Michele Sarà

Metabolic integration between symbiotic cyanobacteria and sponges: a possible mechanism

Metabolic relationships between symbiotic cyanobacteria and host sponge have been investigated in the marine species Chondrilla nucula and Petrosia ficiformis (collected in the Ligurian Sea in 1992). DNA, RNA, total protein, cytosolic protein, total sugar, cytosolic sugar, total lipid, nonprotein sulfhydryl groups, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were assayed in cortex-free sponge tissue, where cyanobacteria are all but absent. For both species, biochemical parameters were determined in specimens living in illuminated habitats and in dark caves, where sponges are virtually aposymbiotic for cyanobacteria. As C. nucula is unable to colonize dark sites, specimens of this species were artificially transferred to a cave and maintained in dark conditions for 6 mo. Results showed that in the absence of light (i.e., in the absence of cyanobacteria) C. nucula undergo metabolic collapse and thiol depletion. In contrast, P. ficiformis activates heterotrophic metabolism and mechanisms which balance the loss of cell reducing power. This suggests that cyanobacteria effectively participate in controlling the redox potential of the host cells by the transfer of reducing equivalents. Only P. ficiformis is capable of counteracting, by means of heterotrophic metabolism, the loss of the contribution from symbionts which is caused by dark conditions. This explains the differences in the ecological requirements of the two species. Because cyanobacterial symbionts release fixed carbon in the form of glycerol and other small organic phosphate (Wilkinson 1979), a model based on the glycerol 3-phosphate shuttle (typically occurring in chloroplasts and mitochondria) is suggested. The mechanism proposed appears to be an ancient biochemical adaptation which arose among ancestral symbiotic systems, and further developed in the relationships between endosymbiotic organelles and cytoplasm.

Parasitic diatoms inside antarctic sponges.

Antarctic sponges may host large populations of planktonic and benthic diatoms. After settling on the sponge, these diatoms enter its body through pinacocytes (1) and form, there, large mono- or pauci-specific assemblages. Yet the total amount of carbohydrates in the invaded sponge tissue is inversely correlated with that of chlorophyll-a. We suggest, therefore, that endobiont diatoms utilize the products of the metabolism of their host as an energy source. This is the first evidence indicating that an endobiotic autotrophic organism may parasitize its animal host. Moreover, this unusual symbiotic behavior could be a successful strategy that allows the diatom to survive in darkness.

Biogeographic traits and checklist of Antarctic demosponges

SummaryThe biogeography of Antarctic demosponges has been studied by dividing Antarctic and circumantarctic areas into geographic entities, and then assigning to these entities all recorded species according to literature reports. Correspondence analysis ordination based on the presence or absence of species shows the existence of a distinct Antarctic Faunistic Complex (AFC), including continental Antarctica, most of the Antarctic and circumantarctic islands and the Magellan area. Such a result has enabled us to drawup a checklist of 352 Antarctic demosponge species. Investigation of within-AFC patterns indicates that the continent is a highly homogeneous area, establishing closer relationships with the Scotia Arc and to a lesser extent with the Magellan region. The AFC has low specific affinities with the other circumantarctic regions (South Africa, temperate Australia and New Zealand), whereas at the generic level relationships appear more pronounced. This biogeographic pattern may lead us to suppose a common Gondwanian origin for Antarctic and circumantarctic sponge faunas, followed by differentiation due to Gondwana fragmentation. Antarctica moved towards polar latitudes and became progressively isolated, only maintaining active interchanges with South America. Climatic changes possibly induced intense processes of speciation in the Antarctic demosponge fauna, thus contributing to its differentiation.

Body Polarity and Mineral Selectivity in the Demosponge Chondrosia reniformis

The skeleton of the common Mediterranean demosponge Chondrosia reniformis lacks endogenous spicules; but exogenous siliceous material is selectively incorporated into its collagenous ectosome, strengthening this layer. Nevertheless, the settling of sponge buds during asexual reproduction necessitates an active incorporation of the calcareous substratum through the sponge lower ectosome. This fact suggests the presence of a polarity in the sponge, with the lower surface selecting primarily carbonates, and the upper surface selecting exclusively silicates and quartz. Our observations under experimental conditions showed that the strong selectivity of the upper ectosome is realized only when the sponge is fixed to the substratum; if detached, the sponge incorporates both quartz and carbonates. In laboratory experiments, the incapacity of both kinds of ectosome to regenerate into a new complete sponge suggests that this polarity arises early in ontogeny.

Diatom invasion in the antarctic hexactinellid sponge Scolymastra joubini

Abstract Sponges often host large amounts of symbionts, mainly represented by cyanobacteria and dinoflagellates. Recent findings show a widespread presence of symbiotic diatoms living inside antarctic demosponges. In this paper, the invasion by large populations of the diatom Melosira sp. into specimens of the hexactinellid sponge Scolymastra joubini is reported. SEM analyses support the hypothesis that the embedded living diatoms have a negative impact on sponge tissues, leading to degenerative processes.

Scanning electron microscope evidence for diatom uptake by two Antarctic sponges

Scanning electron microscopy (SEM) investigation of two Antarctic sponges, Phorbas glaberrima and Tedania charcoti, showed that the exopinacoderm effects a direct uptake of benthic diatoms which settle on the sponge surface. In P. glaberrima, planktonic diatoms were also observed penetrating through the inhalant system, the primary way of feeding in sponges. Benthic diatoms which accumulate in the mesohyl underneath the exopinacoderm help to strengthen the sponge cortex and may be an alimentary source during oligotrophic periods in the Antarctic environment.

A sponge with acrosome: Oscarella lobularis

In this paper we describe the presence of a true acrosome in the Homosclerophorida demosponge Oscarella lobularis. Because it is the first finding of this organelle in Porifera, the general concepts of the acrosomal evolution are reviewed.

Ultrastructural study of spermatogenesis in Oscarella lobularis (Porifera, Demospongiae)

Summary The various phases of spermatogenesis in the demosponge Oscarella lobularis were studied by electron microscopy. Spermatogenesis occurs within spermatic cysts, which are presumed to derive from choanocyte chambers by transformation of choanocytes into spermatogonia. Germ cells develop asynchronously within spermatocysts, and cytoplasmic bridges, indicating incomplete cells division, connect several germ cells. Attached spermatogonia suggest gonial generations. Spermatocytes I typically show the presence of synaptonemal complexes indicating meiotic divisions. Spermatocytes II have a small size probably because of the meiotic divisions of spermatocytes I. Spermatids are characterized by an acrosome, a big mitochondrion and a peripheral sheath of condensed chromatin surrounding a clearer central area in the nucleus. The mature spermatozoon shows a lateral flagellum and a flattened acrosome capping the nucleus. The phylogenetic implications of some features of the spermatozoon are suggested.

Viviparous development in the Antarctic sponge Stylocordyla borealis Loven, 1868

Abstract. The complete larval development of the deep-sea sponge Stylocordyla borealis (from eggs to young sponges) was followed in sponges from the Antarctic waters of Terra Nova Bay. S. borealis shows a viviparous strategy which leads to young complete sponges incubated in the mother body, with cortex, spicules and choanocyte chambers. This development can be considered a K-strategy, which is usually employed by deep-sea organisms and cold-water benthic invertebrates.

The assessment of DNA from marine organisms via a modified salting-out protocol

We developed a rapid, practical and non-toxic salting-out method for the extraction of DNA from marine organisms, and tested it on two representative species of Porifera and Cnidaria, both living in association with symbiotic zooxanthellae. We tested the efficiency of the protocol by comparing the output of the method for fresh tissue, frozen tissue and tissue stored in ethanol. It proved to be effective for extracting DNA in the case of the methods of preservation considered here, and for obtaining quantities of DNA comparable to those obtained via the traditional approach. The DNA from both species was of good quality. The DNA obtained was amplified by PCR using specific primers for the large ribosomal subunit, allowing the identification of the presence of both the host and symbiont genomes.