The Role of Protozoans and Microscopically Small Metazoans in Aquatic Plant Litter Decomposition

Protozoans and microscopically small invertebrates are omnipresent in aquatic systems but, arguably, under-studied, and this is true when it comes to one of the best described ecosystem processes in fresh water—leaf litter decomposition. However, over the last ten years research has been emerging that points to the important role of these microbes (such as ciliates and micro-crustaceans) as consumers of leaf prokaryotes and fungi. In fact, a decomposing leaf can be viewed as a micro-world of a complex food web with tiny organisms interacting with each other—fuelled by energy and nutrients from the leaf and the water interface. Evidence from the latter comes from biofilm studies that highlight the need to incorporate protozoans and micro-metazoans into our understanding of plant litter decomposition. Here, publications on laboratory and field studies are summarised, to show that protozoans and micro-metazoans are not only present on decomposing detritus, but that they can influence leaf litter decomposition. After presenting this empirical evidence, I conclude with some theoretical and more abstract thoughts on why these microbes have an important role in energy and nutrient cycling that links leaves as a substrate for biofilm with surrounding compartments such as the benthic zone and the open water.