Abstract:
Productivity in the Southern Ocean reflects both the spatial and temporal dynamics of
the sea ice ecosystem, as well as the complex cycling of energy through the microbial
community. Marine bacteria are thought to be integral to trophodynamics and the
functioning of a microbial loop within the ice matrix, but there is no clear
understanding of the distribution and diversity of bacteria or the importance of
bacterial production. Understanding the bacterial response to environmental change in
the sea ice ecosystem may provide an insight into the potential changes to the physical
oceanography and ecology of the Southern Ocean.
In this study, a multivariate statistical approach was used to compare the distribution
and abundance of bacteria occurring in pack ice at the tongue of the Mertz Glacier
(George V Coast, Antarctica) with bacteria from fast ice at Cape Hallett (Victoria
Land coastline, Antarctica). Estimates of bacterial abundance were derived using both
epifluorescence microscopy and flow cytometry and correlated with algal and
chlorophyll a data. Significant differences in the vertical distribution of cells within
the ice were observed between the Mertz Glacier and Cape Hallett, but no overall
difference in cell abundance was found between the two locations with 7.6 ± 1.2 x 109
cells per m2 and 8.7 ± 1.6 x 109 cells per m2 respectively. Bacteria and algae were
positively correlated in pack ice of the Mertz Glacier indicating a functional microbial
loop, but no discernable relationship was exhibited in multiyear ice at Cape Hallett.
These findings support the general consensus that the generation of bacterial biomass
from algal-derived dissolved organic matter is highly variable across seasons and
habitats. The tetrazolium salt 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was used to
investigate the bacterial response to experimentally induced changes in light and
salinity in fast ice at Cape Hallett. Two distinct assemblages were examined; the brine
channel assemblage near the surface of the ice and the interstitial or bottom
assemblage. This study presents preliminary evidence that the metabolic activity of
brine bacteria is influenced by light stimulus, most likely as a response to increased
levels of algal-derived dissolved organic matter. No cells were deemed to be
metabolically active when incubated in the dark, while on average thirty-eight
percent of the cells incubated at 150 =mol photons m-2 s-1 were metabolically active.
Additional results indicate that salt concentration is more significant than light
irradiance in influencing the metabolic response of cells present in the interstitial
region of the sea ice profile. When acclimated over a period of eight hours, cells
exhibited a tolerance to changing saline concentrations, but after a further eight hours
there is some evidence to suggest activity is reduced at either end of the saline regime.
Bacterial metabolic activity in each assemblage is thus thought to reflect the
fundamentally different light and saline environments within the sea ice. Metabolic
probes such as CTC will prove useful in providing a mechanistic understanding of
productivity and trophodynamics in the Antarctic coastal ecosystem, and may
contribute to prognostic models for qualifying the resilience of the microbial
community to climate change.
