Monsoon Patterns, Viruses, and HABs Linked in Philippine Mariculture Area

Harmful algal blooms (HABs) are a significant public health concern and can cause severe economic losses. In Bolinao, Philippines, intensive mariculture has led to diatom- or dinoflagellate-dominated HABs since the 2000s, raising risks of paralytic shellfish poisonings and fish kills. In the context of ongoing HAB monitoring and mitigation efforts, we investigated the overall phytoplankton community composition and co-occurring giant viruses during two seasons (Nov–Dec 2021 and Apr–May 2022). Our sampling encompassed a saxitoxin episode lasting 2.5 months (April–June 2022), and a fish kill (May 15–16, 2022). We used a low-cost flow-through camera (PlanktoScope) and 18S rRNA gene amplicon sequencing to identify the dominant phytoplankton. Diatoms were numerically the dominant large eukaryotic algae during most of our time series, but we also detected the dinoflagellate Alexandrium sp., a known saxitoxin-producing alga, coincident with a toxin alert that resulted in a shellfish ban, even though it was not the dominant taxon. Using electron microscopy, we observed diverse morphologies of virus-like particles (VLPs), including “giant” icosahedral VLPs (~170–200 nm capsids). We assembled giant virus genomes from metagenomic data collected during toxin vs. non toxin episodes. Phylogenetic and functional analyses suggest that the recovered genomes are primarily from viruses in the orders Imitervirales and Algavirales, and they encode diverse auxiliary metabolic genes involved in nutrient transport and carbohydrate metabolism. None of the detected giant viruses are likely to have directly influenced the Alexandrium-associated toxin episode. Nevertheless, a network analysis of phytoplankton–giant virus associations suggest that these viruses might contribute to plankton dynamics during HAB events.