Geomicrobiology Journal, 21:45–53, 2004 Copyright C  Taylor & Francis Inc. ISSN: 0149-0451 print / 1362-3087 online DOI: 10.1080/01490450490253455 The Role of Autotrophic Picocyanobacteria in Calcite Precipitation in an Oligotrophic Lake Maria Dittrich, 1 Philipp Kurz, 2 and Bernhard Wehrli 1 1 Swiss Federal Institute for Environmental Science and Technology, EAWAG, Limnological Research Center, Kastanienbaum, Switzerland 2 University of Zurich, Zurich, Switzerland A 1-year field study monitoring depth profiles of picoplank- ton and physicochemical data in the oligotrophic Lake Lucerne (Switzerland) showed that picocyanobacteria play an important role in the CaCO 3 precipitation process. Laboratory experiments with Mychonastes and Chlorella, isolated from Lake Lucerne and Synechococcus using ion selective electrodes, scanning electron mi- croscopy and X-ray powder diffraction clearly demonstrated the potential of picoplankton for fast and effective CaCO 3 precipita- tion. The combination of a field study with laboratory experiments confirmed the previous reports of picocyanobacteria triggering the CaCO 3 precipitation in hardwater oligotrophic lakes. Electron mi- crographs of particles from the water column often reveal the size and shape of picoplankton cells covered by calcite. In addi- tion the results from the laboratory approach indicated that algae and bacteria induced different precipitation mechanisms. Experi- ments with Mychonastes and Chlorella produced crystalline calcite completely covering the cells. Experiments with the cyanobacte- ria Synechococcus, however, yielded amorphous, micritic CaCO 3 , indicating a different precipitation process. Keywords ion selective electrodes, calcite precipitation, oligotrophic lake, picocyanobacteria INTRODUCTION The unicellular autotrophic picoplankton (APP) is an ubiqui- tous component of pelagic ecosystems (Stockner, Callieri, and Received 4 October 2002; accepted 3 April 2003. We thank Erwin Sch¨ affer and Bia Mobwenemo Mampasi for their technical support during our fieldwork, Hansrudolf B ¨ urgi and Heinrich B¨ uhrer for information and data on Lake Lucerne, Christian Dinkel and Beat M¨ uller for their know-how of ion selective electrode measure- ments, Daniel Kobler for assistance with raster electron microscopy, Cristina Callieri and Pieter Vischer for comments on an earlier draft and especially Ren´ eG¨ achter for critically reading the manuscript. Crit- ical and constructive comments from three reviewers helped to improve this article. Address correspondence to Maria Dittrich, EAWAG, Limnologi- cal Research Center, CH-6047, Kastanienbaum, Switzerland. E-mail: dittrich@eawag.ch Cronberg 2000) that dominates the total phytoplankton biomass and production in both oligotrophic lakes and oceans (Weisse 1993), but has often been overlooked due to its small cell size of 0.2 μm to 2 μm in diameter. Considerable efforts have been made to study the ecology and population dynamics of APP (Weisse 1993; Stockner et al. 2000). Cyanobacteria dominate the picoplankton community in most oligotrophic systems and are often more abundant than the larger components of the phy- toplankton (Weisse 1993; Agawin et al. 2000; Stockner et al. 2000; Bell and Kalff 2001). Relatively little is known about the interaction between APP and abiotic processes in ecosystems. Both laboratory experi- ments (Thompson and Ferris 1990; Schultze-Lam et al. 1992; Yates and Robbins 1999; Merz-Preiss 2000) and field observa- tions (Robbins et al. 1996; Thompson et al. 1997; Hodell et al. 1998) suggest that APP may play an important role in calcite precipitation. Because of the small cell size and its high abun- dance picoplankton provides large surface areas for adsorption and heterogeneous nucleation. However, so far no laboratory experiments were performed to investigate these biogenic precipitation processes over time, or to compare precipitation mechanisms induced by two types of picoplankton, eukaryotic green phytoplankton and prokary- otic cyanobacteria. In the present paper we combine field ob- servations with laboratory experiments in order to correlate spa- tial and temporal patterns of picoplankton abundance and calcite precipitation in lake and to provide mechanistic infor- mation about calcite precipitation triggered by different kind of APP. METHODS Study Site, Sampling, and Chemical Analysis Lake Lucerne is an oligotrophic hardwater lake in Central Switzerland that has been studied intensively from 1961 to 1992 (B¨ uhrer and Amb ¨ uhl 1996). At a station located at 47 ◦ 00 ′ 36.8 ′′ N/ 8 ◦ 25 ′ 58.5 ′′ E we collected water samples from five depths (0, 5, 45