A new study by the ICM-CSIC provides evidence that challenges the traditional view of parasites as agents that suppress microalgal blooms, and shows that they are persistent components, mainly in the water column and, in the case of some lineages, also in the sediment.
Algal blooms are often associated with images of waters stained green or brown, as a result of the massive accumulation of microalgae along the coast. For years, the idea that marine parasites are the main agents of mortality capable of contributing to the decline of these blooms has been quite widespread. However, this view, which focuses exclusively on collapse, oversimplifies the diversity of interactions between microalgae and their parasites.
The Coastal Biological Processes (PBL) group at the Institute of Marine Sciences (ICM-CSIC) has long been working to refine this overly simplified view: parasites form part of the same microbial community and their relationships with their hosts are diverse. Although in some cases they can cause highly virulent infections, in others they maintain low-intensity infections and can coexist with the microalgae for prolonged periods.
New scientific evidences
Now, a new study by the ICM-CSIC published in the scientific journal Aquatic Ecology takes this research a step further. The study reveals that beneath the surface of a bloom lies a highly dynamic microbial network, where parasites do not act as occasional "terminators," but rather as persistent regulators driving the evolution of these blooms.
According to the research team, a bloom should not be viewed as a one-on-one battle between an alga and its enemy, but rather as a constantly shifting mosaic of interactions. To demonstrate this, the researchers monitored three dinoflagellate blooms over time at the beaches of La Fosca, El Grau, and L’Estartit (Girona, Spain). Using genetic techniques such as metabarcoding, alongside microscopy and culturing, they analyzed the coexistence of various lineages of zoosporic parasitoids.
The sediment as an active reservoir
The findings confirm a paradigm shift: the proportion of infected microalgal cells remained low over time, and parasite peaks did not coincide with the end of the blooms. This demonstrates that, rather than wiping out the bloom, many parasites sustain low-intensity, chronic infections. This continuous pressure exerts a key influence on the ecosystem: it alters competition among different phytoplankton species and modulates the natural succession of marine communities without requiring mass die-offs.
Another fundamental finding of the study is the direct link between the water column and the seafloor. Coastal sediments are not merely passive sinks; they act as active reservoirs. Microorganisms such as perkinsids—a group of parasites analyzed in the study—demonstrated a clear capacity to persist within the sediment and reinfect the water column once conditions become favorable again.
Furthermore, the research highlights that the same bloom can behave quite differently depending on local conditions. Although the three beaches studied shared similar dinoflagellate species, the composition and timing of the parasites varied significantly due to localized factors such as hydrodynamics and site-specific environmental features.
These findings contribute to a more comprehensive vision of coastal algal blooms. Temporal monitoring reveals that parasitoid presence and composition fluctuate throughout a bloom and between the water column and the sediment. Capturing this variability requires repeated sampling over time across both compartments, combined with complementary methodologies—an approach that promises to deepen our understanding of ecological interactions in coastal ecosystems.