Anaerobic animals from an ancient, anoxic ecological niche
© Mentel and Martin; licensee BioMed Central Ltd. 2010
Received: 16 March 2010
Accepted: 6 April 2010
Published: 6 April 2010
Tiny marine animals that complete their life cycle in the total absence of light and oxygen are reported by Roberto Danovaro and colleagues in this issue of BMC Biology. These fascinating animals are new members of the phylum Loricifera and possess mitochondria that in electron micrographs look very much like hydrogenosomes, the H2-producing mitochondria found among several unicellular eukaryotic lineages. The discovery of metazoan life in a permanently anoxic and sulphidic environment provides a glimpse of what a good part of Earth's past ecology might have been like in 'Canfield oceans', before the rise of deep marine oxygen levels and the appearance of the first large animals in the fossil record roughly 550-600 million years ago. The findings underscore the evolutionary significance of anaerobic deep sea environments and the anaerobic lifestyle among mitochondrion-bearing cells. They also testify that a fuller understanding of eukaryotic and metazoan evolution will come from the study of modern anoxic and hypoxic habitats.
The newly reported tiny marine animals that complete their life cycle in the total absence of light and oxygen are members of the phylum Loricifera, a phylum discovered less than 30 years ago, and they are less than a millimetre in size . They were collected from a deep basin at the bottom of the Mediterranean Sea, where they inhabit a nearly salt-saturated brine that, because of its density (>1.2 g/cm3), does not mix with the waters above. As a consequence, this environment is completely anoxic and, due to the activity of sulphate reducers, contains sulphide at a concentration of 2.9 mM. Despite such harsh conditions, this anoxic and sulphidic environment is teeming with microbial life, both chemosynthetic prokaryotes that are primary producers , and a broad diversity of eukaryotic heterotrophs at the next trophic level [3, 4]. That this ecological niche also supports animals is a surprise that poses all sorts of interesting questions. Despite being unexpected, however, the finding ties together recent developments from several independent fields (marine biology, cell biology, evolutionary theory and geochemistry) that all point to the evolutionary significance of eukaryotic life in anaerobic environments.
The first question raised is 'how?'. How is it possible that animals can inhabit this anoxic and sulphidic environment? This might seem impossible to some, after all one often reads that 'animals have an absolute requirement for oxygen'  or 'sulphide is poisonous' . However, not all animals are strictly dependent upon oxygen. Some use different terminal electron acceptors other than oxygen in their mitochondrial respiratory chains, most commonly fumarate, leading to the excretion of succinate and propionate , often accompanied by acetate excretion as well . Since the mechanism of sulphide toxicity to animals entails the inhibition of cytochrome c oxidase , mitochondria that are not dependent upon that final mediator of an electron transfer to O2 also do not have such a problem with sulphide. Among animal lineages, facultative anaerobic mitochondria have been studied from various free living invertebrates, including the oyster Mytilus (Mollusca) , the peanut worm Sipunculus (Sipuncula)  or the polychaete worm Arenicola (Annelida)  and parasites like Fasciola (Platyhelminthes)  and Ascaris (Nematoda) . However, such oxygen-independent energy metabolism in animals is often restricted to some stages -- albeit sometimes prolonged -- of the lifecycle. The Loriciferans that Danovaro et al.  describe spend their entire life cycle in the sediment: what was once seen as an 'absolute requirement' for O2 among animals should now be seen as a lineage-specific preference, albeit it one that is very pronounced, especially among those animals that, like ourselves, live on land, permanently above the soil line.
Anaerobic mitochondria: more common all the time
It is not yet known what role, if any, the mitochondria of the newly discovered loriciferans play in the energy metabolism of those animals and, unfortunately, the same is true for the myriad of fascinating eukaryotic protists that inhabit the very same environment where the anoxic loriciferans were found. In a series of recent papers, Stoeck and colleagues have surveyed the protistan diversity from L'Atalante and similar habitats with environmental sequencing and metagenomic techniques [3, 4, 27]. Those reports lead to a wealth of representatives from many of the same groups shown in Figure 2 being uncovered: ciliates, fungi and chromalveolates, in addition to representatives of many other eukaryotic groups, including the choanoflagellates , which are regarded as the unicellular sisters to the metazoan clade. The investigation of anaerobic mitochondria remains an area of rapid progress and it will be a challenge to discover what those mitochondria are doing in real-life anoxic and hypoxic environments, which are very widespread among modern habitats  and where eukaryotic anaerobes abound .
What is the evolutionary significance of the new findings? The L'Atalante basin has been anoxic for only about 50,000 years  but have all of its microbial and metazoan inhabitants only recently adapted to life in anoxic conditions during that time? Hardly. Nobody seriously considers that anaerobic prokaryotes dwelling in such anaerobic habitats, such as methanogens and sulphate reducers, have only recently adapted to anaerobic niches. The prokaryote inhabitants have existed for well over a billion years, and have reached this new habitat by dispersal, not by adaptive evolution de novo and in situ. Indeed, geochemical evidence has shown that methanogenesis and sulphate reduction, and the niches in which they occur, are truly ancient [29, 30]. (For marine environments, dispersal is not a fundamental problem because, despite vast distances, similar types of seafloor habitats often harbour similar communities, from microbes to large animals ). However, when it comes to eukaryotes, there is still a curious tendency to assume that eukaryotes only invaded anaerobic niches of late. Perhaps this stems from a tendency (latently anthropocentric, no doubt) to view mitochondria as obligately O2-dependent organelles with the anaerobic forms of mitochondria being rare exceptions, even though the data (for example in Figures 1 and 2 or elsewhere [7, 8, 17, 19, 21]) tell us otherwise. In environments such as the L'Atalante basin, 'normal', O2-dependent mitochondria are the rare exception, if they exist at all, but eukaryotes abound [1, 3, 4, 32, 33]. Hence, further study of mitochondria from such environments should be revealing. That, however, is easier said than done, since, as seen in the present study  as in other work on organisms from anoxic marine sediments , a considerable effort has to be invested in order just to demonstrate that the organisms are even alive and not just sunken carcasses. Work on organisms from these environments poses substantial technical challenges, making every new insight all the more exciting.
While Danovaro et al. opt for the term 'enigmatic' in discussing the evolutionary significance of their findings, we have a decidedly different view. Given that anaerobic forms of mitochondria are widespread throughout the eukaryotic world, we see eukaryotes in anaerobic habitats as evidence for evolution in the Darwinian sense of descent with modification, with the traits that support survival in anaerobic environments having been conserved from earlier phases of Earth's history. This view is underpinned by what geologists and geochemists have been trying to tell biologists over the last 10 years about the prevalence of anoxic and sulphidic environments during the early phase of eukaryotic and metazoan evolution, but with the biologists perhaps not taking as much notice as they should.
The bigger picture: add geological time
What are the geologists trying to tell us about anoxic and sulphidic marine habitats? A readable summary of about 10 year's progress is given in three papers [34–36]. In a nutshell, the geologists are saying that the rise in atmospheric oxygen some 2.4 billion years ago is one thing, but that the oxygen levels in the ocean, where evolution was taking place, is quite another. Several lines of isotope evidence indicate that deep ocean waters were not fully oxygenated until about 580 MY ago, about the time when the first large animals made their fossil debut. The reason for this anoxia, they say, has to do with high levels of sulphide in marine environments, from the workings of sulphate reducers , themselves strict anaerobes. The message for biologists is that the Earth's oceans appear to have been largely anoxic and sulphidic (like the L'Atalante basin, but not hypersaline) below the photic zone (the upper about 200 m) [34, 36] and, possibly, also in the lower photic zone  for the time spanning roughly 1.8 billion years ago to about the beginning of the Cambrian period 560 MY ago. That was the time during which eukaryotes arose and diversified . Hence, it should hardly be surprising that the anaerobic lifestyle is widespread among eukaryotic lineages, right up into the animals . One view is that the oxygenation of deep environments allowed the animals to become larger , which is different from saying that oxygen might in any way be causal to the Cambrian appearance of diverse animal forms . Geologists have not been trying to hide their perspective from us biologists [40–42], nor have we biologists been trying to hide our progress in understanding eukaryotic anaerobes [21, 43–46]. However, it seems that it will still take some time for a new default view of deep marine environments in the Late Precambrian -- a new synthesis of sorts -- to be accepted by biologists. The scenario is one of widespread anoxic and sulphidic habitats, almost certainly (in our view) teeming with little eukaryotic creatures, all with their mitochondria well suited to life with little or no oxygen  and all more or less like the ones we see in anoxic and sulphidic environments today. Such environments staged and witnessed the origin of evolution's greatest early inventions  and the new insights emerging from them deserve our close attention.
MM and WM thank the Deutsche Forschungsgemeinschaft (DFG) and WM is grateful to the European 'Research Council (ERC) for their financial support.
- Danovaro R, Dell' Anno A, Pusceddu A, Gambi C, Heiner I, Kristensen RM: The first metazoa living in permanently anoxic coditions. BMC Biology. 2010, 8: 30-10.1186/1741-7007-8-30.PubMed CentralView ArticlePubMed
- Yakimov MM, La Cono V, Denaro R, D'Auria G, Decembrini F, Timmis KN, Golyshin PN, Giuliano L: Primary producing prokaryotic communities of brine, interface and seawater above the halocline of deep anoxic lake L'Atalante, Eastern Mediterranean Sea. ISME J. 2007, 1: 743-755. 10.1038/ismej.2007.83.View ArticlePubMed
- Alexander E, Stock A, Breiner HW, Behnke A, Bunge J, Yakimov MM, Stoeck T: Microbial eukaryotes in the hypersaline anoxic L'Atalante deep-sea basin. Environmental Microbiol. 2009, 11: 360-381. 10.1111/j.1462-2920.2008.01777.x.View Article
- Edgcomb V, Orsi W, Leslin C, Epstein SS, Bunge J, Jeon S, Yakimov MM, Behnke A, Stoeck T: Protistan community patterns within the brine and halocline of deep hypersaline anoxic basins in the eastern Mediterranean Sea. Extremophiles. 2009, 13: 151-167. 10.1007/s00792-008-0206-2.View ArticlePubMed
- Canfield DE, Poulton SW, Narbonne GM: Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life. Science. 2007, 315: 92-95. 10.1126/science.1135013.View ArticlePubMed
- Grieshaber M, Volkel S: Animal adaptations for tolerance and exploitation of poisonous sulfide. Ann Rev Physiol. 1998, 60: 33-53. 10.1146/annurev.physiol.60.1.33.View Article
- Tielens AGM, Rotte C, van Hellemond JJ, Martin W: Mitochondria as we don't know them. Trends Biochem Sci. 2002, 27: 564-572. 10.1016/S0968-0004(02)02193-X.View ArticlePubMed
- Tielens AGM, van Grinsven K, Henze K, van Hellemond J, Martin W: Acetate formation in the energy metabolism parasitic helminths and protists. Int J Parasitol. 2010, 40: 387-397. 10.1016/j.ijpara.2009.12.006.View ArticlePubMed
- Cooper CE, Brown GC: The inhibition of mitochondrial cytochrome oxidase by the gases carbon monoxide, nitric oxide, hydrogen cyanide and hydrogen sulfide: chemical mechanism and physiological significance. J Bioenergetics Biomembranes. 2008, 40: 533-539. 10.1007/s10863-008-9166-6.View Article
- de Zwaan A: Molluscs. Metazoan Life Without Oxygen. Edited by: Bryant C. 1991, London: Chapman and Hall, 186-217.
- Grieshaber M, Hardewig I, Kreutzer U, Pörtner H-O: Physiologial and metabolic responses to hypoxia in invertebrates. Rev Physiol Biochem Pharmacol. 1994, 125: 143-147.
- Theissen U, Martin W: Sulfide:quinone oxidoreductase (SQR) from the lugworm Arenicola marina shows cyanide- and thioredoxin-dependent activity. FEBS J. 2008, 257: 1131-1139. 10.1111/j.1742-4658.2008.06273.x.View Article
- van Hellemond JJ, Klei van der A, van Weelden SW, Tielens AG: Biochemical and evolutionary aspects of anaerobically functioning mitochondria. Phil Trans Roy Soc London Series B. 2003, 358: 205-213. 10.1098/rstb.2002.1182.View Article
- Barrett J: Parasitic helminths. Metazoan Life Without Oxygen. Edited by: Bryant C. 1991, London: Chapman and Hall, London, 146-164.
- Tielens AGM, van Hellemond JJ: Anaerobic mitochondria: properties and origins. Origin of Mitochondria and Hydrogenosomes. Edited by: Martin W, Müller M. 2007, Berlin: Springer-Verlag, 85-104. full_text.View Article
- Peterson KJ, Cotton JA, Gehling JG, Pisani D: The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records. Phil Trans Roy Soc London Series B. 2008, 363: 1435-1443. 10.1098/rstb.2007.2233.View Article
- Müller M: The hydrogenosome. J General Microbiol. 1993, 139: 2879-2889.View Article
- Cheeseman P, Toms-Wood A, Wolfe RS: Isolation and properties of a fluorescent compound, factor 420, from Methanobacterium strain M.o.H. J Bacteriol. 1972, 112: 527-531.PubMed CentralPubMed
- Giezen van der M: Hydrogenosomes and mitosomes: conservation and evolution of functions. Eukaryotic Microbiol. 2009, 56: 221-231. 10.1111/j.1550-7408.2009.00407.x.View Article
- Hampl V, Hug L, Leigh JW, Dacks JB, Lang BF, Simpson AGB, Roger AJ: Phylogenomic analyses support the monophyly of Excavata and resolve relationships among eukaryotic "supergroups". Proc Natl Acad Sci USA. 2009, 106: 3859-3864. 10.1073/pnas.0807880106.PubMed CentralView ArticlePubMed
- Embley TM, Martin W: Eukaryotic evolution, changes and challenges. Nature. 2006, 440: 623-630. 10.1038/nature04546.View ArticlePubMed
- Risgaard-Petersen N, Langezaal AM, Ingvardsen S, Schmid MC, Jetten MS, Op den Camp HJ, Derksen JW, Pina-Ochoa E, Eriksson SP, et al: Evidence for complete denitrification in a benthic foraminifer. Nature. 2006, 443: 93-96. 10.1038/nature05070.View ArticlePubMed
- Pina-Ochoa E, Hogslund S, Geslin E, Cedhagen T, Revsbech NP, Nielsen LP, Schweizer M, Jorissen F, Rysgaard S, Risgaard-Petersen N: Widespread occurrence of nitrate storage and denitrification among Foraminifera and Gromiida. Proc Natl Acad Sci USA. 2010, 107: 1148-1153. 10.1073/pnas.0908440107.PubMed CentralView ArticlePubMed
- Atteia A, van Lis R, Gelius-Dietrich G, Adrait A, Garin J, Joyard J, Rolland N, Martin W: Pyruvate:formate lyase and a novel route of eukaryotic ATP-synthesis in anaerobic Chlamydomonas mitochondria. J Biol Chem. 2006, 281: 9909-9918. 10.1074/jbc.M507862200.View ArticlePubMed
- Mus F, Dubini A, Seibert M, Posewitz MC, Grossman AR: Anaerobic acclimation in Chlamydomonas reinhardtii anoxic gene expression, hydrogenase induction, and metabolic pathways. J Biol Chem. 2007, 282: 25475-25486. 10.1074/jbc.M701415200.View ArticlePubMed
- Meuser JE, Ananyev G, Wittig LE, Kosourov S, Ghirardi ML, Seibert M, Dismukes GC, Posewitz MC: Phenotypic diversity of hydrogen production in chlorophycean algae reflects distinct anaerobic metabolisms. J Biotechnol. 2009, 142: 21-30. 10.1016/j.jbiotec.2009.01.015.View ArticlePubMed
- Stoeck T, Behnke A, Christen R, Amaral-Zettler L, Rodriguez-Mora MJ, Chistoserdov A, Orsi W, Edgcomb VP: Massively parallel tag sequencing reveals the complexity of anaerobic marine protistan communities. BMC Biology. 2009, 7: 72-10.1186/1741-7007-7-72.PubMed CentralView ArticlePubMed
- Levin LA: Oxygen minimum zone benthos: Adaptation and community response to hypoxia. Oceanography Marine Biol Ann Rev. 2003, 41: 1-45.
- Grassineau NV, Nisbet EG, Bickle MJ, Fowler CMR, Lowry D, Mattey DP, Abell P, Martin A: Antiquity of the biological sulphur cycle: evidence from sulphur and carbon isotopes in 2700 million-year-old rocks of the Belingwe Belt, Zimbabwe. Proc Roy Soc London Series B. 2001, 268: 113-119. 10.1098/rspb.2000.1338.View Article
- Ueno Y, Yamada K, Yoshida N, Maruyama S, Isozaki Y: Evidence from fluid inclusions for microbial methanogenesis in the early archaean era. Nature. 2006, 440: 516-519. 10.1038/nature04584.View ArticlePubMed
- Dubilier N, Bergin C, Lott C: Symbiotic diversity in marine animals: the art of harnessing chemosynthesis. Nature Rev Microbiol. 2008, 6: 725-740. 10.1038/nrmicro1992.View Article
- Bernhard JM, Habura A, Bowser SS: An endobiont-bearing allogromiid from the Santa Barbara Basin: Implications for the early diversification of foraminifera. J Geophysl Res. 2006, 111: G03002-10.1029/2005JG000158.
- Bernhard JM, Buck KR, Farmer MA, Bowser SS: The Santa Barbara Basin is a symbiosis oasis. Nature. 2000, 403: 77-80. 10.1038/47476.View ArticlePubMed
- Lyons TW, Reinhard CT: An early productive ocean unfit for aerobics. Proc Natl Acad Sci USA. 2009, 106: 18045-18046. 10.1073/pnas.0910345106.PubMed CentralView ArticlePubMed
- Johnston DT, Wolfe-Simon F, Pearson A, Knoll AH: Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age. Proc Natl Acad Sci USA. 2009, 106: 16925-16929. 10.1073/pnas.0909248106.PubMed CentralView ArticlePubMed
- Lyons TW, Anbar AD, Severmann S, Scott C, Gill BC: Tracking euxinia in the ancient ocean: A multiproxy perspective and Proterozoic case study. Ann Rev Earth Planetary Sci. 2009, 37: 507-534. 10.1146/annurev.earth.36.031207.124233.View Article
- Mentel M, Martin W: Energy metabolism among eukaryotic anaerobes in light of Proterozoic ocean chemistry. Phil Trans Roy Soc London Series B. 2008, 363: 2717-2729. 10.1098/rstb.2008.0031.View Article
- Shen B, Dong L, Xiao SH, Kowalewski M: The Avalon explosion: evolution of Ediacara morphospace. Science. 2008, 319: 81-84. 10.1126/science.1150279.View ArticlePubMed
- Budd GE: The earliest fossil record of the animals and its significance. Phil Trans Roy Soc London Series B. 2008, 363: 1425-1434. 10.1098/rstb.2007.2232.View Article
- Dietrich LEP, Tice MM, Newmann DK: The co-evolution of life and Earth. Curr Biol. 2006, 16: R395-R400. 10.1016/j.cub.2006.05.017.View ArticlePubMed
- Canfield DE, Poulton SW, Knoll AH, Narbonne GM, Ross G, Goldberg T, Strauss H: Ferruginous conditions dominated later neoproterozoic deep-water chemistry. Science. 2008, 321: 949-952. 10.1126/science.1154499.View ArticlePubMed
- Frei R, Gaucher C, Poulton SW, Canfield DE: Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes. Nature. 2009, 461: 250-253. 10.1038/nature08266.View ArticlePubMed
- Cox CJ, Foster PG, Hirt RP, Harris SR, Embley TM: The archaebacterial origin of eukaryotes. Proc Natl Acad Sci USA. 2008, 105: 20356-20361. 10.1073/pnas.0810647105.PubMed CentralView ArticlePubMed
- Tovar J, León-Avila G, Sánchez LB, Sutak R, Tachezy J, Giezen van der M, Hernández M, Müller M, Lucocq JM: Mitochondrial remnant organelles of Giardia function in iron-sulphur protein maturation. Nature. 2003, 426: 172-176. 10.1038/nature01945.View ArticlePubMed
- Dyall SD, Brown MT, Johnson PJ: Ancient invasions: From endosymbionts to organelles. Science. 2004, 304: 253-257. 10.1126/science.1094884.View ArticlePubMed
- Hrdy I, Hirt RP, Dolezal P, Bardonova L, Foster PG, Tachezy J, Embley TM: Trichomonas hydrogenosomes contain the NADH dehydrogenase module of mitochondrial complex I. Nature. 2004, 432: 618-622. 10.1038/nature03149.View ArticlePubMed
- Lane N: Life Ascending: The Ten Greatest Inventions of Evolution. 2009, London: Profile Books
- Dunn CW, Hejnol A, Matus DQ, Pang K, Browne WE, Smith SA, Seaver E, Rouse GW, Obst M, Edgecombe GD, et al: Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 2008, 452: 745-749. 10.1038/nature06614.View ArticlePubMed
- Kristensen RM: An Introduction to Loricifera, Cycliophora, and Micrognathozoa. Integrative Comp Biol. 2002, 42: 641-651. 10.1093/icb/42.3.641.View Article
- Heiner I, Kristensen RM: Two new species of the genus Pliciloricus (Loricifera, Pliciloricidae) from the Faroe Bank, North Atlantic. Zoologischer Anzeiger. 2005, 243: 121-138. 10.1016/j.jcz.2004.05.002.View Article
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