“ ! …Comment … ? ” stage for the genesis of the first eukaryote and date there are no ‘ancient’ protists (or Microbiology Comment provides a much attention has therefore centred on these Archezoa) which have been shown to be platform for readers of Microbiology to ‘anaerobic’ protozoa. The milestone in this strictly anaerobic. communicate their personal observations evolution is the endosymbiosis of the mito- Strict anaerobes can however be found in and opinions in a more informal way than chondria and thus the passage from an anaer- the crown of the eukaryotic tree. Ciliates, such through the submission of papers. obic to an aerobic way of life. Here, we would as Metopus contortus and Parablepharisma Most of us feel, from time to time, that like to briefly illustrate how this notion of a sp., do not survive for more than a few hours other authors have not acknowledged the link between anaerobiosis and primitism is when exposed to aerobic conditions and work of our own or other groups or have unfounded and how it has misled subsequent require strict omission of O2 for growth omitted to interpret important aspects hypotheses for the evolution of the eukaryotic (15). However, even in these organisms, the of their own data. Perhaps we have cell. capacity for anaerobic biosynthetic metabo- observations that, although not sufficient To discuss anaerobicity it is paramount to lism has not been demonstrated. Phylogenetic to merit a full paper, add a further be clear as to its definition. To describe an studies of these organisms clearly show that dimension to one published by others. organism as anaerobic because O2 is not they have derived, on several occasions, from In other instances we may have a useful required for oxidative phosphorylation is aerobic ciliates (12). Interestingly, examples of piece of methodology that we would practical in many circumstances, but may be microaerobic ciliates behaving as facultative like to share. misleading in this context. Biochemically, a anaerobes have been described in the labora- true anaerobic organism should be able to tory (2). Strict anaerobic ciliates also contain The Editors hope that readers will take carry out all its metabolic functions, such hydrogenosomes and in some cases, such as full advantage of this section and use it as sterol and fatty acid metabolism, anaero- in Cyclidium and Cristigera, both mitochon- to raise matters that hitherto have been bically, without the need to scavenge meta- dria- and hydrogenosome-bearing species are confined to a limited audience. bolic constituents from its environment. A found within the same ciliate genus (16, 14). Jon Saunders, Editor-in-Chief distinction must also be made as to whether an organism is aerotolerant or whether it Problems with current benefits from low levels of O2. mitochondrial hypotheses Primitive anaerobic The first misconception regarding anaero- protozoa: a false concept? Primitive ‘anaerobic’ protozoa bicity and eukaryotic antiquity arose from the somewhat artificial establishment of enjoy just a little O2 Since Bütschli (7), protistologists have tried ‘Anaerobic’ protozoa lack electron-linked the amitochondriate protozoan group, the to not only organize the interrelationships oxidative phosphorylation. They do, how- Archezoa (9). By various phylogenetic means [but mainly by nuclear small subunit (SSU) between the different groups of protists, but ever, take up O2 at high rates and some also to build phylogenetic trees to explain the have been observed to have higher affinities rRNA analysis], the Archezoa were inferred evolution of the eukaryotic cell. In the last 20 for O2 than their aerobic counterparts years, results obtained by molecular tech- containing cytochrome oxidase (4, 21). Low ᭤ GUIDELINES niques and electron microscopy have furnished levels of O have been shown to affect the 2 Communications should be in the form general ideas on what the tree of life may look carbon balance of Giardia and Hexamita (26, of letters and should be brief and to the like. The base of the tree generally includes 5), both belonging to the diplomonads, until point. A single small Table or Figure may groups such as Metamonada (e.g. Giardia, recently generally favoured as forming the be included, as may a limited number of Hexamita and Trepomonas), Microsporidia deepest branching eukaryotic group (20). references (cited in the text by numbers, and Parabasala (e.g. Trichomonas vaginalis, Trichomonas vaginalis, far from being anaer- and listed in alphabetical order at the end Pseudotrichomonas keilini) (9). obic, requires traces of O for optimal growth 2 of the letter). A short title (fewer than 50 These basal ‘anaerobic’ protozoa differ and reproduction (25). Why O is required by 2 characters) should be provided. from aerobic cells by the absence of mitochon- these organisms is not known for certain, but dria and hence oxidative phosphorylation. part of the reason may be to recycle electron Approval for publication rests with Energy generation is derived from glycolysis acceptors such as NAD(P)+ (unpublished the Editor-in-Chief, who reserves the with some unusual features (i.e. some enzymes observations from our laboratories in fact right to edit letters and/or to make a are pyrophosphate-dependent) and also from conclude that in growth conditions where O2 brief reply. Other interested persons may alternative substrate fermentation (i.e. the has been strictly omitted, some diplomonads also be invited to reply. The Editors arginine dihydrolase pathway). Some of them fail to grow). Therefore, the correct term to of Microbiology do not necessarily (e.g. the trichomonads) also possess unusual describe these organisms should be micro- agree with the views expressed in redox organelles known as hydrogenosomes aerophilic. A strict anaerobe in this context Microbiology Comment. which, as well as other products, generate H2 will be defined as an organism which is (giving them their name). highly susceptible to O and which will grow Contributions should be addressed to the 2 Editor-in-Chief via the Editorial Office. Early anoxic Earth has been set as the optimally in an O2-free environment. To Microbiology 146, May 2000 1019 Microbiology Comment as being primitive and anaerobic, urging explain the absence of DNA in hydrogeno- free-living diplomonad Hexamita sp. J Euk Microbiol 44, the notion that the organism which first somes. However, it has been shown recently 447–453. 5. Biagini, G. A., McIntyre, P. S., Finlay, B. J. & Lloyd, D. harboured the mitochondrial endosymbiont that the hydrogenosome-bearing symbiotic (1998). Carbohydrate and amino acid fermentation in the was a primitive anaerobic protozoan. This ciliate Nyctotherus contains DNA (as con- free-living primitive protozoon Hexamita sp. Appl hypothesis, however, has now generally been firmed by immunocytochemistry) (1). Further- Environ Microbiol 64, 203–207. rejected. Over the past few years, genes of more, this recent study revealed that rDNA 6. Budin, K. & Philippe, H. (1998). New insights into the mitochondrial origin have been shown to phylogeny of eukaryotes based on ciliate Hps 70 from Nyctotherus hydrogenosomes shows high sequences. Mol Biol Evol 15, 943–956. be present in these Archezoa (namely the sequence similarity to mitochondrial SSU 7. Bütschli, O. (1880–1889). Protozoa I, II, III. In Klassen microsporidia, Giardia and Trichomonas; see rRNA genes from aerobic ciliates: surely, the und Ordnung des Their-reichs. Leipzig. recent review, 11). Furthermore, there are nail in the coffin for the die-hard sceptic of a 8. Cavalier-Smith, T. (1987). The simultaneous symbiotic growing concerns as to the use of single mitochondrial origin for hydrogenosomes. origin of mitochondria, chloroplasts and microbodies. Ann N Y Acad Sci 503, 55–72. genes to confer phylogenetic relationships. Taking into account the probable mono- 9. Cavalier-Smith, T. (1993). Kingdom protozoa and its Significantly, recent studies now favour the phyletic origin of mitochondria, there is 18 phyla. Microbiol Rev 57, 953–994. ‘Archezoan’ microsporidia species to be more increasing evidence to suggest that the origin 10. Edlin, T. D., Li, J., Visvesvara, G. S., Vodkin, M. H., closely related to fungi than protists (e.g. 10, of the eukaryotic cell and its mitochondrial McLaughlin, G. L. & Katiyar, S. K. (1996). Phylogenetic analysis of beta-tubilin sequences from amitochondrial 19). It has been suggested that the primitive component came into being simultaneously protozoa. Mol Phylogenet Evol 5, 359–367. status enjoyed by the diplomonads is also rather than sequentially as the serial endosym- 11. Embley, T. M. & Hirt, R. P. (1998). Early branching proving increasingly tenuous (e.g. 6). biosis hypothesis would predict (18). There is eukaryotes? Curr Opin Genet Dev 8, 624–629. The demonstration of an anaerobic origin no compelling evidence, however, that this 12. Embley, T. M., Finlay, B. J., Dyal, P. L., Hirt, R. P., for eukaryotes was again perpetuated in Wilkinson, M. & Williams, A. G. (1995). Multiple origins phenomenon occurred in the absence of O2 or of anaerobic ciliates with hydrogenosomes within the the recent ‘hydrogen hypothesis’ (23). This that the symbiont which gave rise to the mito- radiation of aerobic ciliates. Proc R Soc Lond B 262, 87–93. hypothesis argues that the original eukaryote chondrion was anaerobic. It is our interpre- 13. Embley, T. M., Horner, D. S. & Hirt, R. P. (1997). was derived from an association between a tation of the available evidence that (i) the Anaerobic eukaryotic evolution: hydrogenosomes as H -producing proteobacterium and an auto- primitive status of Archezoa is precarious, (ii) biochemically modified mitochondria? Trends Ecol Evol 2 12, 437–441. trophic archaeon, describing how in some ‘ancient’ anaerobic protozoa described to 14. Esteban, G., Guhl, B. E., Clarke, K. J., Embley, T. M. circumstances this association gave rise to date are not true anaerobes, but more likely & Finlay, B. J. (1993). Cyclidium porcatum n. sp.: a free- hydrogenosomes whilst in others it gave rise have adapted to an anaerobic lifestyle (para- living anaerobic scuticociliate containing a stable to mitochondria.