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Petri's dish vs. Winogradsky's column –

shifting boundaries between purity and mixture of microbes

Winogradsky Column - Sediment depth hypothesis

Students: Nikolopoulou Ioanna, Lamprou Andriana

Department of Biological Applications and Technologies, University of Ioannina

Hypothesis

The microbial diversity correlates to the depth of the sediment in an inversely proportional way, with photosynthetic microorganisms being more prominent closer to the surface of the sediment.

Material and methods

The samples were collected from the Logarou lagoon, Koronisia, Greece at 20/10/2017. Sediment from the surface (0-2 cm) and from greater depth (18-25 cm), 700ml each, was enriched with 10 gr white sugar and 1 gr Ca2SO4 .Each sample was then placed in a transparent plastic bottle of 1,5L capacity and then 700ml of ultrapure water were added. The Winogadsky columns were stored at room temperature next to a window and were observed on a weekly basis.

Results

 On the first day of the experiment a difference was observed in the color of the two sediments. The superficial sediment had a dark green color, whereas the deep sediment appeared slightly brown. With the progression of time there was an increase of the sediment's volume as well as the appearance of small gaps in it, due to the production of gas, in both columns. This is mostly due to the presence of sulfate- and sulfur-reducing bacteria that produce H2S, which had a very distinct odor. The column with the superficial sediment presented a change in the color of the water as it obtained a green hue, while there was a slime-like formation in the surface of the water, possibly due to the presence of photosynthetic microorganisms (photo 2). At the same time there was no change in the appearance of the water in the other column.

 Approximately two months later (10/01/2018) an interesting observation was made. Regarding the deep sediment, a distinct stratification occured which was the most significant change in the appearance of this column. Specifically, a big part of the sediment turned from brown to black, while the water also obtained a blackish tint. This is due to the presence of black ferrous sulphide which is produced after the chemical reaction of H2S with any iron that is present in the sediment, due to the strong presence of sulfur- and sulfate-reducing bacteria. At this point the consistency of the deep sediment resembles the natural sratification that was observed in a larger scale in the field during the sampling process. It is important to note that in this column during the experiment, the water did not turn green, whereas in the column with the superficial sediment the water turned greener with time. However, no stratification was observed in the superficial sediment during the experiment. 

The presence of green color in the water of the column with the superficial sediment indicates the presence of photosynthetic microorganisms, which seem to be absent from the other column. Due to the use of ultrapure water, any microorganisms that are present in the column must have originated from the sediment, hence only the superficial sediment seems to have photosynthetic mircoorganisms. The production of gas in both columns is a strong indication that sulfur- and sulfate-reducing bacteria are present in both columns and therefore at both depths. 

 We conclude that the superficial sediment has a greater diversity in microorganisms, specifically more that are involved in photosynthesis. However, we can not estimate accurately the diversity in both samples through optical observation alone, so molecular assessment should be implemented. Furthermore, it is important to note that the sediment can act as a microbial sink, which can provide microorganisms to the ecosystem.

Image 1:Winogradsky columns at 20/10/2017(1),13/11/2017(2), 10/01/2018(3)  Left column: deep sediment. Right column: superficial sediment