Physiology and Function

The first studies on the anaerobic nature and the fermentation of cellulose by termite gut flagellates were performed by Hungate in the 1940s (reviewed by Hungate 1955).

Hungate's work with mixed protozoan suspensions and the few pure culture studies of parabasalid flagellates obtained from the hindgut of Zootermopsis species (Trager 1934; Yamin and Trager 1979; Yamin 1980, 1981; Odelson and Breznak 1985a,b) led to the current hypothesis that the glycosyl units of cellulose and possibly other polysaccharides in the wood particles taken up by the protozoa are fermented to acetate, CO2, and H2 according to the following equation:

(C6H12O6) + 2 H2O ^ 2 CH3COO- + 2 H + + 2 CO2 + 4 H2

The concept is supported by the high hydrogen partial pressure and the dominance of acetate among the short-chain fatty acids in the hindgut of all lower termites investigated (reviewed by Radek 1999; Breznak 2000; Brune 2005).

The acetate produced in the hindgut is resorbed and serves as the main respiratory substrate of the host; this is probably the most straightforward explanation why the well-being and survival of lower termites on a normal, lignocellulosic diet depend on their gut protozoa (Breznak 2000; Brune 2003, 2005). Despite the demonstration of host cellulases in the secretions of midgut and salivary glands of lower termites (see Watanabe and Tokuda 2001 for a review), the cellulolytic activities in the hindgut seem to be largely of protozoan origin (Ohtoko et al. 2000; Nakashima et al. 2002).

However, data on the physiology of gut flagellates are scarce, and the current concept of cellulose metabolism by termite gut flagellates is largely based on assumptions and inferences from other systems. It is assumed that the soluble sugars resulting from polysaccharide degradation in the food vacuoles are oxidized to pyruvate via glycolysis in the cytoplasm. Pyruvate is then imported into the hydrogenosomes - special organelles that enable anaerobic protozoa and fungi to produce molecular hydrogen. Hydrogenosomes were originally discovered by Lindmark and Müller (1973) in the parabasalid flagellates Trichomonas vaginalis and Tritrichomonas foetus, which are parasites of humans or bovines. They contain several key enzymes: pyruvate-ferredoxin oxidoreductase and hydrogenase, which convert pyruvate to acetate, CO2, and H2; and phosphotransacetylase and acetate kinase, which allow the formation of ATP, which is subsequently exported from the hydrogenosomes into the cytoplasm (Müller 1993; Hackstein et al. 2002). The oxidation of glucose to acetate according to Eq. (1) requires that also the reducing equivalents produced during glycolysis are transported into the hydrogenosomes and released as H2.

It should be emphasized that this concept is mainly based on results obtained with trichomonad flagellates that are parasites of mammals. There is no physiological data on the hydrogenosomes of termite gut flagellates. The presence of hydrogenosomes in hypermastigids is based only on microscopic evidence and the fermentation products of a few axenic cultures (see above).

Virtually nothing is known about the physiology of oxymonads, and not a single pure culture has been obtained so far. These protists do not seem to possess hydrogenosomes (Bloodgood et al. 1974; Radek 1994), which means that their metabolism must be different from that of parabasalids. Assuming that oxymonads cannot form hydrogen (which remains to be tested), other reduced fermentation products have to be expected. In Reticulitermes flavipes, where oxymonads are the most abundant flagellates (Cook and Gold 1998), lactate has been identified as a key intermediate in hindgut metabolism (Tholen and Brune 2000). The human pathogen, Trichomonas vaginalis switches to a lactic acid fermentation when pyruvate-ferredoxin oxidoreductase is inhibited with metronidazol (Cerkasova et al. 1986).

The gut flagellate communities of each termite species are stable and host-specific, but also quite diverse (Yamin 1979), which implies also a high degree of functional diversity. There is evidence for a resource partitioning among the different protozoan species colonizing the same gut, which seem to be specialized on cellulose or wood particles of different size classes

(Yoshimura et al. 1993; Inoue et al. 2000). Also a specialization on other wood components such as hemicellulose would create additional niches (Inoue et al. 1997). Certain flagellates do not seem to ingest wood particles (e.g., Yoshimura et al. 1996), and also pinocytosis of dissolved substrates (Hollande and Valentin 1969) or a bacteriovorous lifestyle (Huntenburg et al. 1986) would be alternative strategies of survival.

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