III. Uptake in relation to organic content of the Habitat

Bacterial symbiont diversity in Arctic seep Oligobrachia siboglinids

BACKGROUND

High latitude seeps are dominated by Oligobrachia siboglinid worms. Since these worms are often the sole chemosymbiotrophic taxon present (they host chemosynthetic bacteria within the trophosome organ in their trunk region), a key question in the study of high latitude seep ecology has been whether they harbor methanotrophic symbionts. This debate has manifested due to the mismatch between stable carbon isotope signatures of the worms (lower than -50‰ and usually indicative of methanotrophic symbioses) and the lack of molecular or microscopic evidence for methanotrophic symbionts. Two hypotheses have circulated to explain this paradox: (1) the uptake of sediment carbon compounds with depleted δC¹³ values from the seep environment, and (2) a small, but significant and difficult to detect population of methanotrophic symbionts. We conducted 16S rRNA amplicon sequencing of the V3-V4 regions on two species of northern seep Oligobrachia (Oligobrachia webbi and Oligobrachia sp. CPL-clade), from four different high latitude sites, to investigate the latter hypothesis. We also visually checked the worms' symbiotic bacteria within the symbiont-hosting organ, the trophosome, through transmission electron microscopy.

RESULTS

The vast majority of the obtained reads corresponded to sulfide-oxidizers and only a very small proportion of the reads pertained to methane-oxidizers, which suggests a lack of methanotrophic symbionts. A number of sulfur oxidizing bacterial strains were recovered from the different worms, however, host individuals tended to possess a single strain, or sometimes two closely-related strains. However, strains did not correspond specifically with either of the two Oligobrachia species we investigated. Water depth could play a role in determining local sediment bacterial communities that were opportunistically taken up by the worms. Bacteria were abundant in non-trophosome (and thereby symbiont-free) tissue and are likely epibiotic or tube bacterial communities.

CONCLUSIONS

The absence of methanotrophic bacterial sequences in the trophosome of Arctic and north Atlantic seep Oligobrachia likely indicates a lack of methanotrophic symbionts in these worms, which suggests that nutrition is sulfur-based. This is turn implies that sediment carbon uptake is responsible for the low δ¹³C values of these animals. Furthermore, endosymbiotic partners could be locally determined, and possibly only represent a fraction of all bacterial sequences obtained from tissues of these (and other) species of frenulates.

Prokaryotic Cells in the Hydrothermal Vent Tube Worm Riftia pachyptila Jones: Possible Chemoautotrophic Symbionts

The existence of a symbiotic association between vestimentiferan tube worms from deep-sea hydrothermal vents and chemoautotrophic sulfur-oxidizing prokaryotes, based on histological and enzymatic evidence, is suggested.

Alpha-glucosidase-like activity detected in a siboglinid polychaete, Oligobrachia mashikoi

Siboglinid worms live on carbohydrates produced by symbiotic bacteria. In this study, alpha-glucosidase-like activity was detected in the surface of the body and in the trophosome of Oligobrachia mashikoi. The enzyme exhibiting this activity was partially purified by consecutively applying the crude enzyme extract to Con-A-Sepharose and Sephadex-200 HR columns. The enzyme sample thus obtained gave a single activity peak at a position corresponding to 550 kDa in the Sephadex-200 HR gel filtration column. The enzyme was active in the range of pH 6.0-8.0, with a maximum activity at around pH 6.5. It specifically hydrolyzed maltose, and was inhibited by voglibose and miglitol. Moreover, a glucose transporter 2-like protein was detected by immunohistochemical and Western-blotting analyses using anti-rat GLUT2 polyclonal antibody. These results raise the question how this unique species lives.

Uptake and assimilation of short chain carboxylic acids by Biomphalaria glabrata (Say), the freshwater pulmonate snail host of Schistosoma mansoni (Sambon)

The use of radiolabelling and high performance liquid chromatography (h. p. l. c. ) has shown that the freshwater pulmonate snail Biomphalaria glabrata can achieve a net uptake of short chain carboxylic acid (C 2 -C 4 ) from the medium, and also metabolize them. This appears to be the first time that this phenomenon has been reported for a freshwater invertebrate. The uptake characteristics for acetate and butanoate conform to the Michaelis-Menten model. The transport constants,V max (in micromole equivalents per gram of wet mass per hour) and K s (in micromoles per litre) for acetate (1005 and 2.29 respectively) and butanoate (129 and 1.29 respectively) are similar to those obtained by other workers for marine, detritivorous, polychaete worms. Evidence is given that the carboxylic acids are transported through the body wall, and not taken up to any significant extent by microorganisms or absorbed on the surface mucus. The acids are metabolized by the snail after accumulation, as shown by the appearance of label in respiratory CO 2 and the decline in the mass-specific accumulation rates (m. s. a. r. ), in micromole equivalents per gram of mass per hour, to an asymptote during the course of a 4 h incubation period. Differences in m.s.a.r. values for [1- 14 C]- and [2- 14 C]acetate probably reflect the different fates of the two carbons during metabolism. The m. s. a. r. values are influenced by the metabolic state of the snails. Thus they increase in response to food deprivation and decrease under anoxic conditions. However, as snails continue to accumulate acetate from the medium, even when well fed, or ‘preloaded ’ with acetate, it is clear that this acid is not being used solely as a food supplement. Estimates of the contributions that C 2 and C 4 acids might make to the basal metabolism of the snails have been made using the following param eters: (i) the range of concentrations of these acids in microhabitats, such as the interstitial waters in surface sediments, and an experimentally decayed macrophyte, Lemna paucicostata (ii) transport constants of the carboxylic acids taken up by the snails; (iii) the respiration rates of the snails. It was calculated that the acetate in interstitial waters can contribute only a negligible amount (less than 6%) of the snail’s basal metabolic rate (b. m. r. ). However, if the snails encountered the concentrations of 2250 μm and 400 μm of acetate and butanoate respectively found in supernatant from laboratory decaying Lemna , then each acid could provide more than 50% of the snails’ b. m. r. The significance of these results to the behavioural and chemical ecology of the snails is discussed.

Nutrition of algal-invertebrate symbiosis. I. Utilization of soluble organic nutrients by symbiont-free hosts

The uptake and utilization of organic compounds, known to be present in the photosynthetic products excreted by algal symbionts, has been examined in an aposymbiotic anemone, Aiptasia pallida , and newly hatched, symbiont-free larvae of the turbellarian, Convoluta roscoffensis . Results indicate that these compounds are rapidly assimilated by the hosts, and subsequently metabolized to provide both energy, and a source of stable cell constituents. The role of these metabolic patterns in intact associations is discussed, and the data is further related to the general phenomenon of dissolved nutrients in seawater, and their significance to the nutrition of marine invertebrates.