UCA_1A-9A_1: Day 14 Darker in the dark

Rio San Pedro sediment + 1'48g CaSO4+ filter paper

First of all, I’ll explain how we made our Winogradsky column two weeks ago. We took some sediment from Rio San Pedro and removed all the shells, wood sticks and rubbish that we found. After that, we added 1.48g of CaSO₄. , some filter paper which it’s rich in cellulose, and some water from the same place as the sediment. Then, we mixed all and filled ¾ parts of the bottle with it. Finally, we filled half of the remaining part of the bottle with water. Later, we made two columns. The first one has been exposed to the sunlight and the second one has been in a closed box.

Some components have separated due to their density, which means that most of the CaSO₄.  has moved to the bottom of the column while on the top there is a lot of oxygen. Therefore, we expect aerobic microorganisms to grow on the top of the column while anaerobic microorganisms will grow at its bottom. Dark column should contain chemotroph organisms because it has not been exposed to the sunlight, whereas light column might contain chemotroph as well as phototroph organisms.

Dark column:

We can appreciate some dark zones spread around the bottom of the column where there is less oxygen and more CaSO₄. . This effect occurs because sulfate-reducing bacterias reduce sulfate (SO₄^2- ) to hydrogen sulfide (H₂S) which reacts with the iron contained in the sediment and so producing iron sulfide (FeS) which is black.

Light column:

In this column there are more dark zones than in the dark one, which means that there are sulfate-reducing bacterias. They are found at the bottom of the column as well. However, in this case there are red marks all over the column because the Fe³⁺  is produced by iron-reducing bacterias.

Comparing both columns we found out that there is more Fe³⁺  where sunlight falls. This is why we see more red marks on the left side of the light column (pic. 3). To conclude, iron-reducing bacterias are phototroph, and sulfate-reducing bacterias are both chemotroph and anaerobic because they accumulate at the bottom of the column. However, both bacterias use carbon from filter paper.

Light column

Dark column

Pic. 3