Research

I study phytoplankton (especially diatoms) and how they influence carbon export out of the surface ocean through a process called “the biological pump”.  Currently, I am motivated by three major question:

  1. What adaptations enable phytoplankton to respond to changing ocean conditions?
  2. How are these responses connected to carbon export?
  3. Which particles and cells are transporting carbon through the water column?
Diatom_diversity_micrographs copy2

Collage of diverse diatom species

I use a variety of methods to study phytoplankton, including microscopy and gene expression.  I use these techniques to observe natural communities.  Laboratory experiments with cultured isolates help interpret what we are observing in the field.

To determine the consequence of surface phytoplankton biology on biogeochemistry, I use sediment traps to collect particles sinking out of the surface.  Sediment traps are simply upward-facing tubes that collect sinking material and they come in a variety of models.  Here are the types I have used along with my collaborators:

 

Usually, we include clear gel layers on the bottom of trap tubes to collect sinking particles and make sure they remain intact and distinctly separated from eachother.  These particles can be analyzed in more detail by microscopy.

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example of cells (left) and particles (right) imaged in sediment trap gel layers

Current Projects

Genes expressed by sinking phytoplankton

collaborators: Harriet Alexander (UC Davis/WHOI), Sonya Dyhrman (Columbia University/LDEO), Kit Yu Karen Chan (Hong Kong Science and Technology University)

Phytoplankton physiology plays a role in the transport of cells out of the surface ocean, which can occur by various mechanisms.  For example, nutrient-limited cells may increase their sinking speed or form aggregates, but not all species exhibit the same responses.  We are using molecular signals to indicate 1) what physiological responses are associated with sinking phytoplankton and 2) which species are sinking.

Coscinodiscus

Coscinodiscus from Amazon River plume

We sequenced eukaryotic metatranscriptomes from 4 locations across the South Atlantic Ocean.  We are comparing phytoplankton genes expressed within sinking particles collected in a sediment trap with the genes expressed by the community in the overlying surface seawater.  Based on our previous published microscopy analysis, we know that our locations were characterized by either direct settling of diatom cells or by detrital particle export.  We expect different physiological signals to be reflected in the genes expressed by the sinking plankton.

We also isolated and cultured a diatom that dominated export at a location in the Amazon river plume.  Harriet Alexander is leading a study of the genes expressed by this genus (Coscinodiscus) growing under different nutrient conditions, and how this relates to its sinking speed (measured in the lab by videography, in collaboration with Karen Chan).

related publications:

Durkin et al. 2016. Sinking phytoplankton associated with carbon flux in the Atlantic Ocean. Limnology and Oceanography 61: 1172-1187 (link)

Durkin et al. 2016. The evolution of silicon transporters in diatoms. Journal of Phycology (in press) (link)

Mechanisms of carbon export at the New England Shelf Break

collaborators: Melissa Omand (URI), Meg Estapa (Skidmore College)

Carbon and nutrients are transported out of the surface ocean and sequestered at depth by sinking particles, but this process is still very difficult to quantify and predict, in part because it involves many complex and interacting processes.  To better quantify carbon export in the ocean, we must resolve the underlying mechanisms that produce and transport sinking particles. In November 2015 and June 2016, we deployed a variety of drifting instrument platforms at the New England shelf break, including sediment traps that contain polyacrylamide gel layers on the bottom.

June_geltrap_jars

Microscopic examination of these particles, combined with the high resolution optical and chemical measurements made by Melissa Omand and Meg Estapa, will enable us to characterize how and what types of particles are exported.

related publications:
Durkin et al. 2015. Observations of carbon export by small sinking particles in the upper mesopelagic. Marine Chemistry 175: 72-81 (link)
Estapa et al. 2015. Carbon flux from bio-optical profiling floats: calibrating transmissometers for use as optical sediment traps. (submitted)

Particle and phytoplankton export to the deep seafloor

collaborators: Ken Smith (MBARI), Crissy Huffard (MBARI), Chris Preston (MBARI)

Animals living at the deep seafloor rely on organic particles that sink from the surface for food.  Rapidly settling particles can effectively transport surface phytoplankton and other organic matter to fuel deep sea ecosystems and also lead to long-term carbon sequestration in the deep ocean.  We are testing methods to better characterize these sinking particles, including deploying gel layers in time-series sediment traps deployed at the seafloor for long periods of time.  I am particularly interested in the composition of phytoplankton within particles that reach the seafloor, what types of particles are transporting phytoplankton to the seafloor, how much carbon is exported by phytoplankton containing particles, and how export to the seafloor is changing over time.

Particles identified in a gel layer deployed in the Monterey Canyon

Particles identified (white outlines) in a gel layer deployed in the Monterey Canyon

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