UCA_5B, 6B_1: day 86

After having spent roughly one month in the light, our two columns present highly different characteristics, due to the dissimilar microorganisms that have grown in each one. In order to comprehend the transition from two months of darkness (in both columns) to the light, we'll try to cover their evolution from the very beginning.

4 g sugar column's evolution. The transition from dark to light happened in the last photo.

Control column's evolution. The transition from dark to light happened in the last photo.

At first, in the column containing 4 grams of sugar (and considering it was kept in the dark), chemoheterotrophic were extremely favoured (at first glance, one might think that the osmotic shock would make it impossible for any microorganisms to survive; however, this doesn't seem to be the case, since the last time we checked our column white-yellowish elementary sulfur had appeared, so there had to be some kind of bacteria producing it; we’ll come back to that later).

What the light did to our sugar column.

Despite not being able to see a colour change in the sugar column (owing to the lack of pigments in this kind of microorganisms), we know for a fact that they, indeed, were able to grow: there were huge emissions of gases (be it CO2, H2S or CH4), and this pushed the contents out of the column (along with the water). This could be the action of anaerobic and facultative anaerobic microorganisms (and definitely also organisms performing both aerobic and anaerobic respiration), so it's reasonable to think they could have grown all over the column (perhaps a bit more towards the bottom-center, where the column has been fractured likely due to the gas emission, so considering the oxygen gradient, this was probably the action of facultative anaerobic bacteria). Also, because of fermentation processes, organic acids must have been produced; and it's a possibility that these were used by our next group of anaerobic chemoheterotrophic bacteria (and they might have used the sugar as well).


We could also find FeS precipitates, indicating the presence of sulfate-reducing bacteria that reduce the SO4(2-) (that was obtained from the sulfur source, CaSO4 in our case) to H2S, which then reacted with the iron present in the sediment thus forming iron (II) sulfide. The fact that black sediments could also be seen at the top of the column (and at the bottom, too) seems to reinforce the idea that large quantities of gasses have pushed the column to the top). This didn't happen in our control column, where a black colour was observed only at the bottom, as one would expect due to the anaerobic nature of these microorganisms.

Side by side comparison after 1 month of light.

After we placed both columns in the light, we noticed a very interesting difference between the control and the sugar columns: large quantities of elemental sulfur (which has a yellowish, white tone) were found in the sugar one. We know there are two kinds of sulfur bacteria that can use H2S as a electron donor during anoxygenic photosynthesis: purple bacteria (think Chromatium) and green bacteria (for instance, Chlorobium). Nevertheless, the first one oxidizes H2S to S intracellularly (storing it in cell structures); while the other one produces extracellular sulfur, which is the one which's colour we can observe. Our problem here is that no green hue was to be found (it could have been masked? Maybe the bacteria died after producing sulfur? In theory, it should have been easier for photosynthetic organisms to grow in the sugar column, as the gas emissions produced by chemotrophs would have been used by them). Despite this, it makes sense that the elemental sulfur appeared at the bottom, as this is where H2S concentration is the highest. Yet, we still found something that might have been seen out of place at first: a crystal-like substance at the very top of the column. We think this is none other than sulfur that has made its way out of the column from the bottom propelled by gas emissions.

Sulfur microcrystals?

What happened to the control column the time it spent in the light?

Moreover, we couldn't find cyanobacteria in either column; this makes sense, especially considering that there was no water left in the sugar column (and almost none in the control one), a very important compound for this oxygenic autotrophic bacterium to grow (it's used as an electron donor during light-dependent reactions).


Lastly, photoheterotrophic bacteria (like Rhodospirillum) were found in both columns near the top, judging by the orange colour we observed. It seems they were able to grow more in the control column.

To conclude, if the white compound we found is indeed sulfur, this would support our last hypothesis (photosynthetic bacteria would find it easier to grow in the sugar column), as it indicates the presence of phototrophic bacteria that oxidize H2S.