Globally, there are 3 400 to 4 000 described species of marine microalgae but only 1 to 2percent are considered to be harmful (Shumway et al., 2018).The generic term “harmful algal bloom”1 (HAB) includes proliferations of microalgaein marine or brackish waters that can cause water discolouration and massive fish kills,contaminate seafood with toxins, or alter ecosystems and services in ways that humansperceive as detrimental.The impacts and mass mortalities of marine species caused by harmful algae are not newand have been recorded for decades (Sellner and Rensel, 2018). However, there is growingconcern that these events will increase due to accelerating global warming, climate changeand anthropogenic activities.Toxin-producing HABs, in particular, can contaminate seafood, drinking and recreationalwaters, or kill fish and wildlife. HAB events related with seafood biotoxins represent 48percent of total HAB events globally (Hallegraeff et al., 2021). Some HABs are responsiblefor potentially deadly food-borne illnesses, and some HAB toxins may be aerosolizedand cause respiratory distress in susceptible or high-exposure populations (for example,aerosolized brevetoxins from Karenia brevis blooms in Florida, United States of America).Six human syndromes are presently recognized to be caused by consumption of seafoodcontaminated with marine algal toxins:• amnesic shellfish poisoning (ASP)• azaspiracid shellfish poisoning (AZP)• ciguatera poisoning (CP)• diarrhetic shellfish poisoning (DSP)• neurotoxic shellfish poisoning (NSP)• paralytic shellfish poisoning (PSP)In addition, cyanobacteria are identified as food-borne poisoning causative organisms.The species considered harmful are catalogued in published texts (for example, in Lassuset al., 2016) and now periodically updated in the IOC-UNESCO Taxonomic ReferenceList of Harmful Micro Algae (Lundholm et al., 2021) and in AlgaeBase (Guiry and Guiry,2021).Non-toxin-producing but high biomass blooms, including proliferations of macroalgae,can also be harmful. These “nuisance” blooms can disrupt ecosystem services with impactson the economy and human health; for example, desalination systems, the seafood industry,fisheries and aquculture activites and resources, the tourism sector, and recreation andmaritime facilities such as ports. (Boerlage and Nada, 2015; Jasim and Saththasivam, 2017).High biomass HABs cause environmental stress by reducing dissolved oxygenin seawater, causing hypoxia or anoxia and also expansion of dead zones leading toindiscriminate kills of marine life. Both toxin-producing and nuisance HABs result inthe loss of livelihoods and food security issues for those dependent on seafood and othercoastal or marine resources for their incomes or subsistence. Some HABs occurred in manyregions of the world before coastal ecosystems were altered by human activities. Otherterms that describe these naturally occurring phenomena are phytoplankton blooms,microalgal blooms, toxic algae, red tides, or harmful algae.