A method of isolating and purifying bioactive compounds in an extract obtained from seaweed. The method involves the steps of: (a) circulating the extract through an ultrafiltration membrane having a suitable molecular weight cutoff； (b) collecting filtrate from the extract to obtain a first filtrate fraction and a retentate； and (c) rinsing the retentate to obtain one or more additional filtrate fractions. The bioactivity of the first filtrate fraction and the additional filtrate fractions can then be evaluated to determine their efficacy on plant growth. One or more bioactive molecules isolated from an algal specie are also described in which the one or more bioactive molecules have a molecular weight in the range of about 0.15 k Da to about 1.0 k Da and are capable of enhancing or improving plant growth.
