Abstract
The effect of phytoplankton on the other compartments of a simple microbial loop con- sisting 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). Dur- ing 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 hetero- trophic 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 differ- ences in the resident phytoplankton populations would be reflected in (1) the composition and con- centration 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 hetero- trophic 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 dur- ing the second half of the experiment. Differences in biochemical cycling between mesocosms were predominantly driven by the large quantitative differences in autotrophs.
Highlights
About 20 yr ago, the central role of bacteria and flagellates in many marine pelagic systems was recognized
heterotrophic nanoflagellates (HNAN) had mostly been removed by the pre-filtration step, but a small inoculum remained, growing to significant concentrations by Days 8 to 10 in all 3 mesocosms (Fig. 1b)
The overarching goal of this mesocosm study was to characterize the effect of phytoplankton on the associated microbial food webs
Summary
About 20 yr ago, the central role of bacteria and flagellates in many marine pelagic systems was recognized. Bacterial activity and community composition, for example, modify and are modified by the quality and quantity of dissolved organic matter (DOM), which, in turn, is a result of DOM release, which is a function of phytoplankton composition, growth and exudation (Pinhassi et al 1999, Riemann et al 2000, Castberg et al 2001, Fandino et al 2001, Schäfer et al 2001), grazing (2imek et al 1997, 2001, Lebaron et al 1999), and viral lysis (Fuhrman 1999). These links between organic carbon, bacteria (used here to encompass Bacteria and Archaea), viruses, and protists mean that the species composition and physiological state of the autotrophic community in a given system may profoundly affect the bacterial and protist communities. Controlled experiments that would be required to study the effect of phytoplankton on the microbial loop are impossible in the field due to many potentially confounding physical and biological factors: the patch under investigation might, for example, be diluted due to mixing or advection, or the arrival of swimmers may change the food web
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