Does Microcystis exposure history facilitate adaptation of Diaphanosoma dubium to expanding filamentous cyanobacterium Raphidiopsis raciborskii?

Diaphanosoma, so-called 'tropical Daphnia' are widely distributed in warm waterbodies with Microcystis blooms, and are supposed to have high level of tolerance to Microcystis. However, expanding filamentous cyanobacterium, Raphidiopsis raciborskii is increasingly replacing Microcystis, or becoming co-dominant in warm eutrophic waterbodies. Whether previous experience with Microcystis can facilitate adaptation of Diaphanosoma to R. raciborskii or a shift in cyanobacteria assemblage from Microcystis to R. raciborskii will reduce their tolerance to cyanobacteria is debated. We address this question by investigating the performance of fourteen clones from three Diaphanosoma dubium populations differing in cyanobacteria exposure history. They were fed pure diets of cylindrospermopsin-/non-cylindrospermopsin-producing R. raciborskii, microcystin-producing Microcystis aeruginosa and green alga Auxenochlorella pyrenoidosa. The three cyanobacteria all sustained the growth and reproduction of three D. dubium populations, and both somatic and population growth rates increased with dietary PUFA content for each clone, irrespective of manageability and toxicity. Clones with cyanobacteria exposure history have much better performance than those without exposure history on all food types. Moreover, clones with Microcystis exposure history for >40 years, performed better on all food types than clones with R. raciborskii exposure history for <10 years. The results suggest that D. dubium population with much longer bloom exposure history may adapt to being more tolerant of toxic cyanobacteria in the diet. We infer that previous Microcystis exposure experience might facilitate adaptation of D. dubium to R. raciborskii by enhancing their ability to cope with nutritional constraint and toxicity.