Silica in seaweeds is often present in pockets of silicate (silicon dioxide) - seaweeds is containing calcium and magnesium - in most cases nitrogen phosphorus and potassium are the major minerals required for fertilizing - I have never heard that Si is required in fertilizers (from the European point of view)
I came across this paper discussing biogenic silica in marine macroalgae. It ranges from 0.13% to 39% silica content. So it is possible that some species could be used as a sourceof silicon
Article Marine macroalgae are an overlooked sink of silicon in coastal systems
High quality silica could potentially be extracted from high content macroalgae using chemical and electrochemical methods
Article International Journal of Sustainable Engineering Methods of ...