A water molecule seems 1,000,000 x bigger to a diatom than it does to me

As a (relatively) new mama, I am regularly struck by the fact that my experience of the world as a 5’8” woman seems pretty different from that of my 2’9" daughter. She finds cozy hiding spots under countertops upon which I rest my coffee. Stairs that I blithely hop up she scales as a series of thigh-high obstacles. As I watch her go about her daily business I realize how very differently we experience the same surroundings.

What, then, of the world of microorganisms? Think about it, a diatom experiences water molecules as 10,000 – 1,000,000 bigger, in relation to its body size, than we as humans do. How does that four to six orders of magnitude size difference affect the way diatoms experience their environment, how does it influence the way they carve out a niche?

Well, Amy Hansen, a Ph.D. student with the National Center for Earth-surface Dynamics (NCED) at the University of Minnesota’s Saint Anthony Falls Laboratory, is peering into the world of diatoms to find out.

Cladophora glomerata algal strand colonized by epiphytic diatoms.
Cladophora glomerata algal strand colonized by epiphytic diatoms.Courtesy Amy Hansen
Amy is studying epiphytic diatoms within the Eel River in the Angelo Coast Range Reserve, a research facility in the University of California’s Natural Reserve System. Each spring within the Eel, large mats of the filamentous macroalgae Cladophora glomerata emerge. Over the course of the summer, the mats change color —from green to yellow to rusty brown—as the algal filaments become covered, first with non-nitrogen fixing diatoms, then with thick layers of epiphytic diatoms containing nitrogen-fixing endosymbionts. Amy began to wonder what drives the change in diatom community structure, and therefore color, on the Cladophora filaments.

Amy and her advisors, NCED scientists Miki Hondzo and Jacques Finlay, are guessing that the habitat of the diatoms is governed by micro-scale water flows. The Eel River is a nitrogen-limited ecosystem, and Amy speculates that constrained water flow within densely packed Cladophora stands will limit the amount of dissolved inorganic nitrogen (N) reaching the algal filaments and lead to low local concentrations of N. She is also guessing that water within the nooks and crannies of the rough epiphytic diatom coating will be stagnant and therefore also limited in inorganic N. Hansen hypothesizes that under those conditions, diatom species with endosymbiotic nitrogen-fixing cyanobacteria, such as Epithemia, will come to dominate the epiphyte community.

To test her hypothesis, Amy measured flow velocity, nitrate, and dissolved oxygen profiles within Cladophora stands in the Eel River. She is comparing those measurements with the abundances of different diatoms within the epiphyte community and using the comparisons to determine whether regions of low flow or low nitrate correspond to Epithemia-dominated epiphyte communities.

Particle image velocimetry plot showing flow fields around an un-colonized Cladophora glomerata filament (left).
Particle image velocimetry plot showing flow fields around an un-colonized Cladophora glomerata filament (left).Courtesy Amy Hansen
At a much smaller scale, Amy is using particle image velocimetry (PIV) to record flow fields around colonized and bare Cladophora filaments. By comparing the flow fields between the diatom-covered and bare strands, Hansen can predict the effects of colonization on microscale nitrogen supply to both the epiphytes and the algae. Hansen’s preliminary results indicate that flows within Cladophora stands are significantly slower than the bulk river flow and that nitrate concentrations dip dramatically at the top of the stands.

Just think, wading through a stream, we might experience the water as fast and turbulent, swirling around our feet and pushing us forward. Diatoms in that same stream might be sitting in water that is effectively. It’s like they have a secret world all to their own that we know nothing about!

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