A bloom by any other name

Once a year during the spring, when conditions are juuuuust right, phytoplankton are terrible at social distancing. This annual bloom that takes place in the spring from 35º North in the North Atlantic and reaches all the way to the Arctic Ocean. Why is this relevant to anything? Well, to start, these blooms play a big role in carbon cycling and understanding them has broader implications than microbial community ecology (even though that’s the good stuff…right??). A team of researchers who live both near to and far from the North Atlantic have carefully looked at this phenomenon in those chilly waters to understand microbial community diversity and dynamics.

Figure 1. Map of the sampled stations in the North Atlantic and hierarchical clustering of the samples based on amplicon sequence variant (ASV) profiles.

So who’s responsible for this wild party? As Bolaños and colleagues point out, the main culprit has thought to have been large diatoms at high latitudes. Post the diatom fiesta, coccolithophores, dinoflagellates, and assorted pico-phytoplankton are thought to show up. However, as you might guess, this mixture of microbes changes over time and location, and is influenced by how water is moving and shaking. In fact, the North Atlantic is essentially divided into four different regions (subpolar, temperate, subtropical, and Gulf Stream/Sargasso Sea) and the true intricacies of different community interactions has yet to be completely sorted out.

Figure 2a. Taxonomic and ecological description of the 16S rRNA phytoplankton amplicon datasets. Relative contributions of phytoplankton taxa for depth profiles at each station. 

Bolaõs et al., collected their data as part of The North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), which focuses on studying plankton blooms in the North Atlantic and how they impact other aspects of the planet’s climate (see this video for a more comprehensive intro, or watch this music video…yes, that’s what I said). The authors carefully examined phytoplankton communities at different times and locations in the North Atlantic by collecting samples from two transects: one in early winter, and another in the spring. To get a handle on the community, they used 16S rRNA amplicon sequencing from 5 depths ranging from 5 to 100 meters and multiple stations. They also collected cell counts, looked at nutrients, chlorophyll a, and collected a suite of other environmental parameters.

“… in the western North Atlantic, the differences in abiotic factors that delimit regions also create dynamic ecological borders for phytoplankton.” – Bolaños et al., (2020)

The authors put together a comprehensive view of spring phytoplankton blooms in the North Atlantic. It wasn’t shocking that there were unique and diverse groups of phytoplankton in the bloom, although it was surprising that they found regional levels of consistent complexity. They highlight that understanding how water masses move can help predict phytoplankton community variation, and sampling along a transect opened an interesting window into the progression of the spring bloom over time and space. They also note that essentially…things are complicated, and there are many other factors (zooplankton predation, viral infections, and interactions between microbes) also playing a role.

Figure 2b. Taxonomic and ecological description of the 16S rRNA phytoplankton amplicon datasets. Diagram depicting the spatio-temporal shifts in the subpolar region phytoplankton community composition, derived from phylogenetic taxonomic assignments. 

While it was previously thought that large diatom cells were likely responsible for most of the biomass, Bolaños and colleagues indicated that it was actually made up mainly of pico- and nano-phytoeukaryotes. Bolaños et al., point out that diatoms are quite diverse size-wise, and identifying which are abundant in large blooms can allow for a better understanding of how much they contribute to carbon export.

Overall, this was a nice summary providing a unique view of the diverse microbial communities that make up the spring blooms of the North Atlantic, and fit well within the overall NAAMES project aimed at understanding the role phytoplankton blooms have related to more broad themes such as broad ocean ecosystem function as well as climate.

Kid friendly education links I came across for this article, just in case anyone is interested!

Reference

Bolaños, L.M., Karp-Boss, L., Choi, C.J. et al. Small phytoplankton dominate western North Atlantic biomass. ISME J (2020). https://doi.org/10.1038/s41396-020-0636-0

About Kelle Freel

I'm currently a postdoc working at the Hawai'i Institute of Marine Biology with Dr. Mike Rappé. I'm interested in the biogeography and ecology of microbes, especially of the marine variety. After studying a unique genus of marine bacteria at Scripps Oceanography in grad school, I moved to France, where I worked with a group studying yeast population genomics. In my free time, I like to do outdoorsy stuff, travel, and cook.
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