The harmful algal bloom holobiont

Harmful algal blooms (HAB) are increasing worldwide, including across Australia’s Great Southern Reef, causing mass mortalities of marine life and economic loss. This mini-review examines the phytoplankton species associated with the SA2025 bloom, particularly dinoflagellates, and explores the potential roles of associated bacteria and viruses in shaping bloom dynamics. Karenia cristata was identified during the bloom, with toxin analyses revealing the presence of brevetoxin analogues not previously reported in Australian HABs. Beyond algal toxicity, interactions with bacteria may influence bloom development through nutrient exchange, growth promotion, or algicidal activity. Similarly, diverse marine viruses can infect phytoplankton and may regulate bloom progression or termination through host lysis. Despite growing recognition of these microbial interactions, the bacterial and viral communities associated with the SA2025 bloom remain largely unexplored. Understanding the tripartite interactions between dinoflagellates, bacteria, and viruses alongside oceanography is critical for investigating bloom initiation, persistence, and collapse.