| Summary: | The ecology of tundra terrestrial environments has gained increasing attention due to potential major changes resulting from global warming and climate change. However, the composition of terrestrial microbial communities and their role in the biogeochemical carbon cycle are less well studied. This is the first report of the C-biomass of bacteria, fungi, and representative protozoa (heterotrophic nanoflagellates, naked amoebae, and testate amoebae) in Alaskan tundra soil samples, and the effects of glucose solution enrichment in laboratory studies simulating release of soluble organic compounds as may occur during permafrost melt and increased plant root exudates due to global warming. The data for three moss-rich surface samples, two in spring and one in summer (2011), are reported for C-supplemented (8,000 μg glucose-C) and non-supplemented treatments in laboratory culture. Seven days after supplementation, fungal C-biomass in the glucose-treated and untreated samples were similar in the range of 5 to 11 mg g–1 soil dry weight, the highest values in the summer samples. The bacterial C-biomass was the next highest in the range of 20 to 120 μg g–1 soil dry weight, followed by heterotrophic nanoflagellates (2 to 14 μg g–1 soil dry weight). The naked amoebae (0.13 to 0.94 μg C g–1 soil dry weight) and testate amoebae (2 to 20 ng C g–1 soil dry weight) contributed the least C-biomass. All of the bacterial and protozoan treatments showed increased biomass with glucose supplementation. Based on size, and C-biomass estimates, the phagotrophic protozoa appear to be organized in a classical bacterial-based trophic hierarchy (i.e. bacteria – nanoflagellates – naked amoebae – testate amoebae, in ascending order). Correlations of the C-biomass of bacteria to each of the protozoa, provided further evidence of a trophic pyramid; bacteria vs. nanoflagellates (r = –0.986), indicating top-down control by predatory flagellates, bacteria vs. naked amoebae (r = –0.361) and bacteria vs. testate amoebae (r = –0.131), each of decreasing magnitude as would be predicted for higher level consumers. Estimates of bacterial predation indicated strong predatory pressures on bacteria by the protozoa, greater with C-supplementation compared to the non-supplemented treatments.
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