The global problem of nuisance macroalgal blooms and pathways to its use in the circular economy

Macroalgal blooms are increasing globally as a result of anthropogenic stressors and are leading to negative environmental and socio-economic impacts. There is, however, an opportunity to utilise nuisance macroalgae by creating useful and economically viable products. Thus relieving pressure on diminishing, but sought-after natural resources. Here we have undertaken a systematic review of the literature to synthesise data from 639 peer-reviewed papers to: identify the spatial and temporal nature of macroalgal blooms; quantify the ecological and socio-economic impacts associated with these blooms; outline the conversion of macroalgal biomass into products, in particular for the main blooming genera; provide an assessment of input and product costs for different compounds; and provide feasibility assessments generated from Techno-Economic Analysis (TEA) and Life Cycle Assessments (LCA). Globally, green macroalgae (chlorophyta), predominately Ulva spp. (52%), were found to be the most reported and spatially distributed blooming algae. However, the largest single bloom was produced by Sargassum spp. (phaeophyta), producing >20 million tonnes in 2018. Ecologically, the dominant negative impacts of macroalgal blooms were a decline in species richness and abundance; with seagrasses and fish most impacted. Social and economic impacts of macroalgal blooms were under-reported in peer-reviewed papers and globally under-researched, but where reported, there were clear negative impacts. For nuisance green macroalgal species, biorefining to produce multiple products was the most researched application, yet in the feasibility assessments there was a stronger case for biofuel production. Limited TEA or LCA were identified focussing on bloom material. There is an urgent need for research to provide the tools for the effective utilisation of macroalgal blooms. We recommend increased research into bioprocessing and a TEA of nuisance macroalgal blooms especially for Ulva and other chlorophyta, which are predominantly problematic. If this is achievable then exciting pathways utilising problem macroalgae can be incorporated into the circular economy.