Marine sands often contain large numbers of protzoa and algae. Sands with a particle size of about a tenth of a millimeter tend to be richest, coarser sands occur at more energetic beaches and have fewer organisms, fine sands tend to become muddy and alternative techniques apply.

The organisms live in the spaces between the particles (interices - in the interstial fluid), but often also are attached or cling to the surfaces of the particles. The latter habitat facilitates survival of organisms in marine sands that will be moves around by waves and tides. It also makes it hard to get the organisms out of the sand.

The technique developed by Uhlig flushes the organisms out of the sediments with a combination of gravity, cold (to slow down swimming behaviour) and most significantly a pulse of hypersaline water.

This technique works well in extracting ciliatesand dinoflagellates from marine sandy sediments.

Equipment:

• Rack to suspend Uhlig tubes
• Uhlig tubes. Ideally these are tubes with a mesh of known pore size across one end. Homemade alternatives can be made from plastic cups or tubes from which the bottom has been removed with a sharp blade, and a piece of mesh held in place with an elastic band.
• Nytex nylon mesh approximately 100 microns in size is about right. It can be helpful to have filters of various sizes from 50 - 500 microns. The size of the pores can be adjusted to suit the sediments, but if too much sediment comes out of the tubes, a pad of cotton wool can be used to hold back finer mud.
• Petri dish or culture vessels, two sizes, the smaller one being at least as wide as the extraction tube and will be required for microscopy, and even wider one (150 mm) to collect any spilling water
• Sea water ice, prepared the night before.

Method

• Collect sample, you can use a trowel or a corer. Samples of 100 - 200 mls are usually appropriate. Do not allow the sample to dry out or to drain completely.

• In the lab, place the sample in a Uhlig tube, about two thirds filling the tube

• Fill the tube with crushed sea-water ice

• Suspend the tube so that the bottom of the mesh just touches the smaller petri dish, which itself is inside a lager dish. As the sea-water ice melts, a wave of hypersaline cold water will pass through the sand and flush out various organisms. Check the receiving dish after 30 minutes, one hour and two hours. In the early stages, the collecting dish will contain fairly normal sea water but after a while it will start to receive hypersaline water and this could kill the organisms.

• Cells can be observed and removed under a disssecting scope. As many of the organisms are thigmotactic, this can be a bit of a challenge.

• Sand samples from various depths can be sampled by slicing cored samples and placing each into its own Uhlig extractor.
  
• Ciliates are generally positively geotropic and will migrate out of the sand at different rates of speed and at different times. Therefore it is best to sample the separator at time intervals (e.g. 6,12,24, and 36 hours). The method works well for freshwater, brackish, and marine silicate and carbonate sandy sediments.
  
• Sand samples from various depths can be sampled by slicing cored samples and placing each into its own Uhlig extractor. The technique can be made semiquantitative ... but good and reliable extraction of protists from sediments
  

Notes

• When ice melts, the initial liquid is hypersaline. This technique works because the ciliates in the sand will actively swim away from the hypersaline water. The flow of cold water assists the process. When they arrive in the receiving dish, their thigmotactism keeps them attached to the dish surface, so they are not washed away as more water floods through
• This method is particularly good for karyorelict ciliates which abound in these habitats. It is not easy to distinguish these genera and species. It would be helpful to develop a matrix key for these guys..
• The approach can be made semiquantitative.

Reference

Uhlig, G. 1964. Eine einfache Methode zur Extraction der vagilen, mesopsammeln, Mikrofauna. Helgoländer Wissenschaftlicher Meeresuntersuchungen. 11: 178-185.