Light Microscopy View of the Underwater World: Water Analysis via Light Microscopy

Water quality plays a significant role in almost every body of water. Its assessment often requires a look at the details, as organisms not visible to the naked eye are important indicators. This makes light microscopy an important tool.

Cologne in August: picture-book weather , 33 degrees, shimmering heat. How about jumping in the Rhine to cool down? But would that be a good idea? If we ignore the dangerous currents in Germany’s second-longest river, which can make any ideas of swimming extremely dangerous, what about the water quality? Just 60 years ago the river was so heavily polluted that residents downstream – particularly in the Netherlands as well as in Germany – were worried about their drinking water. Eyewitnesses reported that, downstream of the BASF plant, the Rhine would change color every day or often had a horrible phenol-like smell, or that you could see bubbles from digested sludge rising to the surface where sediments had been deposited. Thankfully, these horror scenarios are now a thing of the past. This is in part thanks to the awareness that has grown among the public for drastic pollution of bodies of water.

Water monitoring via light microscopy

All types of surface waters are investigated in Germany in terms of their quality and usability. Here, the organisms living in the water are also used for the assessment of water quality, as they are easy to collect and offer high value as indicators: thanks to their specialization in specific ambient conditions, they permit fast, long-term conclusions to be drawn about water quality.

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These organisms cannot be seen with the naked eye but can be determined quickly, inexpensively and reliably with routine microscopes. The structure of this type of water monitoring is presented here using river water from Berlin as an example.

Interesting facts about single-cell organisms (flagellates and ciliates)

Flagellates are single-cell organisms with one or more whip-like organelles called flagella. Ciliates are usually slightly larger single-cell organisms with hair-like organelles called cilia that resemble eyelashes. Both belong to the eucaryotic group of organisms called protists. They have a true nucleus and numerous subfunctions and occur in diverse morphological forms. In some cases, it is possible to find hundreds of them within a single drop of water. It has been known for a few decades that these protists can make a huge contribution to reducing levels of bacteria within the microbial loop, as they feed heterotrophically on bacteria (so-called “grazing”) [1, 2]. In addition, they play an important role in the research of the evolution of single-celled organisms into multicellular organisms. It is assumed that all modern multicellular organisms (animals) and the protists group of choanoflagellates have a common ancestor and that the multicellular organisms evolved from such protists (e.g. [6]).

Some representatives, like the choanoflagellates just mentioned, can eliminate very large numbers of bacteria, and presumably also viruses, per unit of time thanks to their filtering mechanism (the interplay between flagellar movement, the frills around the flagella and phagocytosis into the cell) [3, 4]. Because of this ability to eliminate pathogens among others, protists play a significant ecological role and can make a significant contribution to the quality of waters. Routine water monitoring designed to detect the abundancy (number of individuals) and diversity (number of different species) of the protists can make an important contribution here, since it is important to find a method for reducing water-polluting microbes.

Flagellates and ciliates also play an important part in the recycling of dissolved organic matter (DOM) as they consume bacteria that have previously fed on DOM as a nutritional basis. This completes the cycle – and DOM, which is produced e.g. from died organisms, is returned to the nutritional fabric.

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