Vertical Transmission and Successive Location of Symbiotic Bacteria During Embryo Development and Larva Formation in Corticium Candelabrum (Porifera: Demospongiae)
Total Page：16
File Type：pdf, Size：1020Kb
J. Mar. Biol. Ass. U.K. (2007), 87, 1693–1699 doi: 10.1017/S0025315407056846 Printed in the United Kingdom Vertical transmission and successive location of symbiotic bacteria during embryo development and larva formation in Corticium candelabrum (Porifera: Demospongiae) Sònia de Caralt*‡, María J. Uriz† and René H. Wijffels* *Food and Bioprocess Engineering Group, Wageningen University, PO Box 8129, 6700 EV Wageningen, The Netherlands. †Centre d’Estudis Avançats de Blanes (CEAB), CSIC, Accés a la Cala Sant Francesc, 14, 17300 Blanes, Girona, Spain. ‡Corresponding author, e-mail: [email protected] This study reports on the transfer of heterotrophic bacteria from parental tissue to oocytes in the Mediterranean bacteriosponge Corticium candelabrum (Homosclerophorida) and the description of the successive locations of the microsymbionts during embryo development through transmission and scanning electron microscopy. Eight different types of symbiotic bacteria are described morphologically. These eight bacteria morphotypes are found in both adult individuals and larvae. Symbiotic bacteria are transferred to oocytes, but not to spermatocytes. Bacteria are first located at the oocyte periphery below a thick collagen layer and then they migrate to the oocyte cytoplasm, forming spherical clusters. After cleavage, the bacteria can be found in the free space between blastomeres but mainly accumulate at the embryo periphery below the follicular cells that surround the embryo. Once the blastocoel is formed, the symbiotic bacteria move to this central cavity where they actively divide by bipartition, increasing their number considerably. Many examples of phagocytosed bacteria in the proximal zone of the larval cells are observed at this stage. Consequently, bacteria may represent a complementary source of energy for free larvae and settlers before they are able to capture food from the surrounding water. INTRODUCTION primary production. Sarà (1971) suggested that sponges benefit from these micro-organisms by phagocytosis of excess Microbial symbionts are plentiful in many sponge symbionts. On the other hand, the bacterial symbionts can species (e.g. Vacelet, 1975) in such a way that it has been benefit from the nitrogen-rich medium provided by the speculated that remineralization of the organic matter in the host sponge in the form of ammonia (e.g. Jiménez & Ribes, water column would be the main sponges’ role in benthic 2007). Microbial symbionts may also be responsible for the ecosystems, thanks to their symbiotic populations (Jiménez production of bioactive compounds (e.g. Piel, 2006). & Ribes, 2007). Sponge microsymbionts belong to different Microbial symbionts also occur in sponge larvae. taxonomical entities: zooxanthellae (e.g. Rosell & Uriz, 1992), Sponge larvae have been reported to harbour bacteria and cyanobacteria (e.g. Turon et al., 2000), and heterotrophic zooxanthellae (e.g. Mariani et al., 2001). Thus, as in adults, bacteria (e.g. Vacelet, 1975; de Caralt et al., 2003), and can symbiotic micro-organisms might represent a non-negligible represent a significant part of the sponge volume (e.g. up to source of food for larvae, complementary to (or involved in) 30–50%, Vacelet, 1975). Yeasts (e.g. Maldonado et al., 2005), the reported incorporation of dissolved organic molecules and viruses (e.g. Lohr et al., 2005) have also been reported to (Jaeckle, 1995). live within sponges but whether these micro-organisms are One remarkable case is that of Crambe crambe (Schmidt, parasites or symbionts has not been elucidated yet. 1862), in which symbiotic bacteria are reported to be present Due to their abundance, microsymbionts are assumed in the larva but not in the adults (Uriz et al., 2001). Adults to be important for the biology and ecology of the sponge, of C. crambe contain spherulous cells with antibacterial although their functions are unknown in most cases. It has properties, which may prevent the presence of symbiotic been reported that photosynthetic microsymbionts have bacteria. In contrast, spherulous cells are absent in the a role in sponge nutrition (e.g. Sarà, 1971; Wilkinson & larvae and, consequently, larvae can harbour bacteria. As Vacelet, 1979; Rosell & Uriz, 1992). Wilkinson & Vacelet, a result, C. crambe larvae may benefit from their symbionts (1979) showed that cyanobacteria transfer nutrients such as during their pre-feeding stage (Uriz et al., 2001). glycogen to the host sponges. Moreover, Rosell & Uriz (1992) Sponge microsymbionts can be intra- and extracellular. recorded significantly higher growth rates in clionid sponges Zooxanthellae are found inside the archaeocytes (Mariani et cultured under a light regime than in those maintained in the al., 2001), and specialized cells called bacteriocytes harbour dark, and attributed these differences to the zooxanthellae heterotrophic bacteria. Moreover, cyanobacteria and Journal of the Marine Biological Association of the United Kingdom (2007) 1694 S. de Caralt et al. Bacteria transfer from adult to embryo in Corticium candelabrum heterotrophic bacteria can coexist in the same species, spread Bacteria morphotypes (from both adults and embryos) through the sponge mesohyl (e.g. Usher et al., 2004). were characterized through TEM. The main phenotypic The larvae of bacteriosponges also contain abundant characters considered were size and general shape, the bacteria (e.g. Ereskovsky & Tokina, 2004), which can bacteria wall, the presence or absence of periplasm, be acquired by two different ways: vertical transmission the number of membranes, the nuclear region, and the from adults and de novo acquisition from the environment. cytoplasm density. No evidence of de novo acquisition has been documented in sponges, although the finding of similar bacteria in RESULTS taxonomically distant sponges inhabiting the same cave Adults has been interpreted as indirect proof of de novo acquisition Symbiotic bacteria represent ~40.74 ±6.58% (mean ±SE) of (Muricy et al., 1999; Enticknap et al., 2006). Moreover, a the Corticium candelabrum tissue as calculated from the sponge recent study presents some evidence of similar microbial section pictures. The symbiotic bacterial population, which types in different sponges, which pose the question about the is extracellular, is embedded in collagen (Figure 1). Symbiotic mechanism of their acquisition. In contrast, several instances bacteria are isolated from the choanocyte chambers by a of vertical transmission have been reported in recent years thick collagen layer. Eight different phenotypic bacterial (e.g. Usher et al., 2005). types can be recognized in the adult tissue (Figure 2). Homosclerophorids are a particularly suitable sponge Bacteria types B, C, D, F, G and H have complex cell walls, group for studying bacterial transmission because some typical of gram-negative bacteria. genera of this group examined so far, are bacteria- Type (A). Large (to a maximum of ~3.1 µm in length, and rich (Corticium and Plakina, e.g. Ereskovsky & Boury- ~1.2 µm in width), cylindrical to ovoid bacteria. The cell Esnault, 2002). Moreover, larvae of various genera is surrounded by two close membranes (~30–40 nm thick of homosclerophorids (e.g. Plakina, Pseudocorticium and altogether), the inner one (cytoplasm membrane) poorly Corticium) have been reported to contain symbiotic differentiated from the cell cytoplasm. The cytoplasm is bacteria identical to those present in the parent sponge electron-dense, without a distinct nuclear region. This (Boury-Esnault et al., 2003), but transfer of bacteria from bacteria type is relatively abundant and the largest one parents to larvae has only been documented in Oscarella recorded in the adult sponge tissue. spp. (Ereskovsky & Boury-Esnault, 2002). The present Type (B). About 0.7–1.3 µm in length and ~0.6–0.8 µm in study reports on the transfer of heterotrophic bacteria width, bacteria. The cell has a double membrane separated from parental tissue to oocytes in the bacteriosponge by a relatively wide (~60 nm), electron-clear, periplasm. The Corticium candelabrum Schmidt, 1862 (Homosclerophorida), external membrane (cell wall), ~10 nm in thickness, is dark describing the successive locations of bacteria during and well delimited. The cytoplasm is loose at the central embryo and larval development. We also describe and zone and denser at the periphery of the cell. The nucleoid compare the several morphotypes of bacteria present in (electron-clear zones) is spread across the cytoplasm. adults and larvae, through electron microscopy. Type (C). Medium-sized (~1.3–1.5 µm in length and ~0.9 µm in width), variable in shape (from triangular to circular in MATERIALS AND METHODS transversal sections) bacteria with a very wide (up to 180 nm) Samples of Corticium candelabrum were collected during the cover consisting of two layers separated by a well developed, reproductive period (Spring–Summer, 2002) by SCUBA electron-clear periplasm. The internal matrix is granulated, diving in the western Mediterranean (Blanes), at a depth of with one highly electron-dense spot (not always visible in the 7–12 m. Once in the laboratory subsamples of about 3 mm3 in sections). The nuclear region is large and occupies almost size were fixed for transmission electron microscopy (TEM) the whole cell. and scanning electron microscopy (SEM) observations. Type (D). Cylindrical, ~1 µm in length and ~0.7 µm in Samples for TEM were fixed in a mixture of 1% OsO4 width, bacteria. Thick cover consisting of a cytoplasmic and 2% glutaraldehyde in 0.45 M