A direct test of cyanobacterial chemical defense: Variable effects of microcystin‐treated food on two Daphnia pulicaria clones

Abstract

To determine the direct effects of microcystin on the fitness of herbivorous zooplankton, we experimentally added microcystin-LR to freeze-dried cells of palatable Chlorella and fed these compound-treated cells to two clones of Daphnia pulicaria that had shown differing responses to a diet containing a strain of Microcystis aeruginosa that produces microcystin. The Daphnia that performed better on a diet containing live Microcystis showed reduced population growth when exposed to microcystin-LR-treated Chlorella, whereas the Daphnia that performed poorly on the diet containing live Microcystis was not affected by the experimental diet containing microcystin-LR. This is the first study to unambiguously show that some Daphnia strains are and some strains are not harmed by the consumption of microcystin-LR. These surprising results were not generated by interference from lipophilic secondary metabolites in Microcystis. When the crude lipophilic extract of Microcystis was added to dried Chlorella cells, it enhanced the fitness of both Daphnia clones. We hypothesize that the Daphnia clone more tolerant to live cells may upregulate resistance when cued by the presence of the live Microcystis cells, but not by microcystin-LR alone. Alternatively, the Daphnia clone that grew well on a diet containing live Microcystis may sequester compounds from Microcystis that defend the cyanobacteria from autotoxicity; these compounds would have been unavailable to Daphnia consuming freeze-dried Chlorella treated with microcystin-LR alone. Thus, microcystin-LR can suppress Daphnia fitness when consumed; however, the effects of microcystin vary across clones of herbivorous zooplankton and the consequences of this variance should not be overlooked when considering zooplankton-cyanobacteria interactions.Blooms of cyanobacteria threaten aquatic communities and global water supplies (Paerl 1988) because they produce secondary metabolites that can harm or kill fishes, livestock, and humans (Carmichael 1992). Some of the more commonly studied cyanotoxins include cyclic peptides that target mammalian livers (e.g., microcystin and cylindrospermopsin), nerve synapses (e.g., anatoxin-a), and gastrointestinal tracts (e.g., lyngbyatoxin-a) (Carmichael 1992; Chorus and Bartram 1999;Zurawell et al. 2005). Despite a large correlative literature suggesting that cyanotoxins may deter feeding by zooplankton (Lampert 1987;Watanabe et al. 1996;Zurawell et al. 2005), the effect of cyanobacterial secondary metabolites on ecologically relevant organisms, such as herbivorous zooplankton, is equivocal (Wilson et al. 2006a). There are no direct empirical tests of how cyanobacterial secondary metabolites affect grazers consuming these compounds.Cyanobacteria have been proposed to lower zooplankton grazing rates in three basic ways: (1) by occurring as large colonial and filamentous morphologies that clog filtering appendages or are otherwise inedible, (2) by being nutritionally deficient, and (3) by producing toxic secondary metabolites ( (4) fed diets ...

Inducible tolerance to dietary protease inhibitors inDaphnia magna

“…The pronounced reduction of somatic growth in D. magna and the remodelling of the digestive chymotrypsins caused by encapsulated pure BN920 demonstrate that protease inhibitors in cyanobacteria are a potential cause for the low food quality of cyanobacteria for D. magna. Similarly clear evidence for negative effects of cyanobacterial secondary metabolites on Daphnia has so far only been provided for microcystins (Lürling, 2003;Rohrlack et al, 2005a;Wilson and Hay, 2007). Similar to feeding on encapsulated BN920, feeding on ≥20% M. aeruginosa caused a reduction of somatic growth and a remodelling of chymotrypsins in D. magna, which strongly suggests that these effects of feeding on intact cyanobacterial cells are also due to the two known chymotrypsin inhibitors in this cyanobacterial strain .…”

“…Tolerance to toxic cyanobacteria has been shown to vary within Daphnia populations (Gustafsson et al, 2005;Wilson and Hay, 2007;Sarnelle et al, 2010), and it is not clear how representative the clone of D. magna used here is of the original population. The fact that even in Daphnia populations existing without cyanobacteria the capability of switching to more tolerant chymotrypsins is not lost suggests either that maintenance of the respective set of genes is not costly or that in natural phytoplankton the occurrence of dietary chymotrypsin inhibitors is not restricted to cyanobacteria.…”

“…Unequivocal evidence for a detrimental role of these toxins to herbivorous zooplankton has been provided by comparison of a wildtype strain (producing several microcystins) and a mutant strain (with no microcystins) of the same cyanobacterium (Lürling, 2003;Rohrlack et al, 2005a) and by supplementation of a green alga with pure microcystin-LR (Wilson and Hay, 2007). Nevertheless, the effects of the microcystin-free mutants point to the relevance of other bioactive metabolites, and it has been suggested that more attention should be paid to cyanobacterial compounds other than microcystins as causes of the poor quality of cyanobacteria for herbivorous zooplankton (Wilson et al, 2006).…”

Zooplankton richness, abundance and biomass of two hypertrophic shallow lakes with different salinity in central Argentina

“…The former include the availability and quality of food resources, interspecific competition and predation by vertebrates and invertebrates (Lampert & Sommer 1997, Khan et al 2003, Chang et al 2004, Boveri & Quirós 2007, Manca et al 2008. The negative effects of poor quality food supply are known (DeMott et al 2001, Wilson & Hay 2007 as is the fact that the predation by zooplanktivorous fish produces the replacement of larger species by smaller ones is also known (Brooks & Dodson 1965). This, in turn, leads to changes in grazing pressure and phytoplankton biomass (top-down effect) (Bertolo et al 2000, Kalff 2002, Boveri & Quirós 2007, Manca et al 2008.…”

Effects of microcystin-producing and microcystin-free strains ofPlanktothrix agardhiion long-term population dynamics ofDaphnia magna

“…Several hypotheses have been proposed concerning the role of such metabolites at several trophic levels (for reviews, see Vasconcelos and Pereira, 2001;Gross, 2003;Wiegand and Pflugmacher, 2005;Babica et al, 2006;Leflaive and TenHage, 2007;Schatz et al, 2007). However, such a toxic activity is difficult to characterize because of (i) the presence of other molecules that may modify the effect of toxins (Rohrlack et al, 1999b;Buryskova et al, 2006), (ii) the adaptation of target organisms (Wilson and Hay, 2007) and (iii) the appearance of avoidance behaviour in some species of zooplankton (Kurmayer and Ju¨ttner, 1999). As summarized above, several characteristics of cyanobacteria may contribute to lower zooplankton grazing pressure on cyanobacteria.…”