Silicate accumulation in taxonomically diverse seaweeds: morphology, uptake, visualization and environmental responses

Abstract Silicon is required by some microalgae but use and accumulation by seaweeds has been quantified in few taxa. This study aimed to examine biogenic Si (bSi) in taxonomically and morphologically diverse seaweeds, measure Si uptake rates, assess the effects of environmental conditions on bSi, and visualize Si within seaweed tissues. bSi was determined in 57 seaweed taxa across 17 taxonomic orders collected around Tasmania, Australia. There were no clear phylogenetic trends in bSi, but fine filamentous morphologies showed higher bSi content than thicker thalli, suggesting structural roles of Si. Highly variable bSi could relate to epiphytes. Simulated ocean acidification and warming did not affect bSi in taxa with or without active CO 2 uptake mechanisms. Across estuarine-marine gradients, bSi was higher in samples of the same taxa closer to riverine sources, possibly due to higher dissolved Si (dSi) availability. Selected seaweed taxa took up dSi, though slower than rates for macronutrients, and accumulation of bSi and internal dSi was stimulated by higher ambient dSi. Si was visualized in cell walls and marginal cells with Rhodamine 123 staining and epifluorescence microscopy. Si pools in seaweeds can be significant (>0.6 % DW) but more studies on Si growth dependence would be valuable.