Supplementary information for Altermatt et al. Methods in Ecology and Evolution. DOI: 10.1111/2041-210X.12312

“Big answers from small worlds: a user's guide for protist microcosms as a model system in ecology and evolution”

Altermatt F, Fronhofer EA, Garnier A, Giometto A, Hammes F, Klecka J, Legrand D, Mächler E, Massie TM, Pennekamp F, Plebani M, Pontarp M, Schtickzelle N, Thuillier V & Petchey OL

3.4 Nutrient concentration and viscosity of the medium


Manipulating the nutrient content of medium

The level and temporal availability of nutrients are parameters that determine ecological conditions such as resource pulses (Yang et al. 2008), environmental heterogeneity and autocorrelation (Laakso, Loytynoja & Kaitala 2003). Nutrients interact with intrinsic features of the population or community to create resonance (Orland & Lawler 2004), productivity-diversity relationships (Haddad et al. 2008; Altermatt et al. 2011) or relationships between productivity and evolutionary responses (Friman & Laakso 2011). Nutrient levels and the temporal availability of nutrients are easily manipulated in microcosms.

In axenic cultures, the nutrient availability is directly manipulated via the concentration of the medium, whereas in bacterized medium, the nutrients available to the bacteria are manipulated, which then feed back into increased bacteria abundance.

Different numbers of protist pellets were used by Holyoak (2000) (1, 2, and 4; each with a weight of 0.57 g, translating to 0.57, 1.14 and 2.28 g per litre for low, intermediate and high concentrations) whereas Orland & Lawler (2004) manipulated the amount (in grams) of the protist pellet directly (low: 0.2 g/l , high: 1 g/l). Cadotte et al. (2006) used levels of 1g, 0.1g and 0.01g of protist pellet per litre for high, intermediate and low nutrient levels respectively, in addition to different vitamin provisions. Haddad et al. (2008) manipulated nutrient levels by replacing part of the medium with nutrient-free sterile spring water, compared to a nutrient treatment that replaced the original medium with fresh medium of the same type.

Luckinbill 1978 and Luckinbill & Fenton (1979) varied the amount of nutrients available directly via changes in bacterial abundance as well as indirectly via nutrient availability. Friman et al. (2008) manipulated low and high nutrient concentrations by two- versus eightfold dilution of the cerophyll medium to study the effects of productivity on the ecological and evolutionary dynamics of a predator-prey interaction.

Besides, seeds that slowly release nutrients are used to manipulate the carbon sources available to bacteria, which in turn feedback to higher abundances of bacteria as protist prey. These are often added to stabilize the dynamics of the communities (e.g., Haddad et al. 2008; Altermatt, Schreiber & Holyoak 2011), but also to manipulate nutrient concentration (e.g., Fox 2007).

Manipulating viscosity of the medium

Methyl cellulose is well-known for increasing the viscosity of liquid media (Sonneborn 1950). A higher viscosity slows down the movement speed/ability of protists, and this is often used to slow down protists for microscopy purposes (Sleigh 1991). However, it can also be used to manipulate the movement behaviour in the context of behavioural experiments (e.g., to affect the outcome of predator-prey dynamics) or the costs of movement/dispersal due to increased drag in liquid medium. According to Beveridge et al. (2010a; 2010b)(and references therein) the most suitable compound for adjusting the viscosity of microcosm media is Ficoll® [GE Healthcare companies] (Winet 1976; Bolton & Havenhand 1998; Abrusán 2004; Loiterton, Sundbom & Vrede 2004). Ficoll has broadly the same effect as methyl cellulose, however, the handling of the substance is easier than that of methyl cellulose. Ficoll dissolves in water regardless of the temperature (methyl cellulose dissolves better at low temperatures), shows Newtonian fluid properties in solution and only requires small quantities to change the viscosity without being toxic.



Manipulating nutrient concentration of the medium:

  • Microbalance to weigh specific amounts of protist pellet/seeds

Manipulating viscosity of the medium:

  • Microbalance to weigh the amount of methyl cellulose or Ficoll

  • Heater or water bath


Manipulating nutrient concentration of the medium:

  • The same as for the production of the basic medium for dilution.

  • Sources of slow nutrient release such as autoclaved and standardized wheat or millet seeds.

Manipulating viscosity of the medium:

  • Medium prepared according to section 1.2.

  • Methyl cellulose is readily obtained from local pharmacies (often with varying names according to the producer); concentrations of around 3.5 gL–1 are reported in the literature (Luckinbill 1973; Veilleux 1979) to manipulate the swimming/movement of Paramecium aurelia and Didinium nasutum.

  • Ficoll (GE Healthcare companies); Ficoll concentrations of 0, 0.5, 0.7, 1.5, 2 and 2.5% (by mass) produce a viscosity range of 1 x 10–3 to 1.52 x 10-3Nsm–2 at 20 °C, the same as for viscosities expected at temperatures from 20 to 5 °C (Beveridge, Petchey & Humphries 2010a; Beveridge, Petchey & Humphries 2010b).


Manipulating nutrient concentration of the medium:

  • Dilution of the medium to levels of lower nutrient availability.

Manipulating viscosity of the medium:

A) Methyl cellulose:

Because methyl cellulose is a hydrophilic substance and only dissolves in cold water, a special procedure is required to obtain a homogeneous solution:

  1. Add half of the powder into warm medium, let it soak for a moment, then add the remainder till particles are well dispersed in the medium.

  2. Cool down the medium in ice while stirring leads to a much more rapid dissolution of the particles.

B) Ficoll:

  1. Add the selected concentration of Ficoll (by mass) to the medium.

  2. Stir and use directly.


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