microbes are poorly understood.

Methods:

Seawater off Davis Station, Antarctica was incubated in 6 x 650 l polythene tanks (minicosms).  The fCO2 concentration in the tanks was adjusted to values from 343 (ambient) to 1641 µatm.   Samples were periodically collected during the incubation to determine the composition and abundance of microbes via light and electron microscopy.

Findings:

Different taxa responded differently to changes in fCO2. At ambient/low fCO2 levels (343-507ppm) the community had a high diversity of both small and large cells. Relative to the low fCO2  control, treatments exposed to high fCO2  (953-1641 ppm) the abundance of large cells, mainly centric diatoms, declined and the community became dominated by small pennate species. Response curves of the various phytoplankton differ among species, driving changes in community composition and total phytoplankton abundance. Overall the promotion of small cells occurred at lower fCO2 than the inhibition of large taxa, resulting in the net promotion of phytoplankton abundance and biomass at intermediate fCO2  concentrations (506-

953ppm).

Conclusions:

The results from this experiment provide insights potential changes to marine microbial communities in future ocean conditions; changes that may alter biogeochemistry and biogeochemistry in Antarctic waters beyond the end of this century.