AME 49:109-121 (2007)  -  doi:10.3354/ame01138

Microbial dynamics in autotrophic and heterotrophic seawater mesocosms. I. Effect of phytoplankton on the microbial loop

Uta Passow1,*, Christina L. De La Rocha1, Carol Arnosti2, Hans-Peter Grossart3, Alison E. Murray4, Anja Engel1

1Alfred-Wegener-Institut für Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany
2Department of Marine Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3300, USA
3Institute of Freshwater Ecology and Inland Fisheries, Dept. of Limnology of Stratified Lakes, Alte Fischerhütte 2, 16775 Stechlin, Germany
4Division of Earth and Ecosystem Science, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA

ABSTRACT: The effect of phytoplankton on the other compartments of a simple microbial loop consisting of dissolved matter, bacteria and nanoflagellates was investigated in three 1000 l mesocosms. These mesocosms were inoculated with a natural bacterial community and (1) no other additions (No Addition tank), (2) Phaeocystis globosa (Phaeocystis tank), or (3) 2 diatom species (Diatom tank). During the 20 d experiment, autotrophic activity was negligible in the No Addition tank. In contrast a small P. globosa bloom developed in the Phaeocystis tank and a large diatom bloom dominated the Diatom tank. In this paper we describe the experiment, the changes in chlorophyll a and heterotrophic nanoflagellate concentrations, as well as the cycling of nitrogen, phosphorus, and silica. Then we provide a synthesis of the structure and functioning of the microbial loops in these 3 systems using cluster analysis, a statistical pattern recognition tool. The goal was to test the hypothesis that differences in the resident phytoplankton populations would be reflected in (1) the composition and concentration of dissolved organic matter, (2) the composition of the bacterial community, (3) the food web, and (4) the cycling of elements and organic matter. In all 3 mesocosms, nitrate and silicic acid remained abundant. Orthophosphate was preferred by diatoms, whereas Phaeocystis appeared to prefer dissolved organic phosphorus. The hypothesis that phytoplankton composition shapes the structure and functioning of the microbial loop was partially supported: 6 d after inoculation each mesocosm exhibited a distinct organic matter signature. After 10 to 12 d, concentrations of heterotrophic nanoflagellates were high enough to exert significant grazing pressure in all 3 mesocosms. A parallel shift in bacterial community composition was visible in all mesocosms at this time, possibly reflecting grazing pressure. The food-web structure developed divergently in the 3 mesocosms during the second half of the experiment. Differences in biochemical cycling between mesocosms were predominantly driven by the large quantitative differences in autotrophs.


KEY WORDS: Mesocosm · Microbial loop · Structure and function · Food-web structure · Cluster analysis


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Cite this article as: Passow U, La Rocha CLD, Arnosti C, Grossart H, Murray AE, Engel A (2007) Microbial dynamics in autotrophic and heterotrophic seawater mesocosms. I. Effect of phytoplankton on the microbial loop. Aquat Microb Ecol 49:109-121

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