There's Trichoplax adhaerens, a Placozoa, made of a few thousand cells. Then there is Dicyema japonicum, a simple mesozoan, made up of 9 to 41 cells. Arguably, the simplest multicellular organism is the algae Tetrabaena socialis, whose body consists of 4 cells. Then, there's the parasitic Myxozoa which have 7 cells.


This nematode always has either 959 or 1031 cells. https://en.wikipedia.org/wiki/Caenorhabditis_elegans


Species from Gonium genus are typically 4-16 celled. Gonium pectorale is 16 celled. Arakaki, Yoko et al. “The simplest integrated multicellular organism unveiled.”, vol. 8,12, e81641. 11 Dec. 2013, doi:10.1371/journal.pone.0081641 Also, there is four celled Basichlamys sacculifera. See this picture for reference


There is a whole class of organisms called "oligocellular" organisms, see also here on SE.


Hydrogen sulfide can be toxic at high concentrations, however, the amount that your columns will make is likely very low, and will almost certainly degrade/oxidize into something safer before it hurts anyone (who isn't a small organism in the column!). It will noticeably smell bad at levels well below what is dangerous. So maybe just make sure that the ...


From the linked article: We found that hACE2 bound to SARS-CoV-2 SB and SARS-CoV SB with respective equilibrium dissociation constants of 1.2 nM (Figure 2A) and 5.0 nM (Figure 2B). An equilibrium dissociation constant (commonly called Kd) represents the propensity of a complex to dissociate into its constituent pieces. A smaller value represents a smaller ...


The best and most common would be the "multiple fermentation tube” or “most probable number” technique. Rather than testing for all pathogenic bacteria, it uses indicator microorganisms to assess the potability of the water. You'll need a lab to do this but if you have access to one, I'm sure that all the needed materials are common there. Here's a copy ...

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