Two seaweed species were examined (Alaria esculenta and Saccharina latissima) as a fermentation feedstock for the development of a beverage. Three microbial groups were selected for experimentation; a single-species bacterium; Lactiplantibacillus plantarum, a single-species yeast; Saccharomyces cerevisiae and a multi-species culture; kombucha SCOBY (Symbiotic Culture of Bacteria and Yeast). Results showed SCOBY cultures were more successful in fermenting A. esculenta than S. latissima due to the lowered glucose content (4.069 ± 0.174 mg / mL and 12.224 ± 2.249 mg / mL, respectively). Likewise, L. plantarum played a major role in fermenting S. latissima, over A. esculenta, resulting in a lowered pH in the former but not the latter (6.44 ± 0.040 and 4.825 ± 0.045, respectively). S. cerevisiae does not appear to have been suitable as a fermentation medium for these two seaweed species under these conditions due to the lack of pH lowering, glucose reduction and acid production. All treatments showed an increased production of lactic acid, especially the SCOBY treated samples, which is an indicator of fermentation. There is potential for optimization of the seaweed fermentation process through strain selection, fermentation conditions optimization (temperature, time, pH) and seaweed biomass pretreatment. Glutamic acid, the amino acid responsible for umami flavour, was found in greater quantities in the S. cerevisiae treated seaweed, indicating that fermentation using S. cerevisiae cultures may improve the umami flavour profile of a seaweed fermented beverage product. In order to see if the seaweed species used had an impact on the SCOBY microbial makeup, the DNA profiles of the two SCOBYs used were analyzed. There were significant differences between the two SCOBYs. The A. esculenta SCOBY showed slightly rarer species of fungi and the species abundance of fungi was greater than that of S. latissima SCOBY. Furthermore, the number of fungal operational taxonomic units (OTUs) in A. esculenta SCOBY was higher than that in S. latissima SCOBY. This study shows the potential of using brown seaweed species as a fermentation substrate to develop a novel, fermented beverage.