Microfauna–Macrofauna Interaction in the Seafloor: Lessons from the Tubeworm

New research and techniques are beginning to provide intriguing clues into the complex relationships that tubeworms form with other species at hydrothermal vents and deep-sea cold seeps

S ince their discovery in the 1970s and 1980s, giant tubeworms at hydrothermal vents and cold seeps have fascinated biologists and laymen alike-not only for their alien morphology (Figure 1), but also for epitomizing the perfect animal-microbe symbiosis. They are among the biggest worms on this planet-some over 3 m longyet they do not eat other organisms. Tubeworms thrive independently of photosynthetic production [1]. They have even lost their entire digestive tract. One of the most exciting fi ndings in early tubeworm research was the discovery that the worm's food is delivered by bacterial symbionts [2]. The chemoautotrophic symbionts live intracellularly in a specialized worm tissue called the trophosome. They are sulfi de oxidizers, using the free energy yield from the oxidation of sulfi de with oxygen to fi x carbon dioxide with their bacterial Rubisco enzyme. In exchange for providing nutrition for the worm, the symbionts are sheltered from grazing, but most importantly, they receive a steady source of sulfi de and oxygen via the highly adapted blood circulation system of the worm. (I will never forget how horrifi ed I was as a young student by the amounts of almost human-like blood fl owing into my lab dish while dissecting tubeworms to analyze trophosome enzyme activity.) Tubeworm blood physiology, in particular the hemoglobin molecules, are tailored specifi cally to the needs of the symbionts. However, the host metabolism in itself is not different from that of many other animals, the main source of energy being aerobic respiration of carbohydrates. In other words, tubeworms and their symbionts need oxygen as an electron acceptor-so, after all, they are dependent on photosynthesis, the main oxygen-producing process on earth.

Classifi cation of Host and Symbiont
With their strange morphology, vent tubeworms were fi rst classifi ed as a novel phylum, Vestimentifera [3]. Recently they have been regrouped together with the pogonophoran tubeworms ( Figure 2) into a family of annelid polychaetes called the Siboglinidae [4,5]. Vestimentiferan tubeworms of hydrothermal vents grow on chimneys and other hard substrates in the vicinity of active vents, which emit reduced compounds like hydrogen and sulfi de [6]. Vestimentiferan tubeworms living at cold hydrocarbon seeps, i.e., the lamellibrachids and escarpids, are adapted to a sedimentary environment, with a substantial part of the body and tube of many species extending into the mud. All vestimentiferan tubeworms found today at vents, seeps, and a few other reduced submarine habitats harbor sulfi de-oxidizing endosymbionts in their trophosome. These symbionts belong to bacteria of the gamma-proteobacteria clade and are phylogenetically related to each other [7]. (For the only known exception see [8].)

Tubeworm Mysteries
The study of tubeworms is now in its fourth decade, and there are still many fascinating problems to be solved. One of the most interesting-but also most diffi cult-questions in tubeworm symbiosis is how this obligate and highly integrated interaction between microbes and animals evolved. How can a worm evolve into a perfect home for chemosynthetic bacteria? What are the main evolutionary steps towards this symbiosis, and in which order did they occur? Another intriguing problem is how the worms acquire their endosymbionts, which appear to be taken up from the environment-but so far have not been detected as free-living forms. How does the host recognize its specifi c symbiont from the vast diversity of gamma-proteobacteria and sulfi de oxidizers in the environment? Furthermore, how do tubeworms populate new vents, seeps, and other reducing environments emerging from the ever-changing ocean fl oor-how do their larvae migrate and settle, and what determines the distribution and lifetime of tubeworm populations in the different mid-ocean ridge and continental margin habitats? Although these questions are still to be answered, new research and techniques are beginning to provide intriguing clues.

Seep Vestimentifera and Their Energy Source
At some seeps the vestimentiferan tubeworms are so abundant that they form a special habitat that is attractive for a host of other marine species [9]. Seep vestimentiferans are usually thinner, have slower growth rates, and have greater longevity than their vent relatives [10]. For example, a 2-m-long Lamellibrachia luymesi individual is estimated to be more than 200 y old and hence represents the longestlived animal on earth [11,12]. At seeps, geological processes causing fl uid and gas seepage can last hundreds to millions of years, whereas hydrothermal vents often have a lifespan on the order of decades. Vent tubeworm colonies will die when their chimneys stop venting, i.e., delivering sulfi de, so they are adapted to a rapidly changing environment, as typifi ed by their fast growth and high reproduction.

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Like vent vestimentifera, seep vestmentifera also depend on the availability of sulfi de in their direct vicinity, but they are sessile, and anchor on hard substrates such as carbonates. Individual aggregations at seeps can consist of hundreds to thousands of worms, requiring sulfi de fl uxes of half a mole per day-and this for more than 200 y [12]. So an ecological problem that has always intrigued biologists and geochemists alike is how these tubeworms obtain their energy over the long term. Because vent and seep vestimentifera depend on sulfi de-oxidizing symbionts, their distribution is limited to habitats with high sulfi de fl uxes lasting for at least a few reproductive cycles. However, at cold seeps, unlike hydrothermal vents, most of the chemical energy occurs in the form of hydrocarbons. Cold seeps are characterized by high fl uxes of methane, higher hydrocarbons (such as ethane, propane, butane), and/or petroleum from deep subsurface reservoirs. Often the source fl uids and gases do not contain much sulfi de, because there are no hightemperature seawater-rock interactions involved in their formation, as there are at vents. Some pogonophoran tubeworms at seeps have teamed with methane-oxidizing symbionts to profi t from the high availability of hydrocarbons, but seep vestimentiferans do not appear to be able to directly tap this resource. However, seep vestimentiferans are still capable of producing enormous biomass over many years with the help of their sulfi de-oxidizing symbionts. So where does the supply of sulfi de come from at seeps that enables such large aggregations to be maintained for so long?
Only recently was it realized that anaerobic microbial processes, namely, the oxidation of hydrocarbons with sulfate, could produce astonishingly high fl uxes of sulfi de in cold seep settings [13,14]. At methane seeps, methanotrophic microbial communities inhabiting the surface sediments oxidize methane with sulfate, which results in very high sulfi de fl uxes [13]. If the seepage consists of other hydrocarbons such as petroleum, their degradation with sulfate supports an even higher production of sulfi de [14]. In some seep sediments, sulfi de concentrations can reach 25 mM in subsurface sediments (5-10 cm below the sediment surface). Such concentrations are not known from tubeworm habitats at hydrothermal vents.
However, the zones of high hydrocarbon turnover and sulfi de fl ux at seeps are often limited to only a few centimeters below the seafl oor, depending on hydrocarbon fl ows and the rate of sulfate transport from the bottom water into the sediments. Sulfate is crucial because the freeliving hydrocarbon-degrading microbes in seep sediments depend on this electron acceptor for an energy yield. Without sulfate to fuel the oxidation of hydrocarbons, sulfi de production stops, even if there is still an enormous reservoir of hydrocarbon available. How might tubeworms, sulfi deoxidizing symbionts, and benthic hydrocarbon degraders overcome these limitations?

Ménage à Trois-A Model Solution
Cordes et al. [15] have now provided an answer to how the stability of sulfi de production is maintained over such long periods and how the worms optimize sulfi de uptake. Seep vestimentifera have specifi c adaptations to their habitat. A main adaptation is the subsurface part of the lamellibrachids called a "root." The tubeworm root appears to have a special function in the energy cycle of the organism-as in plant  roots. Several authors have proposed that the worm roots are not only important in sulfi de uptake, but generally in geochemical engineering of the sediments in the direct environment [16,17,18]. Obviously such hypotheses are very diffi cult to test-today it is still hardly possible to measure gas, petroleum, and sulfi de fl uxes in the seafl oor in situ at depth, especially below tubeworm aggregations. But it is also not possible to recover whole aggregations of worms and to keep them alive in the lab for biochemical and biogeochemical measurements-this would require simulation of seepage under pressure. Instead, Cordes et al. [12,15] have used geochemical and biological modeling to solve the intriguing question of seep vestimentiferan longevity and how they might also interact with free-living anaerobic microbes to increase sulfi de availability.
To explain the persistence of the large tubeworm colonies in the Gulf of Mexico, Cordes et al. suggest a broader mutualistic interaction between the tubeworm, its endosymbiont, and benthic hydrocarbon-degrading and sulfi de-producing microbes. Seep tubeworms take up sulfi de from the sulfi de-rich subsurface sediment zones through the roots, but, crucially, they may also release sulfate through the roots as a byproduct of sulfi de oxidation by the tubeworm's endosymbiont. Sulfate may also be ventilated through the tube into the sediments. Since anaerobic microbial communities in subsurface hydrocarbon-rich sediments are limited by sulfate infl ux, any additional supply of sulfate enhances their production of sulfi de. Furthermore, the removal of sulfi de by the worm will thermodynamically favor anaerobic hydrocarbon oxidation coupled to sulfate reduction. Hence, the tubeworm roots may provide an excellent habitat for anaerobic hydrocarbon oxidizers. For example, Cordes et al. predict in their model that nearly all of the sulfate released through the root will be utilized by benthic microbes for anaerobic hydrocarbon degradation in the direct vicinity of the worm. This process could provide 60% of the sulfi de needed by a tubeworm aggregation to persist for 80 y. Hence, it may even be concluded that tubeworms farm anaerobic hydrocarbon degraders to provide a steady supply of sulfi de to their endosymbionts. Especially at petroleum seeps, this would guarantee a lifelong energy source and help explain the extraordinary longevity of the worms. The mutual benefi t arising from the association of sulfi de oxidizers, sulfate reducers, and a host worm is known to be exploited by the oligochaete Olavius algarvensis [19]. In this very effective "ménage à trois" the sulfate reducer has even become an endosymbiont of the worm. Interestingly, some of our recent studies at the methane seeps of Hydrate Ridge (Cascadia margin) also show that certain populations of anaerobic methane oxidizers are specifi cally associated with seep organisms-such as the symbiotic clam Calyptogena and the giant fi lamentous sulfi de oxidizer Beggiatoa [20]. But many more examples may be out there, of bacterial and archaeal populations specifi cally growing in the "rhizosphere" of benthic organisms, potentially profi ting from bioturbation, bioirrigation, fecal deposits, and exudates.
The association and interaction between benthic fauna and sedimentary microorganisms is a very interesting fi eld of study, although inevitably still very speculative. So far it has been limited by a lack of appropriate technologies, not only for in situ biogeochemical and biological measurements, but also for quantitative investigation of specifi c functional microbial populations. Some insight can be provided by clever environmental modeling approaches-such as the one developed by Cordes et al., but ultimately the models need empirical verifi cation. Only very recently has it become possible to combine visually targeted sampling ( Figure  2) and high-resolution measurements of geochemical gradients with molecular tools for the identifi cation of microbes, such as 16S rDNA and organic-biomarker-based techniques. For the study of continental margin and deepsea ecosystems, this requires the availability of underwater vehicles (Figure 3) as well as multidisciplinary research platforms and extensive, highly detailed lab work-so this is very expensive research. Yet this is the future, if we want to determine whether such an intriguing ménage à trois as proposed by Cordes    longevity of these extraordinary tubeworms, and possibly also other chemosynthetic symbioses, forming some of the most fascinating marine ecosystems at continental margins.