If expected values are relatively high (1-2% or more) you can use a refractometer (Brix method).

Brix method is more preferable as Yaro suggests but u must the values.

Thank you guys :)

I think you must at first lyophilisated your algae before to extract your sugar, because the macroalgae are a little bit huge ;-)

Thank you Nathalie.

°Brix had good correlation for this analysis in seaweed. however you should wash the algae to remove any influence of salts

Method of Dubois et al  is  accurate and relatively simple  method for  detecting soluble sugar. Wash and drain the tissue  thoroughly before  weighing the tissue for extraction.