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The albatross and the phytoplankton

Laura Nielsen

An albatross soaring over the wide open ocean doesn’t just rely on chance sightings of prey; it actually follows its nose. Dimethyl sulfide (DMS) is a biological sulfur compound that can result from the activity of microorganisms called phytoplankton. Not only does airborne DMS provide a wind-map for foraging seabirds, it also also aids in the formation of clouds which help cool our warming planet.

Phytoplankton?

University of California Davis graduate student Jesse Krause traveled to Alaska to research migratory birds with a team from the department of Neural Biology, Physiology and Behavior. Before that, he studied elephant seals. Krause had never pictured himself researching birds or elephant seals.

”The more that you learn about them the more you realize the many cool things that they can do, then you start to appreciate them a lot more.” ~Jesse Krause

For me, phytoplankton fit that description to a T. These humble microscopic organisms hold a surprising amount of sway over the carbon balance of our changing world. They are yet another piece of a complex puzzle.

Though the Greek word phyton, ‘plant’, influenced the word phytoplankton, the term actually designates any microorganism which drifts in water and engages in photosynthesis. Some phytoplankton are plants, others are photosynthesizers not designated as plants, like protists or fungi. They include diatoms, coccolithophorids, some dinoflagellates, and cyanobacteria. We sometimes call phytoplankton simply ‘algae’. Though the single-celled organisms are tiny, they are plentiful; phytoplankton perform half of all photosynthetic activity on Earth. Since they require sunlight for growth, you’ll find them most plentiful near the surface of the ocean where masses of phytoplankton (called ‘blooms’) can be so dense and widespread that the chlorophyll each one uses for photosynthesis appears in satellite photos, staining large ocean swatches an intriguing green.

Photosynthesis is a process by which organisms harnesses solar energy, converting sunlight into fuel. Phytoplankton convert water and carbon dioxide into molecules composed of oxygen, carbon and hydrogen called carbohydrates, as well as a byproduct, oxygen. Carbohydrates are basically sugars, providing the phytoplankton with a way to store energy and also with the building blocks they need to build and maintain their structure.

Massive impact

The photosynthesizers are primary producers, transforming energy from the Sun into a form usable by organisms which do not perform photosynthesis but instead must hunt and eat to gain energy. Phytoplankton are eaten by zooplankton and krill, which in turn are eaten by fish, shellfish, cephalopods, seabirds and even whales. That makes them the base of the marine food chain – other creatures are dependent on them.

As if that weren’t enough, phytoplankton also serve as an important part of the global carbon cycle.

The ocean absorbs carbon dioxide CO2 from the atmosphere very efficently; so much so, in fact, that it has been lessening the impact of anthropogenic fossil fuel emissions. Phytoplankton utilize CO2 during photosynthesis. While some phytoplankton are eaten by creatures which then respire (breathe out) the CO2 and other phytoplankton are decomposed by bacteria which also loose CO2, the phytoplankton that die and sink all the way to the depths of the ocean effectively isolate or sequester the CO2 at the sea floor. Millions and millions of these transactions performed over time help remove CO2 from the atmosphere.

More than 99.9% of the CO2 that has ever been incorporated by living organisms during the history of our planet is now buried in marine sediments, at the bottom of the sea.

Crushed plankton, cloudy skies

Phytoplankton produce dimethylsulfoniopropionate, a compound that plays a role in maintaining cellular structures. When phytoplankton are gobbled up by zooplankton or krill, their structure is crushed and dimethylsulfoniopropionate becomes more available for bacteria to utilize. Bacterial marine microbes break down dimethylsulfoniopropionate, releasing dimethyl sulfide and acrylic acid.

Dimethyl sulfide (DMS) is a biological sulfur compound that is immensely important; it helps regulate Earth’s climate.

Marine DMS emissions are the largest natural source of atmospheric sulfur. Sulfur-based gases, especially sulfuric acid, are ingredients that help form aerosols which form the seeds of clouds – that is, biologically induced cloud condensation nuclei. So, vast quantities of phytoplankton produce the first step in a chain that leads to more clouds being formed. Clouds are lighter in color than open ocean water; they have higher albedo (reflectiveness). Clouds are efficient at bouncing the Sun’s light back into space, while ocean water instead absorbs solar energy and heat. That means that clouds stimulated by the death of phytoplankton help cool the Earth and combat climate change.

Something else interesting about DMS is that it has a smell.

The importance of olfactory

It doesn’t seem intuitive for birds to use smell, but they do. Humans have about 400 genes coded to help us smell. In contrast, nocturnal kakapos and kiwis have over 600. Not all birds smell well – songbirds might only dedicate about 3% of their brain space to their olfactory bulb. Seabirds, though, can have olfactory bulbs that take up 37%.

Dr. Simone Meddle, associate professor at the University of Edinburgh, studies brains. She seeks to understand what drives behavior.

“If you look at a bird brain and a human brain you can identify structures that perform the same function for either a human, or a bird, or a frog for example.” “If you are a bird that predominately feeds on the ocean, they have a very acute sense of smell. So they have a very large olfactory bulb, which is a part of the brain that controls the sensory information, the olfactory information. So they have a well developed olfactory bulb.” ~Simone Meddle

Her paper ‘An intrinsic vasopressin system in the olfactory bulb is involved in social recognition’ published in Nature describes investigations of rat brains which suggest that a hormone helps process and integrate olfactory information, then cues an appropriate behavioral response.

“The hormone vasopressin is important for regulating social cues at the level of this part of the brain – at the level of the olfactory bulb itself – before it goes into the rest of the brain.” ~Simone Meddle

Dr. Gabrielle Nevitt is a professor at the University of California Davis in the Department of Neurobiology, Physiology, and Behavior. She is plotting out ways that birds use smell to recognize individuals and to forage (look for prey) over the open ocean. Her previous work has shown that prions detect the odor of their mates, and storm-petrel chicks pinpoint their nest through smell. Her new paper ‘Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators’ published in Proceedings of the National Academy of Sciences of the United States of America explores how DMS facilitates interaction between seabirds and phytoplankton.

Navigating like a tube-nosed sea bird

Tube-nosed seabirds like albatrosses, shearwaters and petrels routinely fly thousands of miles over open ocean searching for krill to prey on. Even in fickle wind over open ocean, wandering albatrosses can track down a scent 12 miles away. Their sensory environment isn’t defined by the endless wave tops; they’re following their noses to patches of DMS being released where schools of krill or zooplankton are grazing on phytoplankton.

Nevitt described investigating a map of DMS odors over water to Audubon Magazine. She realized that the smells could feasibly inform seabirds of a kind of landscape atop the ocean.

"I could see peaks and valleys of DMS over shelf breaks, seamounts, and other underwater features, and I realized the ocean's surface wasn't featureless to the birds." "They have their own map, an odor landscape, in the air above the water." ~Gabrielle Nevitt

Why specifically seamounts? Phytoplankton need nutrients to grow and thrive. Yet nutrients are not plentiful near the surface of open ocean. They can feed into the ocean from rivers, or land atop the water carried by windblown dust, or cycle to the surface carried by upwelling water coming from the nutrient-rich ocean floor (landscape of dead organisms and fecal pellets). Relatively shallow parts of the ocean, like shelf breaks and seamounts, are often hotspots of productivity because they uphold nutrients that are normally found in waters too deep for sunlight to reach.

Iron is a nutrient that benefits phytoplankton. Once, whale feces provided iron enrichment. Phytoplankton grew with the help of iron, krill consumed phytoplankton and their iron, and baleen whales ate huge amounts of krill. Yet commercial whaling has made whales much more rare. Instead, the same cycle benefits phytoplankton with iron released from sea bird droppings. Birds are attracted to the DMS emitted when phytoplankton are eaten. The tube-nosed seabirds eat the predators (especially krill) that were eating the phytoplankton, which reduces pressure on the little organisms. The birds’ droppings provide fertilizer. It’s a mutually beneficial relationship which has the potential to boost primary productivity, and therefore benefit the entire marine ecosystem.

It benefits us as well, by reaffirming a cycle which releases DMS and results in cloud formation.

The wrench of ocean acidification

The ocean is a powerhouse, absorbing CO2 emissions in huge quantities which would otherwise be atmosphere-bound. However, when the ocean absorbs CO2 it also decreases the pH levels of ocean water, making the water more acidic. Like other climate amplification, ocean acidification is happening faster in the Arctic and at high latitudes than it is elsewhere. Ocean acidification has wide ranging consequences.

When the ocean is more acidic, phytoplankton manufacture less dimethylsulfoniopropionate – the precursor to dimethyl sulfide (DMS); ocean acidification leads to lower concentrations of DMS, and therefore fewer cloud-prompting sulfur based aerosols, cloud condensation nuclei. With less DMS, it is projected that fewer clouds will form over the vast oceans to block out the Sun’s warming rays. Katharina Six, Max Planck Institute for Meteorology, has a paper published in Nature Climate Change ‘Global warming amplified by reduced sulphur fluxes as a result of ocean acidification’ which projects that reduced cloud cover due to lower concentrations of DMS could could raise projected global average temperatures about 0.4 to 0.9 degrees Fahrenheit (0.23 to 0.48 degrees Celsius).

For action, against extinction

From Gabrielle Nevitt's ‘Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators’:

”To our knowledge, no studies have definitively resolved how marine ecosystems will respond to the extinction of marine top predators and the loss of their contribution to trace-nutrient recycling. Procellariiform seabird numbers are declining rapidly: Nearly half (46.5%) are listed as vulnerable, endangered, or critically endangered. Results presented here illustrate a fundamental, albeit understudied, link between apex predators and the base of the pelagic food web, suggesting that a decline in seabird populations could negatively affect overall marine productivity.”

Acting to lower CO2 emissions could reduce ocean acidification, lessening the loss of DMS. It can also benefit species which are facing major stressors as their environments change to the rapid pace of rising temperatures. Preserving predatory seabirds can benefit phytoplankton: CO2 sequesterers, primary producers and vital base of the marine food chain. Here’s to our oceans’ mighty photosynthesizers.

Laura Nielsen

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

References:

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{*} ‘Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators’ Matthew S. Savoca and Gabrielle A. Nevitt, Proceedings of the National Academy of Sciences of the United States of America, PNAS 2014 ; published ahead of print March 3, 2014, doi:10.1073/pnas.1317120111 http://www.pnas.org/content/early/2014/02/26/1317120111.full.pdf+html

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Laura Nielsen
Anchorage Daily News Bloggers