Dynamics and interactions of the microbial communities in Lake Geneva with a dominant viral axis S Personnic1, I Domaizon2, A Millery2, S Jacquet1 1. INRA, Equipe de Microbiologie Aquatique, BP 511, 74 203 Thonon Cedex, France 2. Université de Savoie , Equipe de Microbiologie Aquatique, 73376 Le Bourget du Lac cedex - France Introduction: It is now well established that viruses play a crucial role in the regulation of aquatic microbial populations/communities. Viruses are the most abundant particles in both marine and freshwater environments (with typically 107-109 part.ml-1), and they exert an important control on bacterial population dynamics and diversity. There is a general agreement in saying that viral studies are fundamental to a better comprehension of the microbial food web. Complete microbial communities studies are required to understand the dynamics and the role of the viruses at the basis of the microbial food webs. In this study, we investigated for the year 2004, between 0 and 50 m of Lake Geneva, the distribution of virioplankton, bacterioplankton, protozoan (ciliates, flagellates) and small phytoplancton (PE-rich picocynaobacteria and small eukaryotes) using flow cytometry (FCM) and epifluorescence microscopy (EFM). Some in situ experiments were performed in order to assess flagellate protozoan and virus-induced mortality of heterotrophic bacteria. Our results reveal that the flagellates and the viruses could be responsible for between 31 and 42% of the total daily mortality of heterotrophic bacteria in this lake. In May 2004, viruses could explain up to 10% of the mortality of the bacteria whereas flagellates were responsible for 32% of bacterioplankton removal. A comparison with an experiment made in Lake Bourget is also provided.
Microbial community dynamics Heterotrophic bacteria display 2 peaks of cells abundance in April and September. Virus-like particles 1 (VLP1) appear to be well correlated with the second peak of the heterotrophic bacteria suggesting a possible and visible control of VLP1 on the bacterial compartment.
10
DEPTH (m)
Heterotrophi c Bacteria
VLP2
VLP1
Heterotrophic Bacteria
50 Mar
May
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Sep
Heterotrophic Bacteria Viruses Zooplankton Protozoa n flagellate Higher Predators
Protozoa n ciliates
Viruses
Fig 1. Simplified view of the aquatic microbial loop. Viruses may affect each microbial compartment
VLP1
Small Eukaryotes May
Jul
Sep
May
Jul
Sep
Flagellates Total Mar
May
Jul
Sep
50 Mar
May
Jul
Sep
Nov
20
30
2.0e+5
40
8.0e+5 1.0e+6
4.0e+5 6.0e+5
VLP3
1.2e+6
Mar
May
Jul
Sep
30 40
2e+6 4e+6 6e+6 8e+6 1e+7
VLP2
50 Mar
Nov
May
Jul
Sep
Nov
Date
10 0 500 1000 1500 2000 2500 3000 3500 4000
PE-rich picoyanobacteria
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Nov
30
40
Nov
50 Mar
0 2e+4 4e+4 6e+4 8e+4 1e+5
30 0 10 20 30 40
20
50
50
40
Mar
2.0e+7 4.0e+7 6.0e+7 8.0e+7 1.0e+8 1.2e+8
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Nov
10
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Cyanobacteria
DOM
20 1e+6 2e+6 3e+6 4e+6 5e+6 6e+6 7e+6
10
Small Eukaryotes
Microbial loop Phytoplankton (APP)
10
20
40
Fig 2. Typical FCM signatures of the picocyanobacterial, bacterial and viral communities
Allochtonous DOM
Microbial Community Dynamics in surface waters of Lake Geneva in 2004 30
VLP3
Viruses
5 10 15 20 25
20 30 40
30 35 40 45
Ciliates
50 Mar
Nov
Date
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Date
The small autotrophic eukaryotes peaked in April, May and September. Although it is not detailed here, these peaks corresponded to different populations. High concentrations of VLP2 and VLP3 were also recorded in May and September, suggesting these groups could be associated to small phytoplankters. Interestingly, this was already shown elsewhere (Larsen et al. 2003). In April and in September, we observed an important increase of the ciliate group, clearly associated to the bloom of the small eukaryotes. Picoyanobacterial biomass was important in summer and it is likely that this community was controlled by heterotrophic and ciliate grazing as well as VLP2 and/or VLP3. We are currently analyzing these data to go further in the description and the comprehension of the microbial dynamics of Lake Geneva.
Fig 3. Distribution of the PE-rich picocyanobacteria, small eukaryotes, heterotrophic bacteria, flagellates Total (Pigmented and heterotrophic), ciliates and viruses between 0 and 50 meters in Lake Geneva from January to December 2004. Data were obtained with a FACSCalibur FCM and concentrations are expressed in cells or particles per ml-1
Lytic activities Experiments using microcosms incubated in situ in the lake have been carried out in 2003 and 2004 (Fig. 4) with the goal to estimate effects of viral enrichment on bacterial mortality and diversity, to calculate viral production (using production inhibitor), and to evaluate the % of mortality due to lytic activity (from dilution experiments, see below). Using TEM, parameters such as the burst size (BS, numbers of viruses liberated by lytic events) and the % of bacterial cells visibly infected by viruses (FVIC), we can access to the % of bacterial cells infected (FIC) and in fine the % of bacterial production removal due to viral lysis (VIBM). We observed that 9.5 to 60 % of the heterotrophic bacteria mortality could be attributed to viral induced lysis. Flagellates were responsible of 18 to 63 % mortality through grazing of the bacterial community.
Microcosms . Dialysis Bags . Bottles
Heterotrophic Bacteria Mortality
Some of these results will be published in a forthcoming issue of Freshwater Biology
Fig 4. Experiences microcosms is situ
Viral lyses
Flagellates Grazing
EXP. Lake Geneva May 2004
9.5 %.d-1
32 %.d-1
EXP. Lake Le Bourget April 2003
* 60 %.d-1
56 %.d-1
EXP. Lake Le Bourget May 2003
* 35 %.d
-1
63 %.d-1
EXP. Lake Le Bourget August 2003
* 52 %.d-1
18 %.d-1
in
Table 1. Results of 4 experiments held in Lake Geneva and in Lake Bourget indicating the role of viruses and flagellates in driving 9-34 11.51% heterotrophic bacterial mortality. Burst size and % of 10-30 15.70% infected cells obtained with TEM have also been reported BS
FIC
6-14
8.30%
* In predator free enclosures
Conclusion: We know almost nothing on aquatic microbial ecology of French pre-alpine lakes. Our study is the first dealing with the dynamics of the different biological entities at the basis of the food network in surface waters of Lake Geneva. It clearly revealed possible relationships between the different groups in terms of predator-prey or parasiteprey relationships. In the future, it will be critical to assess the diversity among these different microbial communities and how each of them are regulated by their biological, chemical and physical environment throughout the year. New experiments will be performed to assess the relative impact of predators and viruses on both bacterial and picocyanobacterial mortality and diversity. Acknowledgments: Thanks to U Dorigo, T Sime-Ngando and S Duhamel for their collaboration to this work. SP is funded by a French Ministry fellowship. FCM has been funded by INRA and University of Savoie
