A transparent soil substitute usable without sterility
The superabsorbent polymer is a material that acts as a soil substitute while offering excellent transparency. It makes it possible to observe biological processes that normally occur underground and that, under natural conditions, are inaccessible to the eye. Its generic chemical composition makes it poorly assimilable by most microorganisms, which gives it a natural resistance to contamination. It can therefore be used without resorting to sterility, which represents a considerable advantage for approaching the conditions that prevail in the natural environment.
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- Physico-chemical properties
- Not all superabsorbent polymers are identical.
- How to obtain superabsorbent polymer
- Polymer preparation
- Two final tips
Physico-chemical properties of superabsorbent polymers
The term “superabsorbent polymer” actually refers to a large family of chemical products. These materials are made up of long chains of monomers whose properties vary depending on the nature and proportions of the constituent monomers as well as the crosslinking molecules. The following properties are particularly interesting for culture in a Petri dish:
Chemical properties:
- Contains little or no carbon available to most microorganisms
- High cation exchange capacity, favorable for plant nutrition
- Strong buffering capacity toward neutrality (pH close to 7)
- Shrinks significantly in the presence of dications (notably Ca²⁺ and Mg²⁺)
- Little or non-toxic (subject to consulting the manufacturer’s safety data sheet)
Physical properties
- Transparent, with a refractive index close to that of water
- Absorbs water and nutrient solutions, then releases them back to the plants
- Can be dehydrated and rehydrated repeatedly
- Insoluble in water
- Flexible texture when hydrated, easy to cut and reshape
- Roots and fungal hyphae can penetrate inside the hydrated granules
- Adheres slightly to plastic and glass when partially dehydrated
- Relatively transparent to 365 nm ultraviolet light, used for fluorescence
C) Medium-sized hydrated granules D) The same granules with water in the Petri dish
Non-toxic
As with any chemical product, it is recommended to consult the safety data sheet (SDS) provided by the manufacturer. In practice, the polymer can be handled with the usual basic laboratory precautions. It is important to note that superabsorbent polymers are not authorized for growing plants intended for food consumption. The recommendations presented on this site apply exclusively to laboratory, teaching, or research use.
Broad biocompatibility
Numerous observations made over the course of experiments have shown that the superabsorbent polymer is compatible with a wide diversity of living organisms. Protists, microscopic algae, amoebae, nematodes, rotifers, planarians, myxomycetes (Physarum polycephalum), and insect larvae have all been observed in Petri dishes containing the polymer — whether introduced deliberately or appearing as contaminants — and all of them seemed to develop without difficulty. Furthermore, since the Petri dishes are not sterile and the roots produce organic exudates, some bacterial growth nevertheless occurs, allowing the progressive establishment of a modest microbiota after a few weeks.
Not all superabsorbent polymers are identical.
It is essential to understand that two products bearing the same CAS numbers can have very different properties depending on the proportions of monomers used by each manufacturer. In practice, Petri dish culture trials conducted with polymers from different suppliers have produced results ranging from excellent to very poor. No supplier explicitly mentions using its product for laboratory culture in Petri dishes. This is therefore a new, marginal use, similar to that of the Sheaffer company, which does not recommend using its pen ink for staining hyphae and arbuscules (see the Staining techniques).
The superabsorbent polymer Hortasorb, sold by Horticultural Alliance, has been used for nearly all of the work presented on this site. It is marketed in granule form in three sizes: small, medium, and large. The “medium” granule size works well for work in Petri dishes.
How to obtain superabsorbent polymer
Hortasorb = Aquasorb
“Hortasorb” and “Aquasorb” are two trade names for the same chemical product.
Horticultural Alliance is part of the Hello Nature group, a US-based company specializing in biostimulants and beneficial microorganisms for plants and soils. Hello Nature can provide Aquasorb samples to researchers or experimenters wishing to use this polymer for work on endomycorrhizal culture.
For any sample request for research or academic purposes, please contact directly: helene.reynaud@hello-nature.com.
Preparing the polymer for culture.
The polymer can be used in two forms depending on the intended application: as granules or as a suspension.
Preparing the polymer in granule form
This is the simplest method: simply sprinkle the polymer granules into the nutrient solution and wait for hydration. Full hydration takes a few hours, so it is best to prepare the granules in advance, ideally 12 hours before use.
The amount of liquid absorbed by the granules depends directly on the mineral composition and pH of the solution used:
| Nutrient solution | Swelling rate |
| Demineralized water | Up to 200X or more the dry weight |
| mMS1 solution (the most common) | About 75X the dry weight |
| Acidic solution (vinegar) | Less than 50X the dry weight |
The rate of hydration also varies depending on the size of the granules and the nature of the nutrient solution.
Standard recipe:
- 1) Pour 500 ml of mMS1 solution into a beaker.
- 2) Sprinkle 5 g of dry polymer onto the solution
- 3) Cover and let hydrate at room temperature for 12 hours
- 4) Drain off the excess liquid
- 5) The granules are ready to be placed in a Petri dish.
Preparing the polymer as a suspension
This form of preparation is useful for certain specific operations, particularly germination tests or when greater flexibility is needed for handling hyphae or spores (this technique will be detailed later). The principle involves finely grinding the hydrated granules to obtain a translucent suspension of polymer microparticles.
Standard recipe:
- 1) Pour 500 ml of mMS1 solution into a beaker.
- 2) Sprinkle 5 g of dry polymer onto the solution
- 3) Cover and let swell at room temperature for 12 hours
- 4) DO NOT DRAIN the excess liquid
- 5) Pour the hydrated granules AND the liquid into a blender
- 6) Blend for a few minutes — typically 6 minutes total, working in 2-minute intervals to avoid overheating the motor
- 7) Let stand for at least 12 hours to allow the bubbles to escape
- 8) The suspension will have a creamy texture and will be ready for use
(brightfield), the suspension would be transparent and invisible.
Two final tips
Handling dry powder polymer: The polymer in dry powder form can release fine microparticles that may become airborne in a draft. It is recommended to wear gloves and protective goggles when handling it to avoid any contact with the eyes or respiratory tract
Since the hydrated polymer is insoluble in water, it will not dissolve in the sink or drain and could accumulate there. Please dispose of waste in a way that does not clog the drains.