In unity, there is strength 

See, a macrophytum! Well, not really, since the specimen fits under the microscope without the panorama stack. If these tiny plants join together and form an almost compact unit, we can't say that. In this case, duckweed negatively affects the life underneath its leaves. The Lemna canopy lowers the level of transmitted light throughout the water column. It also leads to a worsening of gas exchange. Lower oxygen concentrations are associated with higher levels of harmful hydrogen sulphide (H2S). Although sulphur is a biogenic element, hydrogen sulphide in particular is an effective inhibitor of respiratory chain proteins in the mitochondria.

The presence of H2S is obvious – it smells terrible and colours the surface of the sediment black. It is largely produced by bacteria reducing sulphur compounds for energy, because oxygen-poor environments contain other types of electron acceptors that can be used for respiration – in the case of sulphur, sulphates. These bacteria try to avoid places with oxygen because it is toxic to them. In other words, they may not have all the enzymatic machinery to eliminate reactive oxygen species (ROS), which are highly destructive to living cells. So while others suffer when Lemna dominates, some bacteria benefit. 

Surprisingly, neither sulphate reducing bacterias like hydrogen sulphide. Therefore, they tend to work together in different bacterial consortia that remove hydrogen sulphide. Once it has leaked from sediments or neighbouring cells, it is subsequently converted by photolithotrophic bacteria. This pathway depends on solar light, which may not be found in a puddle with overpopulated duckweed. Lithotrophic bacteria remain. Nevertheless, the oxidation of hydrogen sulphide doesn't have to take place. Electrons can be removed by nitrates or could be removed by ferric ions, but this is reliant on the availability of source compounds to the microbiome structure. 

Ferric ions are unstable because in the absence of oxygen they reduce back to bivalent iron, which becomes unusable. In addition to local communities and chemistry, metals maintain a certain equilibrium between gaseous, dissociated and bound forms of hydrogen sulphide. Their sulphides are black. 

S železem přichází potíž se stabilitou, jelikož v nepřítomnosti kyslíku se jako žadané trojmocné redukuje do nevyužitelné podoby. Rovnováhu plynného, disociovaného a vázaného sulfanu regulují mimo zdejší společenstva a chemismus konečně přítomné kovy, jejichž sulfidy jsou odpovědné za ono zabarvení dna. 

Valdés, J., Pedroso, I., Quatrini, R. et al. (2008): Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications. BMC Genomics 9(597): 1–24. Dostupné z: doi: 10.1186/1471-2164-9-597

Wang, R., Lin, J. Q., Liu, X.M., Pang, X., Zhang, C. J., Yang, C. L., Gao, X. Y., Lin, C. M., Li, Y.Q., Li, Y., Lin, J. Q., Chen, L. X. (2019): Sulfur Oxidation in the Acidophilic Autotrophic Acidithiobacillus spp. Front Microbiol. 9(3290): 1–20. Dostupné z: doi: 10.3389/fmicb.2018.03290 10;9:3290