Observations on the role of dissolved organic compounds in the nutrition of benthic invertebrates

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.

The phylogeography and ecology of oligobrachia frenulate species suggest a generalist chemosynthesis-based fauna in the arctic

We used ancient DNA (aDNA) extraction methods to sequence museum voucher samples of Oligobrachia webbi, a frenulate siboglinid polychaete described from a northern Norwegian fjord over fifty years ago. Our sequencing results indicate a genetic match with the cryptic seep species, Oligobrachia haakonmosbiensis (99% pairwise identity for 574 bp mtCOI fragments). Due to its similarity with O. webbi, the identity of O. haakonmosbiensis has been a matter of debate since its description, which we have now resolved. Furthermore, our results demonstrate that chemosynthesis-based siboglinids, that constitute the bulk of the biomass at Arctic seeps are not seep specialists. Our data on sediment geochemistry and carbon and nitrogen content reveal reduced conditions in fjords/sounds, similar to those at seep systems. Accumulation and decomposition of both terrestrial and marine organic matter results in the buildup of methane and sulfide that apparently can sustain chemosymbiotic fauna. The occurrence of fjords and by extension, highly reducing habitats, could have led to Arctic chemosymbiotic species being relatively generalist with their habitat, as opposed to being seep or vent specialists. Our stable isotope analyses indicate the incorporation of photosynthetically derived carbon in some individuals, which aligns with experiments conducted on frenulates before the discovery of chemosynthesis that demonstrated their ability to take up organic molecules from the surrounding sediment. Since reduced gases in non-seep environments are ultimately sourced from photosynthetic processes, we suggest that the extreme seasonality of the Arctic has resulted in Arctic chemosymbiotic animals seasonally changing their degree of reliance on chemosynthetic partners. Overall, the role of chemosynthesis in Arctic benthos and marine ecosystems and links to photosynthesis may be complex, and more extensive than currently known.

Cryptic frenulates are the dominant chemosymbiotrophic fauna at Arctic and high latitude Atlantic cold seeps

We provide the first detailed identification of Barents Sea cold seep frenulate hosts and their symbionts. Mitochondrial COI sequence analysis, in combination with detailed morphological investigations through both light and electron microscopy was used for identifying frenulate hosts, and comparing them to Oligobrachia haakonmosbiensis and Oligobrachia webbi, two morphologically similar species known from the Norwegian Sea. Specimens from sites previously assumed to host O. haakonmosbiensis were included in our molecular analysis, which allowed us to provide new insight on the debate regarding species identity of these Oligobrachia worms. Our results indicate that high Arctic seeps are inhabited by a species that though closely related to Oligobrachia haakonmosbiensis, is nonetheless distinct. We refer to this group as the Oligobrachia sp. CPL-clade, based on the colloquial names of the sites they are currently known to inhabit. Since members of the Oligobrachia sp. CPL-clade cannot be distinguished from O. haakonmosbiensis or O. webbi based on morphology, we suggest that a complex of cryptic Oligobrachia species inhabit seeps in the Norwegian Sea and the Arctic. The symbionts of the Oligobrachia sp. CPL-clade were also found to be closely related to O. haakonmosbiensis symbionts, but genetically distinct. Fluorescent in situ hybridization and transmission electron micrographs revealed extremely dense populations of bacteria within the trophosome of members of the Oligobrachia sp. CPL-clade, which is unusual for frenulates. Bacterial genes for sulfur oxidation were detected and small rod shaped bacteria (round in cross section), typical of siboglinid-associated sulfur-oxidizing bacteria, were seen on electron micrographs of trophosome bacteriocytes, suggesting that sulfide constitutes the main energy source. We hypothesize that specific, local geochemical conditions, in particular, high sulfide fluxes and concentrations could account for the unusually high symbiont densities in members of the Oligrobrachia sp. CPL-clade.

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.

Endosymbionts of Siboglinum fiordicum and the phylogeny of bacterial endosymbionts in Siboglinidae (Annelida)

Siboglinid worms are a group of gutless marine annelids that are nutritionally dependent upon endosymbiotic bacteria. Four major groups of siboglinids are known-vestimentiferans, moniliferans, Osedax spp. and frenulates. Although endosymbionts of vestimentiferans and Osedax spp. have been previously characterized, little is currently known about endosymbiotic bacteria associated with frenulate and moniliferan siboglinids. This is particularly surprising given that frenulates are the most diverse and widely distributed group of siboglinids. Here, we molecularly characterize endosymbiotic bacteria associated with the frenulate siboglinid Siboglinum fiordicum by using 16S rDNA ribotyping in concert with laser-capture microdissection (LCM). Phylogenetic analysis indicates that at least three major clades of endosymbiotic gamma-proteobacteria associate with siboglinid annelids, with each clade corresponding to a major siboglinid group. S. fiordicum endosymbionts are a group of gamma-proteobacteria that are divergent from bacteria associated with vestimentiferan or Osedax hosts. Interestingly, symbionts of S. fiordicum, from Norway, are most closely related to symbionts of the frenulate Oligobrachia mashikoi from Japan, suggesting that symbionts of frenulates may share common evolutionary history or metabolic features.

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.

The carboxylic and amino acid chemoreception niche of Bulinus rohlfsi (Clessin), the snail host of Schistosoma haematobium

The response of the freshwater snail, Bulinus rohlfsi, to gradients of carboxylic acids, amino acids and some related compounds was investigated by means of diffusion olfactometers. The snails proved to be very discriminating in their responses to the chemical species tested and there were also very marked ontogenetic differences. Although 15 distinctive types of amino and carboxylic acids proved to be attractants and arrestants, only octanoic acid proved effective for snails of all ages. Possible mechanisms involved in the chemoreception of active compounds, the ecological, physiological and biochemical relevance of the responses and possible application to controlled release technology are discussed.

D-Glucose transport across the apical membrane of the surface epithelium in Nereis diversicolor

Epidermal D-glucose transport was investigated in vivo in the brackish-water polychaete worm Nereis diversicolor. Transfer across the apical membrane is rate-limiting to D-glucose uptake, but the cuticle and/or mucus presents some resistance to D-glucose diffusion between bulk solution and transporting membrane. Maximal D-glucose influx is about 10(-12) mol sec-1 per cm2 of apical plasmalemma. Under natural conditions (approximately 1 microM D-glucose in the medium), backflux from the epidermal transport pool is negligible, but a significant paracellular outflux may occur. D-glucose influx across the apical membrane is Na+-dependent and completely inhibitable by phlorizin and harmaline; phloretin is less effective, and cytochalasin B has no effect. Influx is moderately depressed by KCN and iodoacetate, alpha-methyl-D-glucopyranoside is an effective substitute of D-glucose in transport. Animals acclimated to a low salinity, in which epidermal salt transport takes place, show a marked decrease of D-glucose transport capacity. On transfer of animals from a high to a low salinity, or vice versa, the corresponding change of influx occurs after a time-lag of at least an hour. Permeability of the epidermis to simple diffusion of D-glucose is 8 X 10(-8) cm sec-1 (on basis of gross epidermal area).