Ocean's Tiny Architects: Unveiling the Hidden Forces Shaping Phytoplankton Blooms
- Ray Sullivan
- Jul 1
- 3 min read
Phytoplankton blooms, vibrant explosions of microscopic life, are fundamental to marine ecosystems, driving global primary production and fueling the biological carbon pump. Traditionally, our understanding of these massive oceanic events has often been simplified, categorizing species like diatoms as "high nutrient, turbulent water" specialists and dinoflagellates as "low nutrient, stratified water" dwellers. Bess Ward’s Lab at Princeton, along with researchers from the University of Cape Town in South Africa, performed a recent mesocosm experiment simulating a coastal upwelling event, which has revealed a much more intricate picture, demonstrating how complex nutrient dynamics and previously underappreciated biotic interactions sculpt the successional patterns of these vital communities.
Vineis, et al. observed a remarkably rapid conversion of dissolved inorganic nitrogen into phytoplankton biomass within just six days. As nitrate and ammonium concentrations plummeted, particulate nitrogen and particulate carbon biomass steadily climbed across all experimental setups. Interestingly, the highest carbon dioxide uptake rates lagged behind nitrate uptake rates by a full 24 hours. This suggests that diatoms, known for their ability to store excess nitrate internally, leverage this stored nutrient for subsequent carbon assimilation, even when external nitrate is nearly depleted. This finding challenges the simplistic view that nutrient availability alone dictates bloom progression.
Contrary to traditional frameworks like Margalef's mandala (a conceptual framework that has traditionally simplified the underlying ecology shaping the succession of phytoplankton blooms), the experiment showed that diatoms bloomed successfully during both the early and late phases of the bloom. Furthermore, mixotrophic dinoflagellate genera like Akashiwo, Heterocapsa, and Prorocentrum were observed to co-exist and even bloom alongside diatoms in the later stages. This co-occurrence is particularly surprising, as dinoflagellates are generally considered poor competitors for nitrate and ammonium. While overall community alpha diversity was highest early on and declined as the bloom progressed, diatom diversity peaked during periods of high nitrate uptake, suggesting a dynamic relationship between nutrient availability and species richness.
Perhaps the most compelling insights from this research come from the realm of biotic interactions, which appear to play a deterministic rather than stochastic role in successional trends.
The study identified the order Syndiniales, classified as putative parasites, co-occurring with several diatoms, particularly in the initial phase of the bloom. Molecular ecological network analysis showed positive connections between twelve Syndiniales Amplicon Sequence Variants (ASVs) and abundant early-blooming diatoms like Chaetoceros protuberans and Thalassiosira sp.. While the short duration of the experiment may have prevented observation of a large infection and lysis event, this suggests that parasitic interactions could be a significant, yet understudied, factor in the successional patterns of coastal diatom populations, warranting further investigation.
The consistent co-occurrence of diverse diatom and dinoflagellate species, despite the principle of competitive exclusion, strongly suggests the presence of multiple "unseen niches" or resource-based metabolic networks (syntrophy). For instance, late-blooming diatoms like Thalassiosira sp. and Minidiscus sp. were positively connected to abundant late-stage dinoflagellates such as Prorocentrum and Heterocapsa, indicating potential biotic connections.
The mixotrophic dinoflagellate Akashiwo sanguinea emerged with a potentially unique ecological role. It was positively connected to numerous ASVs, including seven Thalassiosira ASVs. A compelling hypothesis for this co-occurrence is that Akashiwo might feed mixotrophically on bacteria that support diatom growth through auxotrophy. This indirect connection could reduce competitive pressure on diatoms, facilitating their coexistence even under high nutrient conditions.
Harmful Algal Bloom species like Akashiwo sanguinea, Heterocapsa pygmaea, and Prorocentrum, though minor contributors to total biomass in this study, possess adaptations like mixotrophy (feeding on prey when inorganic nutrients are limited) that provide a competitive advantage later in a bloom. Some, like Akashiwo sanguinea, are known for allelopathic activity, inhibiting the growth of other plankton, which could influence community structure.
This study, integrating microscopy, pigment analysis, 18S rRNA amplicon sequencing, and biogeochemistry, provides a crucial high-resolution view of bloom dynamics. It underscores that phytoplankton bloom successional patterns are not merely a function of nutrient availability but are critically shaped by complex and often indirect biotic interactions. These findings challenge oversimplified conceptual frameworks and highlight the imperative to incorporate diverse diatom adaptations and intricate biotic networks into future models to accurately estimate productivity and carbon cycling in dynamic upwelling systems.
Vineis, J.H., Burger, J.M., Fawcett, S.E. and Ward, B.B. (2025), Co-occurrence and successional patterns among diatoms, dinoflagellates, and potential parasites in a coastal upwelling experiment. Limnol Oceanogr, 70: 1481-1498.
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