Survival, growth and reproduction of Daphnia galeata feeding on single and mixed Pseudomonas and Rhodomonas diets

Abstract

1. Bacteria can be an important resource for zooplankton production in aquatic food webs, although the degree to which bacteria sustain zooplankton growth and reproduction is not clear. We performed a growth experiment with Daphnia galeata feeding on different ratios of P-replete Pseudomonas and Rhodomonas, ranging from a 100% bacterial to a 100% algal diet. 2. A pure bacterial diet did not support survival, growth or reproduction of D. galeata. While a 20% share of Rhodomonas in the food allowed survival of daphniids, the occurrence of offspring on a 50% algal diet indicated that the threshold for successful reproduction was between those two proportions of algal food. Increasing the proportion of the alga further increased growth and reproductive output, indicating that Rhodomonas was a higher-quality food than Pseudomonas. A subsequent labelling experiment demonstrated that D. galeata incorporated phosphorus fromPseudomonas and Rhodomonas with similar efficiency, whereas carbon was incorporated more efficiently from Pseudomonas than from Rhodomonas. 4. Hence, we hypothesise that inadequate levels of essential biochemicals in pure bacterial diets led to decreased Daphnia performance. Concentrations of fatty acids in general, and especially of polyunsaturated fatty acids, were much lower in Pseudomonas than in Rhodomonas. This difference could explain the different growth and reproduction responses, although limitation by other essential biochemicals (e.g. sterols) cannot be ruled out. 5. Hence, where they dominate, bacteria may provide a significant part of the elemental flux to species feeding higher in the food web on the short term. However, the performance of consumers may be constrained by essential biochemicals.

Differing Daphnia magna assimilation efficiencies for terrestrial, bacterial, and algal carbon and fatty acids

“…Heterotrophic bacteria, such as Actinobacteria and Proteobacteria are typical in the oxic epilimnia of the lakes (Zwart et al 2002, Jezberova´et al 2010 where they utilize organic carbon. Several studies have shown qualitative differences between bacterial taxa as diet sources for zooplankton (Deines and Fink 2011, MartinCreuzburg et al 2011, Taipale et al 2012, Wenzel et al 2012. The results obtained in the present study with Actinobacterium VicMua1 were similar with the previous results on Daphnia fed with heterotrophic Micrococcus luteus (Taipale et al 2012), indicating that Daphnia can maintain high somatic growth rates when Actinobacteria is mixed with high-quality phytoplankton.…”

“…Our laboratory experiment showed that Daphnia are able to assimilate bacterial carbon with similar efficiency as phytoplankton if the required essential FA and sterols are obtained from other sources (also see Taipale et al 2012, Wenzel et al 2012. Daphnia consuming strains of Actinobacterium VicMua1 and Betaproteobacterium Mekk-D6 bacteria were able to survive and to sustain positive growth rates with as little as 2% of the diet originating from Cryptomonas, which contained 0.2 lg sterols, and 0.2 lg x-6 and 1 lg x-3 PUFAs per mg C. However, Daphnia was not able to reproduce with only 2% Cryptomonas in their diet.…”

“…Bacteria alone are known to be incomplete dietary resources for zooplankton (Martin-Creuzburg et al 2011, Taipale et al 2012, Wenzel et al 2012, in part because they do not synthesize PUFAs (excluding some marine bacteria; Russell and Nichols 1999) or sterols (Volkman 2003). However, recent studies have revealed that Daphnia feeding on bacteria-phytoplankton mixtures can obtain similar somatic growth and reproduction as on pure phytoplankton cultures (Taipale et al 2012, Wenzel et al 2012.…”

“…However, recent studies have revealed that Daphnia feeding on bacteria-phytoplankton mixtures can obtain similar somatic growth and reproduction as on pure phytoplankton cultures (Taipale et al 2012, Wenzel et al 2012. This is presumably related to the fact that zooplankton benefit from the high phosphorus content of bacteria ).…”

Carbon transfer from dissolved organic carbon to the cladoceranBosmina: a mesocosm study

“…highly-unsaturated fatty acids, HUFA), and have thus been deemed unsuitable as a sole food source (Martin-Creuzburg et al, 2011). However, mixed algae-bacteria diet can support the growth and reproduction of zooplankton (Wenzel et al, 2012). Besides, microbial food for zooplankton consists of additional heterotrophic microbes in addition to bacteria, e.g.…”