Hydrogen Metabolism

The symbiosis with methanogens is likely based on the interspecies transfer of molecular hydrogen. Little is known about the physiology of the protozoa, but from thermodynamic considerations, any fermentative metabolism releasing part of the reducing equivalents as H2 will become more exergonic if this product is not allowed to accumulate (Schink 1997; see also Chapter by B. Schink). Virtually all 16S rRNA gene clones of methanogens obtained from lower termites fall into the phylogenetic radiation of the genus Methanobrevibacter. Since all members of this genus, including the three species isolated from the gut of Reticulitermes flavipes, perform methano-genesis from H2 and CO2, but use other substrates less efficiently, it is reasonable to assume that also the hitherto uncultivated Methanobrevibacter strains associated with the protozoa are potential hydrogen consumers (Lee et al. 1987; Tokura et al. 2000).

Based on the results of various treatments that selectively eliminate or affect certain members of the gut microbiota of Zootermopsis angusticollis, Messer and Lee (1989) concluded that the large protozoa of the genus Trichonympha are the most important hydrogen source in the hindgut, whereas the methanogenic symbionts of Trichomitopsis termopsidis produce most of the methane.

It is therefore possible that the bacterial symbionts of the larger protozoa are also involved in hydrogen metabolism. Reductive acetogenesis plays a major role as hydrogen sink in the guts of virtually all lower termites (Brauman et al. 1992; Breznak 1994). So far, only a few homoacetogens have been isolated from termite guts (Breznak 1994; Boga and Brune 2003; Boga et al. 2003). The recent demonstration of reductive acetogenesis in Treponema primitia and other termite gut isolates (Leadbetter et al. 1999) makes them likely candidates for this function, especially since spirochetes are often associated with certain gut flagellates (see Sect. (8.)4.1.2).

However, in view of the high H2 partial pressures in the hindgut lumen of Reticulitermes flavipes (Ebert and Brune 1997), it is unlikely that - at least in this termite - an improved hydrogen transfer is the basis for the close association. The fact that spirochetes and methanogens are also attached to oxymonad flagellates, which do not possess hydrogenosomes (Bloodgood et al. 1974; Radek 1994), indicates that there may be other reasons for these associations.

Other Possible Functions

The fermentative metabolism of termite gut flagellates is poorly studied, and especially in the case of oxymonads, where hydrogenosomes are lacking, further work is sorely needed (see above). Reduced fermentation products other than hydrogen would also open the possibility of different metabolic links between the protozoa and their prokaryotic symbionts. Microinjection of radiolabeled metabolites into the hindgut of Reticulitermes flavipes has revealed that lactate is an important intermediate in the carbon flux from polysaccha-rides to acetate, but the microorganisms responsible for the production and consumption of lactate remain to be identified (Tholen and Brune 2000).

There are also other hypotheses concerning possible roles of the prokaryotic symbionts of gut flagellates. One of them would be the production of cellu-lases, as suggested for the endosymbionts of the larger flagellates (Bloodgood and Fitzharris 1976). However, after elimination of the endosymbionts by antibiotics, the hypermastigote Trichonympha agilis is still capable of degrading cellulose (Yamin 1981), and the hindgut protozoa seem to posses their own cellulase genes (Ohtoko et al. 2000).

Another possible function would be in the fixation of atmospheric nitrogen. Nitrogen fixation is an important process in wood-feeding termites (reviewed by Breznak 2000). It has been estimated that 30-60% of the total nitrogen in Neotermes koshunensis is derived from dinitrogen (Tayasu et al. 1994), and a few diazotrophs have been isolated from termite guts (for references, see Breznak 2000). Recently, also spirochetes - including the termite gut isolate Treponema azonutricum - have been shown to fix atmospheric nitrogen (Lilburn et al. 2001; Graber et al. 2004).

There is an enormous diversity in the nifH genes present in the guts of each termite species, and most of them cannot be assigned to described species of prokaryotes (Ohkuma et al. 1996, 1999a; Lilburn et al. 2001). In addition, the analysis of nifH gene expression in the gut of Neotermes koshunensis has shown the preferential expression of nifH from an alternative nitrogenase (anf), which could not be assigned to a known microorganism (Noda et al. 1999) and may belong to one of the epibiotic or endosymbiotic prokaryotes.

It has been postulated that the epibiotic bacteria of Streblomastix strix are chemotactic sensors for acetate (Dexter and Khalsa 1993), but in view of the high acetate concentrations in termite hindguts (see Breznak 2000 for review), it is not convincing that a chemotaxis towards acetate should be of any advantage for the flagellate. Leander and Keeling (2004), who investigated the ultrastructure of this fascinating association, could determine at least three different morphotypes among the epibionts that differed mainly in size. Elimination of epibionts by antibiotic treatment resulted in a modified morphology of the flagellate, which suggests an influence of the symbionts on the shape of the host.

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