Finding the Unicorn

by Harriet Alexander, Woods Hole Oceanographic Institution

Harriet Alexander and Sarah Hurley pull a plankton net  up
through the water from the side of the ship. (Colleen Durkin, WHOI)

One week ago, R/V Knorr stumbled across the elusive unicorn of the open ocean. Before anyone gets too excited, no, we did not just prove the existence of a mythical creature, nor did we run across a pod of narwhals. I am referring to what my advisor, Sonya Dyhrman, and I have laughingly called the unicorn of my Ph.D. research: a large diatom bloom in the middle of the open ocean.

The open ocean might be generally described as a nutrient-poor region, where small picophytoplankton are better able to acquire the things they need to grow than large phytoplankton, such as diatoms. As such, picophytoplankton tend to be numerically dominant over larger diatoms. However, there is mounting evidence that a confluence of events occasionally sparks a large bloom of these bigger phytoplankton in regions where they usually have a hard time growing. Imagine poppies suddenly blooming in the middle of the Sahara Desert.

Everyone, including Harriet, was surprised by the color and smell
of the phytoplankton that we captured. (Carly Buchwald, WHOI)

Why do we care about whether picoplankton or diatoms are blooming? As Gwenn Hennon explained in her blog post on the biological pump, phytoplankton convert carbon dioxide to energy and oxygen in the surface waters. Some of these organisms sink, taking their carbon to the deep ocean away from the atmosphere, and some phytoplankton are better at sinking than others—this is due to both their size and their density relative to seawater. Bigger, denser things, as you might intuit, are better at sinking. Diatoms are not only bigger than the small picoplankton that tend to dominate the open ocean, but are also denser, due to their silica frustule (a glass-like shell). Thus, diatoms may be a particularly efficient vehicle for the movement of carbon to the deep ocean.

So, back to the unicorn.

Up until about a week ago, we had been traveling through the subtropical gyre, which is sparsely inhabited by large diatoms. The morning of April 17, I had been trying to snag a little bit more sleep, as I did not have any planned sampling that day. Suddenly, my roommate came and woke me, saying that we had found something interesting . I stumbled upstairs, slightly blurry-eyed to find everyone in a frenzy deploying a CTD at an unplanned station.

Harriet sampled a small amount of "goo" to
identify it. (Colleen Durkin, WHOI)

Evan Howard and Gwenn Hennon, who, respectively, monitor oxygen and picoplankton communities continuously through underway systems, had identified a huge peak in oxygen production and a major shift in the picoplankton community composition. Lowering the CTD reinforced what they had found: there was a ton of chlorophyll in the surface water.

While everyone was gearing up for some on-the-spot sampling, we decided that a net tow might prove interesting. By dragging a net up and down through the water column we can capture and concentrate all the organisms larger than a certain size into a smaller volume. In this region, we might expect a typical net tow to contain a few animals and maybe some algae. When we recovered the net, it was full of (to use the scientific terminology) goo—greenish-yellowish-brownish goo, smelling strongly of sulfur. Gwenn Hennon and Colleen Durkin grabbed a sample to look at under the microscope and reported that we had, in fact, stumbled upon my unicorn—a huge bloom of diatoms.

My Ph.D. research is focused on trying to better understand the nutritional physiology of diatom growth in a nutrient-limited environment. By sampling the RNA (the genetic material being expressed) of the diatom community in the environment and in on-deck incubations, I will be able to create a metabolic fingerprint for the diatoms that are growing in the system. Coming across this large bloom of diatoms provides me with the opportunity to gauge what these organisms are doing and experiencing during such an event.

Examination of the sample under a microscope revealed it to be
a bloom of diatoms--large cells encased in glass-like shells. The
left photo shows how abundant these cells were and the left shows
a cell about to divide. (Colleen Durkin, WHOI)

Because these blooms only occur episodically and are spatially patchy, we were very lucky to cross paths with it during our cruise (and even luckier to have a great underway team to notice it). The opportunity to do research at sea makes these fortunate encounters possible, giving us the flexibility to study the unexpected and potentially discover something new.

This type of discovery is nearly impossible to make intentionally. Kind of like finding a unicorn.