Ó Springer 2006
Hydrobiologia (2006) 563:225–230 DOI 10.1007/s10750-005-0008-9
Primary Research Paper
Morphological response of Microcystis aeruginosa to grazing by different sorts of zooplankton Zhou Yang1,2,3,*, Fanxiang Kong1,*, Xiaoli Shi1 & Huansheng Cao1 1
Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, 210008, Nanjing, China 2 Jiangsu Key Laboratory for Bioresource Technology, School of Biological Sciences, Nanjing Normal University, 122 Ninghai Road, 210097, Nanjing, China 3 Graduate School of the Chinese Academy of Sciences, 100039, Beijing, China (*Authors for correspondence: Tel.: +86-25-86882191; Fax: +86-25-57714759; E-mail:
[email protected];
[email protected]) Received 29 August 2005; in revised form 1 December 2005; accepted 26 December 2005; published online 10 April 2006
Key words: Microcystis aeruginosa, zooplankton, flagellate, grazing, colony formation, inducible defense
Abstract In the experiment we investigated the effect of grazing by different sorts of zooplankton on the induction of defensive morphology in the cyanobacterium Microcystis aeruginosa. The results showed that protozoan flagellate Ochromonas sp. grazing could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the grazing treatments of copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus were still strongly dominated by unicells and paired cells and no colony forma occurred. In the protozoan grazing treatment, the proportion of unicells reduced from 83.2% to 15.7%, while the proportion of cells in colonial form increased from 0% to 68.7% of the population at the end of the experiment. The occurrence of a majority of colonial M. aeruginosa being in the treatment with flagellates, indicated that flagellate grazing on solitary cells could induce colony formation in M. aeruginosa. The colonies could effectively deter flagellate from further grazing and thus increase the survival of M. aeruginosa. The colony formation in M. aeruginosa may be considered as an inducible defense against flagellate grazing under the conditions that toxin cannot deter flagellate from grazing effectively. Introduction Microcystis aeruginosa Ku¨tz, a bloom-producing cyanobacterium found in both fresh and marine waters (Reynolds & Walsby, 1975; Dokulil & Teubner, 2000; Tsujimura et al., 2000; Landsberg, 2002; Lehman et al., 2005), which occurs as large colonial morph under natural conditions, exists mainly as single cells and a few paired cells in axenic cultures in the laboratory (Reynolds et al., 1981; Bolch & Blackburn, 1996). Colonial algal isolates lose their typical colonial appearance after some generations in the laboratory suggesting that the factor responsible for the typical colonial
form is absent in the culture media but is present in the nature lake water. Colony formation in M. aeruginosa may be a phenotypic response of individual cells to current environmental conditions. These individuals should then have a high phenotypic plasticity in the ability of adaptations. To answer the question what triggers the change in morphology of the phytoplankton species it is necessary to understand the effect of various environmental factors, including abiotic and biotic factors. Some studies have shown that abiotic factors could be very important in inducing changes in morphology in phytoplankton ( Grossart & Simon, 1993; Trainor, 1993; Quesada
226 & Vincent, 1997). In addition, infochemicals released by herbivorous zooplankton also seem to be responsible for morphological changes in some species of algae (Lu¨rling & van Donk, 1996; Fialkowska & Pajdak-Sto´s, 1997; Pajdak-Sto´s et al., 2001; Jakobsen & Tang, 2002; Tang, 2003; Lu¨rling, 2003a, b; van Holthoon et al., 2003). All these environmental factors might function as a synergistic complex system resulting in phenotypic responses among some phytoplankton species. To test whether colony formation in M. aeruginosa is associated with zooplankton grazing, we conducted an experiment in which filtered lake water with abundant or few zooplankton was added to an axenic M. aeruginosa culture. Filtered lake water with abundant zooplankton induced colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the treatment of filtered lake water with few zooplankton were still strongly dominated by unicells and paired cells and no colony was formed (Yang et al., 2005). In the M. aeruginosa suspension added with filtered lake water with abundant zooplankton, some colonies of several, dozens, and sometimes even hundreds of M. aeruginosa cells were formed. This result indicated that herbivorous zooplankton, which are potential grazers, maybe also one of the factors that are responsible for the colony formation in M. aeruginosa. However, the colonyinducing effect of different zooplankton may be not identical. It is therefore of interest to clarify which zooplankton (copepods, cladocerans, rotifers, and protozoans) play the most important role in inducing colony formation in M. aeruginosa. The aim of this study was to investigate possible influence on colony formation in M. aeruginosa by different potential grazers. There are because many studies about zooplankton feeding on M. aeruginosa and the effect of M. aeruginosa on zooplankton have been extensively investigated (eg Fulton III & Paerl, 1987; Nandini, 2000; Nishibe et al., 2002), here we just present the results of morphological response of M. aeruginosa to grazing by different zooplankton.
Materials and methods Toxic M. aeruginosa Ku¨tz was obtained from Institute of Hydrobiology, the Chinese Academy
of Sciences. The microcystin level was above 130 lg g)1 dry weight (Li et al., 2003), which is slightly higher than that in Lake Taihu which is densely populated with Microcystis in summer (Xu et al., 2005). The alga was batch cultured axenically in liquid BG)11 medium (Rippka et al., 1979) in 1.0 l Erlenmeyer flasks at 25 °C under fluorescent light at an intensity of 40 lE m)2 s)1 with a light–dark period of 12: 12 h. The alga in its late exponential growth phase was used in the experiments. The zooplankton used in the experiments were taken from established cultures of animals originally isolated from Lake Taihu. Metazoan zooplanktons, including copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus, were separately cultured in beakers and fed with Scenedesmus as food. The protozoan flagellate Ochromonas sp. was cultivated in flasks with addition of autoclaved wheat seeds. All zooplankton were grown at 25 °C under fluorescent light at an intensity of 40 lE m)2 s)1 with a light–dark period of 12:12 h. For the determination possible morphological response of the M. aeruginosa to direct zooplankton exposure, a total of 12 batches of 100 ml M. aeruginosa were transferred into 250 ml flasks, followed by addition of copepod E. graciloides (100 individuals l)1; 1.8±0.25 mm), cladoceran D. magna (200 individuals l)1; 2.1±0.10 mm), rotifer B. calyciflorus (300 individuals l)1; 0.22±0.03 mm), and flagellate Ochromonas sp. (approximately 6000 individuals ml)1; 7.8±0.90 lm), respectively. The control was the algal culture on its own without any zooplankton. The initial algal concentration in all treatments (on cell number basis) was approximately 5.53106 cells ml)1. We used high cell densities in this experiment because we were interested in the effects of M. aeruginosa at densities typically occurring in algal blooms and the possible colony formation induced by different zooplankton under laboratory conditions. The test was run in triplicate for 6 days under the conditions as described above. The algal cultures within the flasks were shaken three times a day. Samples were daily taken and preserved in Lugol’s solution. The numbers of cells per unit were counted under a light-microscope by counting at least 600 units (i.e. unicells, two-celled as well as colonies). The mean number of cells per unit and the mean
227 proportion of different cells (unicells, two-celled, and colonies) were computed from these counts, respectively. The biomass of M. aeruginosa in different treatments and the control were also recorded. All data were presented as mean±1 SD. Two-way analysis of variance (ANOVA) was used to determine the significance of differences in the biomass and mean number of cells per unit between control and the treatments.
Results Algal biomass expressed as cell density increased over the course of the experiment for three metazoan grazing treatments (Copepod, Cladoceran, and Rotifer) and control (Fig. 2). No significant difference was detected among them (p>0.05). However, algal biomass in the protozoan treatment decreased gradually (Fig. 2) and was significantly lower than those in the three metazoan grazing treatments and control (p
