Abstract
Larval settlement of the polychaete Hydroides elegans Haswell, 1883 is mediated by marine biofilms; complex agglomerates of bacteria, diatoms, fungi and protozoa enmeshed in a matrix of extracellular polymers (EPS). In our previous investigations, benthic diatoms were demon- strated to be potent mediators of larval settlement in H. elegans. The putative diatom-derived settle- ment cues were heat-stable components in close association with the diatom surface. For an in-depth investigation of the chemical nature of diatom-derived larval settlement cues, the EPS of the induc- tive (Achnanthes sp., Nitzschia constricta) and non-inductive (Amphora tenerrima, Nitzschia frustu- lum) diatoms were bioassayed for their effect on larval settlement. When EPS fractions larger than 100 kDa were immobilized in stable hydrogels to mimic their association to a solid substratum, they evoked an effect on larval settlement similar to that of the respective monospecies diatom films. The crude exopolymer samples mainly consisted of polysaccharides with a small proportion of proteina- ceous sample components. After enzymatic removal of proteinaceous EPS components, the samples evoked an effect on larval settlement similar to that of the crude EPS samples, indicating the negligible role of large proteins or glycoproteins as settlement cues.
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