Award: OCE-2149606

Diatoms are the basis of the most productive food chains in the ocean.  These silica-shelled phytoplankton are able to grow rapidly in the presence of their favorite nitrogen source, nitrate, and to escape predation by grazers and parasites long enough to produce large blooms and support globally significant fisheries.  The motivation for this project was the report that diatoms were the exception to findings based on a global survey of phytoplankton communities that found that 1) half of the total diversity of marine eukaryotic microbial communities was due to parasites and 2) the structure of phytoplankton assemblages was dominated by biotic interactions, i.e., with other organisms rather than by bottom up factors (i.e., environmental factors such as nutrient supply).  Highly productive diatom blooms had been traditionally thought to be bottom up controlled, and to result in low diversity biomass accumulations, dominated by one or a few species.  Interestingly, the new global survey did not include any samples from highly productive upwelling regions, where the classic diatom based blooms and big fisheries occur. In light of the global survey findings, we hypothesized that those bloom situations might also include parasites as an unrecognized control mechanism, although we suspected that blooming diatoms might be an exception. Using DNA sequencing technology, we analyzed two simulated diatom blooms, one in Monterey Bay, California, and one in Chesapeake Bay, Maryland.  In both cases, “large” volumes of seawater (200 and 20 Liters, respectively) were incubated in triplicate for about a week.  Both experiments resulted in diatom blooms, as evidenced by cell counts (microscopy), pigments (chromatography) and amplicon (ribosomal rDNA sequence) analysis.  Nitrate, the limiting nutrient, was completely utilized and most of the nitrogen was recovered in the phytoplankton biomass. In the Chesapeake Bay experiment, grazers contributed to the demise of the bloom. In both cases, the DNA analysis was able to detect that many different species of diatoms contributed to the bloom, especially multiple species of the genus Chaetoceros and a few other globally important diatom species.  The blooms were much more diverse, i.e., contained many more species, than usually assumed on the basis of microscopy or pigment analysis alone.  In both cases, DNA sequences representing eukaryotic parasites were detected, but in neither case were the parasites highly connected to the diatoms. There was no indication that direct biotic interaction between the parasites and the diatoms affected the community structure during the development and denouement of the bloom.  Thus, we conclude that diatom blooms are bottom up controlled at their initiation and grazing or nutrient depletion lead to their demise, more like the classical view of diatom blooms than the picture suggested by the highly interactive biotic control found by the open ocean global DNA survey.  The DNA approach did, however, uncover the much more diverse and complex nature of diatom blooms than had been concluded from traditional methods, suggesting complex and diverse evolutionary pathways for diatom success.

Last Modified: 05/05/2025
Modified by: Bess B Ward