This study explores some of the mechanisms that may underlie the positive diversity-productivity relationship observed in multi-species microcosm experiments. Specifically, we aimed to identify the possible facilitation of diatom productivity by the use of organic exudates produced by other species in the community focusing on within-species variation in this response for Cylindrotheca closterium. Three different strains (VD 18, VDO5 and VDO6) collected from intertidal mudflat of "Plaat van Doe1", the Westerschelde estuary of C. closterium (Ehrenberg) were cultured in different treatments: F/2 nutrient, in the polyculture of other Navicula species and in spent medium. Yields of Cylindrotheca strains were significantly different from each other as well as for between treatments. Some strains of C. closterium showed significantly better growth in polycultures and in spent medium. Proportional deviation of strain VD 18' s yield from its expected value was 1.5, showing a positive interaction with other species and higher than the other strains. To test the bacterial activity that may influence degradation of organic compounds in spent medium, another experiment was performed under axenic and non axenic conditions. C. closterium aggregates were found in spent medium treatment when cells were grown under both conditions. As these cultures received half of F /2 nutrient or spent medium effect to the clustering, we tested F/2 nutrient concentration. Statistically, no significant differences of different F/2 nutrient concentration on C. closterium production were found. Lastly, there was significantly positive effect of galactose, mannan and spent medium on C. closterium production. Our combined results demonstrate that heterotrophy may contribute to facilitative interactions in benthic diatom communities and hence help explain positive diversity-productivity relationships in microphytobenthos. In addition, significant variation between strains was observed in their response, contributing to the growing realization of considerable variation in phenotypic traits among genotypes of microalgae.
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