Aquatic Toxicology 83 (2007) 223–237 Planktonic microbial community responses to added copper Anne-H´ el` ene Le Jeune a,b , Marie Charpin a,∗ , Denis Sargos a , Jean-Franc ¸ois Lenain b , V´ eronique Deluchat b , Nadine Ngayila b , Michel Baudu b , Christian Amblard a a UMR CNRS 6023, Laboratoire de Biologie des Protistes, Universit´ e Clermont-Ferrand II, 24, avenue des Landais, 63177 Aubi` ere Cedex, France b Laboratoire des Sciences de l’Eau et de l’Environnement, Facult´ e des Sciences et Techniques, 123, avenue Albert-Thomas, 87060 Limoges Cedex, France Received 24 January 2007; received in revised form 11 April 2007; accepted 27 April 2007 Abstract It is generally agreed that autotrophic organisms and especially phytoplanktonic species can be harmed by copper through its effect on photo- system. However, the impact of copper on other components of the pelagic food web, such as the microbial loop (autotrophic and heterotrophic picoplankton, pigmented and non-pigmented flagellates and ciliates) has received little attention. Indoor experiments were conducted to evaluate the direct and indirect effects of copper, supplied in the range of concentrations used to control cyanobacteria growth in ponds, on non-targeted organisms of natural microbial loop communities sampled in spring and summer. Two copper concentrations were tested (80 gL -1 and 160 gL -1 final concentrations), set, respectively, below and above the ligand binding capacity of the water samples. Both caused a significant decrease in the biomass and diversity of pigmented organisms (picophytoplankton and pigmented flagellates). Conversely, the heterotrophic bacterioplankton and the heterotrophic flagellates did not seem to be directly affected by either copper treatment in terms of biomass or diversity, according to the descrip- tor chosen. The ciliate biomass was significantly reduced with increasing copper concentrations, but differences in sensitivity appeared between spring and summer communities. Potential mixotrophic and nanoplanktorivorous ciliates appeared to be more sensitive to copper treatments than bacterivorous ciliates, suggesting a stronger direct and (or) indirect effect of copper on the former. Copper sulphate treatments had a significant restructuring effect on the microbial loop communities, resulting in a dominance of heterotrophic bacterioplankton among microbial microorganisms 27 days after the beginning of the treatment. The spring microbial communities exhibited a greater sensitivity than the summer communities with respect to their initial compositions. © 2007 Elsevier B.V. All rights reserved. Keywords: Microbial loop; Copper; Pigmented organisms; Non-pigmented organisms 1. Introduction Copper sulphate treatments have long been used to control phytoplanktonic biomass (Haughey et al., 2000), by virtue of the high sensitivity of autotrophic organisms to cupric ions (Sunda and Lewis, 1978). With the rising frequency of Cyanobacteria blooms, this type of treatment is still largely employed at the early stage of bloom formation in ponds used for recreational activities to avert potential human exposure to cyanotoxins. The effects of copper sulphate on single autotrophic species (toxicity tests) and phytoplanktonic communities (field stud- ies) have been well researched (McKnight, 1981; Winner and Owen, 1991; Gustavson and W¨ angberg, 1995; Franklin et al., 2002; Peˇ na-Castro et al., 2004; Soldo et al., 2005; Le Jeune et ∗ Corresponding author. Tel.: +33 4 73 40 77 12; fax: +33 4 73 40 76 70. E-mail address: Marie.CHARPIN@univ-bpclermont.fr (M. Charpin). al., 2006). In contrast, little work has been reported on its effects on non-targeted organisms. In freshwater media, the food web interactions among autotrophic and heterotrophic bacteria and their protozoan grazers (flagellates and ciliates) recycle particu- late and soluble nutrients released by the classical pelagic food chain (phytoplankton, zooplankton, fish) (Azam et al., 1983). This complementary trophic food chain, called the microbial loop, can act as a significant mediator of energy transfer to upper trophic levels by recovering part of the pelagic produc- tion that would otherwise be lost from the systems (Amblard et al., 1998). Trace elements such as copper sustain essential key metabolic functions in a wide range of organisms, but exces- sive concentrations can cause adverse toxic effects (Bar´ on et al., 1995). The picoplankton (auto- and heterotrophic bacteria) have been thought to play an important role in the trophic transfer of trace elements by scavenging dissolved trace metals through their high surface-area-to-volume ratios (Twiss and Campbell, 1995; Twiss et al., 1996). In the case of rising concentrations of 0166-445X/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2007.04.007