Abstract
It is known that cyanobacteria negatively affect herbivores due to their production of toxins such as protease inhibitors. In the present study we investigated potential interspecific differences between two major herbivores, Daphnia magna and Daphnia pulex, in terms of their tolerance to cyanobacteria with protease inhibitors. Seven clones each of D. magna and of D. pulex were isolated from different habitats in Europe and North America. To test for interspecific differences in the daphnids’ tolerance to cyanobacteria, their somatic and population growth rates were determined for each D. magna and D. pulex clone after exposure to varying concentrations of two Microcystis aeruginosa strains. The M. aeruginosa strains NIVA and PCC− contained either chymotrypsin or trypsin inhibitors, but no microcystins. Mean somatic and population growth rates on a diet with 20% NIVA were significantly more reduced in D. pulex than in D. magna. On a diet with 10% PCC−, the population growth of D. pulex was significantly more reduced than that of D. magna. This indicates that D. magna is more tolerant to cyanobacteria with protease inhibitors than D. pulex. The reduction of growth rates was possibly caused by an interference of cyanobacterial inhibitors with proteases in the gut of Daphnia, as many other conceivable factors, which might have been able to explain the reduced growth, could be excluded as causal factors. Protease assays revealed that the sensitivities of chymotrypsins and trypsins to cyanobacterial protease inhibitors did not differ between D. magna and D. pulex. However, D. magna exhibited a 2.3-fold higher specific chymotrypsin activity than D. pulex, which explains the observed higher tolerance to cyanobacterial protease inhibitors of D. magna. The present study suggests that D. magna may control the development of cyanobacterial blooms more efficiently than D. pulex due to differences in their tolerance to cyanobacteria with protease inhibitors.
Highlights
The frequency of cyanobacterial blooms in many marine and freshwater environments has increased world wide during the last century, partly due to increasing temperatures as a consequence of global warming and partly due to the eutrophication of lakes [1]
For each treatment (20% NIVA, 50% NIVA and 10% PCC2) as well as for both growth rate calculations all D. magna and D. pulex clones were categorized into five groups according to their respective relative growth rate reduction (Figure 1a–f), whereas values #0 were regarded as no growth rate reduction on respective diet and were grouped in the category 0–20%
On a mixture of 80% Chlamydomonas sp. and 20% NIVA the categorized growth rates of the D. pulex clones were significantly more reduced than those of the D. magna clones (Figure 1a, d). This applies for the reduction of population growth rates (U-test: U = 6, p,0.05) and of somatic growth rates (U-test: U = 6, p,0.05): While five of seven D. magna clones were grouped into the first category (0%–20%) of relative somatic and population growth rate reduction, no D. pulex clones were grouped in this category when grown on a mixture with 20% NIVA
Summary
The frequency of cyanobacterial blooms in many marine and freshwater environments has increased world wide during the last century, partly due to increasing temperatures as a consequence of global warming and partly due to the eutrophication of lakes [1]. When the temperature of the epilimnion reaches its maximum in late summer and early fall [4], the phytoplankton of many eutrophic lakes and ponds is often dominated by bloomforming cyanobacterial species of the genera Microcystis, Anabaena and/or Oscillatoria [5]. During this time cyanobacteria are often an important food source for herbivorous zooplankton in freshwater ecosystems, such as for Daphnia, which often provides an important link for the transfer from primary production, e.g. from cyanobacteria to higher trophic levels. Since in eutrophic lakes growth of Daphnia is rather constrained by food quality than by food quantity, bloom-forming cyanobacteria in those habitats have been claimed to be a major factor for a constrained mass and energy transfer from primary producers to organisms of higher trophic levels [8,9]
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