Direct quantification of unicellular algae sinking velocities reveals cell size, light, and nutrient-dependence
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Abstract
Eukaryotic phytoplankton, also known as algae, form the basis of marine food webs and drive marine carbon sequestration when their biomass sinks to the ocean floor. Algae must regulate their vertical movement, as determined by motility and gravitational sinking, to balance access to light at the surface and nutrients in deeper layers. However, the regulation of gravitational sinking velocities remains largely unknown, especially in motile species. Here, we directly quantify single-cell masses and volumes to calculate sinking velocities according to Stokes’ law in diverse clades of unicellular marine microalgae. Our results reveal the cell size, light, and nutrient-dependency of sinking velocities. We identify motile dinoflagellate and green algal species that increase their sinking velocity in response to starvation. Mechanistically, this increased cell sinking is achieved by photosynthesis-driven accumulation of carbohydrates, which increases cell mass and density. Moreover, cell sinking velocities correlate inversely with proliferation rates, and the mechanism regulating cell sinking velocities integrates signals from multiple nutrients. Our findings suggest that the regulation of cell composition according to environmental conditions contributes to the vertical movement of motile cells in the oceans. More broadly, our approach for sinking velocity measurements expands the study of gravitational sinking to motile cells and supports the modeling of marine carbon pump and nutrient cycles.
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Finally, our work is a proof of methodology that could support marine ecology research and modeling of carbon fluxes. With the development of automated sample preparation methods that separate large algae aggregates from single cells of different sizes, the SMR could be adapted for measurements of algae directly from the ocean. Such future approaches could provide high throughput sinking velocity measurements in situ.
Thank you for sharing this work as an open source preprint! I'm very intrigued by the nutrient-dependent changes in buoyancy and I'm excited to see the future use of this technique!
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Unialgal cultures were grown in filter-sterilized L1 or L1-Si medium, as appropriate for cell type (see table 1).
It would be really cool to see how the presence of silica is impacting the buoyancy. P. tricornutum (which looks like one of the slowest sinkers measured here) can survive without the presence of silica. It would be interesting to see if there are notable differences in P. tricornutum buoyancy in L1-Si compared to standard L1.
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Phaeodactylum traceroute
I think this is a typo? According to wikipedia, the only member of the Phaeodactylum genus is P. tricornutum and P. traceroute isn't mentioned again in the text.
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As the highly elongated diatom Phaeodactylum tricornutum was the only species where sinking velocities were influenced >5% by cell shape (Supplemental dataset S3), all sinking velocities reported in figures are assuming a spherical object.
I love the inclusion of a diverse panel of algal species. The P. tricornutum strain used in this study (CCMP632) was reported to be fusiform 95-100% of the time (DOI: 10.1111/j.1529-8817.2007.00384.x). It'd be interesting to see how buoyancy is impacted by morphology of the same species. In my hands, I always find triradiate cells to be harder to pellet.
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