Poster ID 1550 12/2006
Microbial Metabolic Activity and Bioavailability of Dissolved Organic Matter Under the Impact of Intense UV Radiation in Pony Lake, Antarctica
Center for Biofilm Engineering
M. Dieser1, C.M. Foreman1, D. McKnight2, P. Miller3, Y.-P. Chin4 1Montana State University, 2University of Colorado, 3Rose-Hulman Institute of Technology, 4The Ohio State University a National Science Foundation Engineering Research Center in the MSU College of Engineering
ABSTRACT
METHODOLOGY
McMurdo Sound area of Antarctica. This shallow lake is ice-covered except in midsummer, when strong winds typically cause thorough mixing of the water column. The source of water appears to be accumulated snow; water is lost by ablation of the ice cover and evaporation of lake water in midsummer. In the west the pond is bordered by an Adelie penguin rookery. Previous studies have shown that Pony Lake can have very high dissolved organic carbon (DOC) concentrations (~100 mg C liter-1). Furthermore, Pony Lake is unique because it lacks terrestrial carbon inputs in the watershed, which makes this an excellent example of a system containing autochthonous, microbially (algae, cyanobacteria, bacteria and viruses) derived organic matter. Over two field seasons we have investigated the influence of photolytic processes on the microbial utilization of DOM from Pony Lake, Antarctica. We have determined that the intense ultraviolet radiation in Antarctica rapidly photo-bleaches DOM, resulting in the loss of UV absorbing compounds, and rendering fractions of the DOM pool less biologically available to microbes. We monitored microbial community structure, abundance and primary and secondary production over the austral summer as the lake transitioned from fully ice covered to ice free. Bacterial numbers ranged from 1.10 x 106 – 4.09 x 107 cells ml-1 in ice core samples and from 2.15 x 105 to 3.22 x 107 cell ml-1 in ice free Pony Lake due to seasonal shifts in melting as well as alterations in the DOM pool. Secondary production was higher in the ice free lake compared to the ice core samples. 24 h exposure experiments of natural bacterial assemblages to in situ sunlight demonstrated that tritiated thymidine incorporation was significantly less than compared to dark controls. Our findings indicate that the intense solar radiation down in Antarctica affects the microbial community, alters DOM bioavailability and composition, and that the microorganisms are not only overwintering in the frozen ice cover of Pony Lake, they are metabolically active.
SITE
1
Samples were collected during the austral summer seasons 2004/05 and 2005/06. Our research team chose three time points to sample the transition from an icecovered to partially ice-free lake: First, early November (lake is solid frozen, bacteria are encased in ice), second, mid-December (ice cover is thinning), and third, mid-January (lake is partially ice-free). With this sample strategy and applying standardized microbial analyses and specially designed experiments we hope to construct seasonal profiles as well as to take into account the importance of icecover in controlling the lake biogeochemistry.
ICE
Ice Core
BP
Abundance
PPR
[cells d-1]
[cells ml-1]
[µg C l-1 d-1]
Table 1. Temporal trends in selected Pony Lake parameters during 2004/05 as the lake transitioned from fully frozen to open water. For additional comparison icecore analyses for 2005/06 are added. [Bacterial production (BP), Primary production (PPR)]
2004 Top
6.08E+08
2.20E+06
931
Middle
3.44E+09
1.10E+06
306
Bottom
1.64E+08
2.97E+07
31
Top
2.13E+08
2.13E+07
31
Middle
1.95E+08
4.09E+07
15
Bottom
1.95E+08
2.40E+07
40
2005
Lake 11. Dec 04
2.52E+09
4.05E+05
668
21. Dec 04
2.77E+09
2.15E+05
2658
29. Dec 04
1.79E+09
3.14E+05
1490
15. Jan 05
1.85E+09
1.36E+06
1232
PIGMENTS
1e+10 PLP t=0 PLP t=24 P PLP t=24 UP
1e+9
pigmented Bacs (5)
H2O2 Labile photoproducts
OH• Refractory DOM
MICROBIAL LOOP
CFU [cells ml-1]
108 107 106
non-pigmented Bacs (3)
105 103 E.coli
100 0
1
2
6
10 15 20 30 40 50 70 100 Cycles
Figure 5: Response of bacterial isolates to freeze-thaw cycles. Both controls, Escherichia coli and Serratia marscens, died after 40 and 50 cycles respectively. Our pigmented isolates showed some variability, but in general survivability was unaffected by the freeze-thaw regime. In comparison, the non-pigmented isolates decreased three orders of magnitude within the first 20 cycles and then leveled off.
Figure 10: Bacterial diversity was assayed using 16S rDNA PCR. Amplicons were analyzed by denaturing gradient gel electrophoresis (DGGE). Differences occurred between the ice-core samples from consecutive seasons as well as during the transition period from an ice covered to partially ice free lake.
General summary of site conditions: 1e+6
Ice-
1e+5
2004: Ice core samples collected represent refrozen lake water in the form of annual year ice. 2005: Samples collected represent multi-year ice.
1e+11 2004/2005
2005/2006
1e+10
UP t=0 P t=0 UP t=12 P t=12
1e+9 1e+8 1e+7 1e+6 WW XAD WW Source
SRFA
Serratia
102
left to right) 1: Top core 2004 Middle Core 2004 Bottom Core 2004 2: Top core 2005 Middle Core 2005 Bottom Core 2005 3: Lake: 11.12.04 Lake: 21.12.04 Lake: 29.12.04 Lake: 14.01.05
1e+7
WW
104
101
Figure 2: Scheme showing direct (yellow) and indirect (red) photochemical processes that can degrade dissolved organic matter (DOM) into labile photoproducts or modify DOM into refractory DOM. Indirect processes are often mediated by reactive oxygen species (ROS), such as H2O2, hydroxyl radical (OH), and superoxide (O2-). These DOM fractions are important to the microbial food web.
Bacterial Production [cells d-1]
Figure 4: Microbes from icy environments must be able to withstand dramatic thermal and radiative fluxes on varying timescales. In situ these organisms survive daily and annual freeze-thaw cycles, and exposure to high levels of UV radiation. To examine the potential role of bacterial pigments as cryo- protectants, pigmented and non-pigmented isolates (shown above) from Antarctic ecosystems were exposed to 100 six hour freeze-thaw cycles.
(From
CONCLUSIONS
2004/2005 2005/2006 Season
109
DOM
WATER
1e+8
Figure 8: Bacterial productivity was measured via the incorporation of tritiated thymidine into DNA. Pony Lake water was photolyzed for ~24hr and bacterial productivity assayed. After 24hr productivity in the light exposed samples was significantly less than in the dark controls, demonstrating that photolysis inhibited microbial production. P= photolyzed UP= unphotolyzed samples.
PHOTOCHEMICAL REACTIONS
ICE
3
Figure 7: In situ solar radiation produces an effect on the lake DOM due to direct and indirect photochemical processes. To measure the effects of UV radiation on DOM alterations and consequently bacterial production, Pony Lake water was subject to photolysis in the Antarctic sun for ~24hrs. Dark controls were stored underneath the photoreactors. Light (UV and PAR) were monitored over the course of the experiments along with chemical actinometers.
Ross Island
Figure 1: Map of Antarctica showing the location of Cape Royds, the site of Pony Lake.
2
METABOLIC ACTIVITY
Bacterial Production [Cells d-1]
Pony Lake is a saline and hypereutrophic coastal pond located on Cape Royds in the
BIODIVERSITY
PHOTOLYSIS
Figure 9: In this experiment bacteria-free water was photolyzed for ~24hrs, then Pony Lake bacteria were reintroduced. Bacterial productivity was assayed after a 12 day incubation period. The results from 2004/2005 demonstrate that the decrease in bioavailability was directly related to changes in the DOM, and was not just due to damage to bacteria exposed to the sunlight. Experiments during the season 2005/06 did not show as clear of an effect. This difference appears to be related to a three fold decrease in UV intensity during the second season compared to the first.
Lake Water2004: The lake transitioned from fully frozen to partially open water. 2005: Only a small area along the lake edge melted by mid-late January. 2005: There was a 3 fold decrease in UV radiation compared to the previous year.
Conclusions: • Temporal differences exist in microbial abundance and production as Pony Lake transitions from being completely frozen to open water, as well as between annual and multi-year ice. • Investigations indicate that microorganisms are not only over wintering in the frozen ice cover of Pony Lake, they are metabolically active. • Pigmentation seems to afford an advantage to bacteria isolated from Antarctic environments to withstand freeze-thaw-cycles. • Intense in situ UV-radiation decreases bacterial activity. • Photolyzing DOM renders it less bioavailable. • Community composition differs between annual and multiyear ice. • Community composition also changes during the transition period from a fully ice covered to a partially open lake during the austral Antarctic summer. ACKNOWLEDGEMENTS: Support for this project was provided by a grant from the National Science Foundation’s Office of Polar Programs (OPP-0338342). Logistical support was made available by Raytheon Polar Services and Petroleum Helicopters Inc. We are grateful for the help of our colleagues: C.L. Jaros, R. Cory, K. Cawley, J. Guerard, R.L. Fimmen, and D. Rosenberger.
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