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Bacteria – the smallest life forms in the ocean: How many are there?

Helge-Ansgar Giebel bedient das Durchflußzytometer, ein Gerät zur Bestimmung der Bakterienzellzahlen.
On board the Sonne: Helge-Ansgar Giebel works with the flow cytometer to count the number of bacterial cells in the water column.

The smallest forms of life in the ocean, bacteria, are the central focus of our research on board. Bacteria are single celled life forms; they are so small that we cannot see them without a microscope. Nevertheless, we want to know how many bacteria there are in the ocean; how are they counted?

Helge-Ansgar Giebel an unserer Äquator-Station.
Helge-Ansgar Giebel at our equator station.

Today’s report comes from Helge-Ansgar Giebel, from the ICBM research group “Biology of geologic processes – Aquatic Ecology” in Oldenburg. Here on board he is part of the team looking at the surface waters, which we define as waters to a depth of 1000 m. Helge also is responsible for all of the logistics for our cruise; he has, for example, just organized the return transport of our valuable samples. Thank you, Helge!

Although they are very small, bacteria play a very important role for life in the ocean. Although they consist only of a single cell, bacteria are survival specialists, and have developed unique capabilities. And they carry out very important “service functions” for other forms of life in the ocean. A large percentage of the available carbon and nutrients in the ocean is in dissolved form. Bacteria can take up these dissolved substances, and transform them into biomass as they grow and multiply. Through this process, dissolved food becomes particulate food. These bacteria (as small particles) can be eaten by other organisms in the food chain. Small organisms that filter to collect food (such as bacteria) are the next step in the food chain.  And when we have a nice piece of fish on our plate, we are also benefitting from the activities of marine bacteria.

Für die Zählung werden die Bakterienzellen im Meerwasser angefärbt. Links die angefärbte Probe, rechts unbehandeltes Meerwasser.
Bacteria in seawater are stained with a dye before they are counted. Left, the sample that has been stained; right, the unstained seawater.

How many bacteria are there in the ocean? Since we can’t see them by eye, we use a microscope to count them. Using a microscope on board a rolling ship is one of the most unpleasant jobs for a scientist – it is an easy way to get seasick. So we have brought a special instrument aboard the Sonne, a flow-through cytometer. Samples flow through the instrument, and the cells are counted through use of a laser. To make the bacteria more easily counted by instrument, we stain the cells with a special stain that binds to their DNA. The stained samples are sucked into the instrument, and pass in front of a laser. When a stained cell goes through the instrument, the laser produces a green color, and the cell is counted by the detector.

And there are many cells to count: at the surface of the ocean, there are approximately 1 million bacteria in a milliliter of water – which translates to a billion bacteria per liter! At a depth of 5000 meters, there are far fewer bacteria: a few thousand per milliliter. In sediments, there are typically far more bacteria (a billion per cubic centimeter – see earlier blog post). The sediment bacteria have to be counted under the microscope, however, since sediment particles interfere with the flow-cytometer so that the bacteria can’t be counted. Fortunately, the scientists responsible for counting them are not very prone to sea-sickness…