Screening for competition effects and allelochemicals in benthic marine diatoms and cyanobacteria isolated from an intertidal flat (southern North Sea)

Abstract

Scholz B. and Liebezeit G. 2012. Screening for competition effects and allelochemicals in benthic marine diatoms and cyanobacteria isolated from an intertidal flat (southern North Sea). Phycologia 51: 432–450. DOI: 10.2216/11-80.1Marine intertidal flats are colonized by diatom-dominated biofilms, whereas coccoid and filamentous cyanobacteria usually occur only at some stages during the growth season. Increasingly, cyanobacteria were most abundant during warmer seasons, outcompeting other taxa and we assumed direct competition between diatoms and cyanobacteria. Therefore, the most abundant species (25 diatoms and 10 cyanobacteria) were isolated from an intertidal flat in the Lower Saxonian Wadden Sea (southern North Sea) and the isolates were screened for allelochemical production (e.g. alkaloids, flavonoids) and allelopathic activities (e.g. algicidal, antimicrobial). Screenings were performed in single-species and mixed cultures of two, three and six species. Only polyunsaturated fatty acids, terpenes and sterols were found in single-species cultures, but aldehydes, flavonoids, alkaloids and phenols were detected in biomass extracts from mixed cultures. Chroococcus dimidiatus and Anabaena torulosa (Cyanophyceae) dominated other species in mixed cultures of cyanobacteria and diatoms; alkaloids were present in these mixed cultures. Cyanobacteria outgrew diatoms in cultures with two or three species, whereas the diatom Navicula phyllepta (Bacillariophyceae) outcompeted other diatoms and accumulated aldehydes. Six species (three diatoms and three cyanobacteria) were combined in the final test, which was conducted under different nitrogen regimes (800 µM and 250 µM). Navicula phyllepta outgrew the cyanobacteria and other diatoms under low-nitrogen conditions. Although on the basis of the current screening it was impossible to draw conclusions about the effective organisms in the mixed-species treatments, we highlighted both various bioactivities and potential bioactive chemicals in single-species and mixed cultures for a variety of benthic intertidal diatom and cyanobacterial strains.