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what's to eat?

Greek Origin
Word of the Day:


from “protistos” or “proto” meaning first (or earliest)

Protists get hungry, too

December 6, 2011 (posted December 7, 2011)
by Cherie Winner

While I was sitting at my place in the main lab early this morning, checking e-mail, Dr. Maria Pachiadaki opened the door from the starboard deck.

“Hera, Cherie, come look! There’s a rainbow!”

It had to be a good sign. Maria was out on deck to deploy the Deep SID in an attempt to learn about the basic ecology of protists in the deep sea. Her original plan was to do the experiment with SID-ISMS, but when that instrument had problems, Maria couldn’t do her experiment. Fortunately, Deep SID saved the day.

Maria’s goal was to do a groundbreaking experiment to study how bacteria are affected by protists grazing on them. When ecologists study large land animals such as wolves or deer, one of the things they try to find out is what the animals eat and how much of it they eat. They need that information to understand the role of each species in the ecosystem and how nutrients and energy are cycled through the system.

Scientists who study microscopic organisms that live in the sea such as protists want to know the same information. But how do they find out what and how much microbes deep in the ocean eat?

They know that protists in the deep sea consume bacteria, archaea, other protists, or cellular debris from organisms that have died. But there are many kinds of protists in the ocean. Which ones eat what? How much do they eat? If they find a new source of food, like a strand of filamentous bacteria, how fast can they chow down?

Until now, the primary way to find out was to bring water samples to the surface, add fluorescent “labels “ to natural prey as tracers, and watch what happens.

For samples from shallow depths, that might be OK. But as you know by now, that is not a good approach to use with samples from very deep in the sea, because simply bringing the organisms to much lower pressures at the surface changes them in ways we don’t fully understand yet.

So Maria set out on this cruise to use the SID-ISMS to do a “grazing experiment” with protists and bacteria previously collected from deep waters near this DHAB. On the ship, the bacteria were labeled with a fluorescent dye, then sent back down in SID-ISMS. When the instrument reached the DHAB again, it would add the labeled bacteria to a fresh sample of DHAB water containing protists. At different time points it would sample and preserve some of the water.

After a few hours, SID-ISMS would return to the ship and Maria would count the fluorescent bacteria. Since she knew how many bacteria were there at the beginning, that would tell her how many had been eaten.

She might also be able to see the labeled bacteria inside the protists that had eaten them. That might give her an idea of what kind of protists were eating the most.

Although Maria wasn’t able to use SID-ISMS to complete her experiment, the instrument did bring back good samples of bacteria and protists (see video slideshow below). She used Deep SID to do a modified version of the experiment.

Deep SID can’t go deep enough to reach the DHABs, so instead Maria is looking at grazing by protists at several depths. She has successfully done her experiment at 40 meters, which is in the photic zone where sunlight can penetrate; at 200 meters, below the photic zone; and at 500 meters, far below the photic zone.

At midday Wednesday, Deep SID went on a mission to 950 meters—about as deep as it is has ever gone before. It was successful! At 2 p.m. Deep SID was sent down again to try the experiment a final time at 2,000 meters. We’ll see what happens!

Chief scientist Ginny Edgcomb says that as far as she knows, no one has ever done an in situ (in place) grazing experiment at such depths using bacteria from the original environment. The results are sure to tell us something new about some of the ocean’s tiniest residents.

Maria’s work fits in between the work of two other scientists on this cruise. Dr. Konstantinos Kormas is studying the bacteria from DHABs, and Dr. Hera Karayanni is studying what eats the protists that have eaten the bacteria. Together, they will be able to assemble a good picture of the first few links in the food chain deep in the Mediterranean Sea, where protists are the link between bacteria and archaea below them on the food chain and the larger animals above them, such as fish larvae, jellyfish, and squid.

On its first deployment to a DHAB halocline, SID-ISMS collected water and fixed (preserved) the cells in it. Back on the ship, Maria Pachiadaki stained the cells with DAPI, a fluorescent dye that binds to DNA. Chief scientist Ginny Edgcomb took these photos of the cells through a fluorescence microscope.


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