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[1] Viruses are an abundant and dynamic constituent of microbial communities in aquatic ecosystems. In this study we characterized the abundance of viruses associated first with the bottom sediment and overlying water of cryoconite holes and second with shallow ice cores of two different glaciers in Svalbard. Viral abundances were ca. 10-100 times lower than the average for marine and freshwater ecosystems in temperate regions. Virus to bacterium ratios (VBR) (average > 10, range between 0.7 and 74 in the water and ice samples) and a strong positive correlation between viral and bacterial abundance (r = 0.93, p < 0.01, N = 57) indicate that viruses most probably play an important role in controlling bacterial mortality and hence biogeochemical cycling on glaciers. Samples taken along a transect from the glacier ablation area to proglacial ponds in its forefield showed that viral abundance increased in response to a higher host availability, which in turn probably resulted from an increase in temperature and higher mineral levels in the ponds. In a transplantation experiment, viruses from cryoconite holes were incubated with a bacterial community from a proglacial lake. Results from the transplantation experiment showed that viruses from cryoconite holes were able to infect bacteria from proglacial lakes and thus influence biogeochemical cycles across different glacial ecosystems. Our data therefore suggest that viruses in cryoconite holes may be able to infect a broad range of bacterial species.

Bacterial production in glacial runoff and aquatic habitats along a c. 500 m transect from the ablation area of a Svalbard glacier (Midre Lovénbreen, 791N, 121E) down to a series of proglacial lakes in its forefield were assessed. In addition, a series of in situ experiments were conducted to test how different nutrient sources (glacial flour and dissolved organic matter derived from goose faeces) and temperature affect bacterial abundance and production in these ecosystems. Bacterial abundance and production increased significantly along this transect and reached a maximum in the proglacial lakes. Bacterial diversity profiles as assessed by denaturing gradient gel electrophoresis indicated that communities in glacial runoff were different from those in proglacial lakes. Heterotrophic bacterial production was mainly controlled by temperature and phosphorus limitation. Addition of both glacial flour and dissolved organic matter derived from goose faeces stimulated bacterial production in those lakes. The results suggest that glacial runoff sustains an active bacterial community which is further stimulated in proglacial lakes by higher temperatures and nutrient inputs from bird faeces. Thus, as in maritime temperate and Antarctic settings, bacterial communities developing in the recently deglaciated terrain of Svalbard receive important inputs of nutrients via faunal transfers from adjacent ecosystems.

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The effect of different phosphorus loads (L P ) on the phosphorus (P) and carbon (C) content (biomass) of algae and bacteria was assessed in continuous culture. We tested if a mixed freshwater microbial assemblage co-cultured with a phytoflagellate (Cryptomonas phaseolus) would comply with the 'phytoplankton-bacteria paradox ' (sensu Bratbak & Thingstad 1985). This hypothesis states that the ratio of bacterial to algal abundance changes to the benefit of bacteria with decreasing L P . However, the phenomenon was originally investigated by simultaneously altering L P and microbial growth rates, and it is unclear to which extent it can be assigned to either parameter. Therefore, we set up 3 chemostat systems in triplicate at equal dilution rates, but with daily L P of 21, 41 or 62 µg l -1 d -1 (corresponding to 50, 100 and 150 µg P l -1 ). Higher L P led to a 5-fold increase in total algal abundance and biomass but to less than a doubling of these parameters in the bacterial assemblage. Total biomass ratios of bacteria to algae changed from 0.18 to 0.06 with increasing L P , while the bacteria-algae total phosphorus ratios decreased from 0.80 to 0.17. The cellular C:P ratio of algae remained similar at all P concentrations, whereas the molar C:P ratios of bacterial cells significantly increased at higher L P (from 44 to 73). An enrichment experiment with the 50 µg P l -1 treatment demonstrated that bacteria at the lowest L P were co-limited by P and C, and that increased P stimulated mainly the algal fraction. The phytoplankton-bacteria paradox at the level of a mixed microbial assemblage is thus characterised by the following aspects: (1) bacteria profit from their high affinity to P and are better competitors at lower L P ; (2) although algae compete with bacteria for P, P-limited algae release extracellular C that stimulates growth of their bacterial competitors; (3) when bacteria depend on algae as their sole source of organic C, this provides a feedback mechanism by which algae limit the abundance of their competitors at higher L P ; (4) large oscillations in the bacteria-algae ratios at the lowest L P point to a greater instability of this interaction with stronger P competition. However, bacteria were not able to outcompete C. phaseolus, as algae were their only C source.KEY WORDS: Algae-bacteria interaction · Chemostat · Continuous cultivation system · Cryptomonas phaseolus · Eutrophication · Phosphorus · Phytoplankton-bacteria paradox Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 38: [203][204][205][206][207][208][209][210][211][212][213] 2005 nutrients, such as nitrogen or phosphorus, the relationship between bacteria and algae is not considered mutualistic, but competitive (Currie & Kalff 1984b, Elser et al. 1995, Vadstein 2000.The competition for P between bacteria and algae cannot easily be studied in situ, i.e. in a planktonic food web of high complexity. Continuous cultivation systems have the advantage that algae and bacteria experience...