Marine sponges often contain diverse and abundant microbial communities, including bacteria, archaea, microalgae, and fungi. In some cases, these microbial associates comprise as much as 40% of the sponge volume and can contribute significantly to host metabolism (e.g., via photosynthesis or nitrogen fixation). We review in detail the diversity of microbes associated with sponges, including extensive 16S rRNA-based phylogenetic analyses which support the previously suggested existence of a sponge-specific microbiota. These analyses provide a suitable vantage point from which to consider the potential evolutionary and ecological ramifications of these widespread, sponge-specific microorganisms. Subsequently, we examine the ecology of sponge-microbe associations, including the establishment and maintenance of these sometimes intimate partnerships, the varied nature of the interactions (ranging from mutualism to host-pathogen relationships), and the broad-scale patterns of symbiont distribution. The ecological and evolutionary importance of sponge-microbe associations is mirrored by their enormous biotechnological potential: marine sponges are among the animal kingdom's most prolific producers of bioactive metabolites, and in at least some cases, the compounds are of microbial rather than sponge origin. We review the status of this important field, outlining the various approaches (e.g., cultivation, cell separation, and metagenomics) which have been employed to access the chemical wealth of sponge-microbe associations.

Sponge-Associated Microorganisms: Evolution, Ecology, and Biotechnological Potential

Why write a book on sponge taxonomy; who would read such a book; what is the relevance of sponges; and what are sponges anyway?

Systema Porifera. A Guide to the Classification of Sponges

Earth’s biota produces vast quantities of polymerized silica at ambient temperatures and pressures by mechanisms that are not understood. Silica spicules constitute 75% of the dry weight of the sponge Tethya aurantia, making this organism uniquely tractable for analyses of the proteins intimately associated with the biosilica. Each spicule contains a central protein filament, shown by x-ray diffraction to exhibit a highly regular, repeating structure. The protein filaments can be dissociated to yield three similar subunits, named silicatein α, β, and γ. The molecular weights and amino acid compositions of the three silicateins are similar, suggesting that they are members of a single protein family. The cDNA sequence of silicatein α, the most abundant of these subunits, reveals that this protein is highly similar to members of the cathepsin L and papain family of proteases. The cysteine at the active site in the proteases is replaced by serine in silicatein α, although the six cysteines that form disulfide bridges in the proteases are conserved. Silicatein α also contains unique tandem arrays of multiple hydroxyls. These structural features may help explain the mechanism of biosilicification and the recently discovered activity of the silicateins in promoting the condensation of silica and organically modified siloxane polymers (silicones) from the corresponding silicon alkoxides. They suggest the possibility of a dynamic role of the silicateins in silicification of the sponge spicule and offer the prospect of a new synthetic route to silica and siloxane polymers at low temperature and pressure and neutral pH.

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Silicatein α: Cathepsin L-like protein in sponge biosilica

Cell types are the basic building blocks of multicellular organisms and are extensively diversified in animals. Despite recent advances in characterizing cell types, classification schemes remain ambiguous. We propose an evolutionary definition of a cell type that allows cell types to be delineated and compared within and between species. Key to cell type identity are evolutionary changes in the 'core regulatory complex' (CoRC) of transcription factors, that make emergent sister cell types distinct, enable their independent evolution and regulate cell type-specific traits termed apomeres. We discuss the distinction between developmental and evolutionary lineages, and present a roadmap for future research.

The origin and evolution of cell types

Sponge remains have been identified in the Early Vendian Doushantuo phosphate deposit in central Guizhou (South China), which has an age of approximately 580 million years ago. Their skeletons consist of siliceous, monaxonal spicules. All are referred to as the Porifera, class Demospongiae. Preserved soft tissues include the epidermis, porocytes, amoebocytes, sclerocytes, and spongocoel. Among thousands of metazoan embryos is a parenchymella-type of sponge larvae having a shoe-shaped morphology and dense peripheral flagella. The presence of possible amphiblastula larva suggests that the calcareous sponges may have an extended history in the Late Precambrian. The fauna indicates that animals lived 40 to 50 million years before the Cambrian Explosion.

https://science.sciencemag.org/content/sci/279/5352/879.full.pdf

Precambrian Sponges with Cellular Structures

Morphology.- 1. Functional Morphology and Morphological Variation.- General Features.- Functional Morphology.- Variation in Morphology: The New Systematics.- Skeletal Morphology.- Spicule Geometry as a Major Morphological Feature.- Other Morphological, Reproductive and Biochemical Features.- Morphological Variation in the Phylum.- Cellular Structure.- 2. The Epithelia.- Pinacocytes (Excluding Hexactinellids).- Choanocytes (Excluding Hexactinellids).- Variation in Cell Shape.- Pinacoderm and Choanoderm in Hexactinellida.- Contractile Vacuoles and Water Balance.- 3. Cellular Components of the Mesohyl.- Cell Types and Structure.- Archeocytes, Polyblasts, Thesocytes.- Spongocytes, Gemmular Epithelium, Lophocytes, Trophocytes, Megasclerocytes.- Histoblasts, Collencytes, Fiber Cells, Myocytes, Neuroid Cells.- Microsclerocytes.- Special Cells-Cellules a Inclusions.- Summary of Special Cells.- The Cortex.- Cords of Cells.- Symbionts.- Cyanobacteria.- Zooxanthellae.- Zoochlorellae.- Bacteria.- Some Problems in Symbiont-Host Relationships.- Pigment.- Cellular Functions.- 4. Calcification, Silicification, Bioerosion.- Calcareous Deposits: Morphology, Chemistry, Distribution.- Calcareous Spicules.- Calcareous Scales.- Cemented Calcareous Spicules.- Aspicular Calcareous Deposits.- The Secretion of Calcareous Spicules.- Factors Affecting Calcareous Spicule Secretion.- Morphogenetic and Developmental Aspects of Calcareous Spicule Secretion.- The Secretion of Other Calcareous Structures.- Siliceous Deposits: Morphology, Chemistry, Distribution.- Siliceous Spicules of Demosponges.- Siliceous Scales.- Siliceous Spicules of Hexactinellids.- Cemented Siliceous Spicules.- Entrapped Siliceous Spicules.- The Secretion of Siliceous Spicules.- Demosponges.- Hexactinellids.- Factors Affecting Siliceous Spicule Secretion.- Morphogenetic and Developmental Aspects of Siliceous Spicule Secretion.- The Secretion of Other Siliceous Structures.- Iron Containing Deposits.- Excavation of Calcareous Substrata.- General Features of Burrowing and the Species Involved.- Cellular Basis of Etching.- Mechanisms of Burrowing.- Morphogenetic Aspects of Burrowing.- 5. Collagen Fibrils, Spongin, Matrix Substances.- Collagen: Morphology, Chemistry, Distribution.- Collagen Fibrils.- Spongin Fibers.- Gemmule Coats.- Collagen Synthesis and Secretion.- Biochemical Aspects of Collagen Synthesis.- Cellular Aspects of Collagen Secretion.- Other Organic Products.- Biochemical Studies.- Cytochemical and Morphological Studies.- Fibronectin.- Morphogenetic and Developmental Aspects of Organic Matrix Substances.- 6. Canal System Structure and Activities.- Ostia.- Structure in Calcarea.- Structure in Demospongiae.- Structure in Hexactinellida.- Development.- Incurrent Canals.- Inhalant "Organs".- Papillae.- Other Patterns.- Cellular Features.- Development.- The Terms Prosopyles and Apopyles.- Aphodal, Diplodal, Asconoid, and Syconoid Arrangements.- Pinacocytic Prosopyles.- Choanocytic Prosopyles.- Choanocyte Chambers.- Structural Heterogeneity of Chambers.- Development.- Choanocytic Apopyles.- Pinacocytic Apopyles.- Excurrent Canals.- Structure.- Development.- Morphological Patterns.- Oscules.- Morphological Types.- Cellular Features.- Development.- Canal System Elements in Reproductive Stages.- Water Currents.- Origin and Direction.- Transport Rates and Velocities.- Types of Changes in Water Transport.- Effectors Which Directly Influence Water Currents.- Responses of Effectors to Experimental Stimuli.- Spontaneous Changes in Water Currents.- Environmental Factors Influencing Water Currents.- Intrinsic Control of Water Currents.- Coordination by Neuroid Cells.- Feeding.- Retention of Particulates.- Sites of Entrapment-Ingestion.- Distribution and Digestion.- Elimination.- Development.- 7. Gamete, Embryo, Larval Development.- General Features of Sexual Reproduction.- Gametogenesis.- Spermatogenesis.- Oogenesis.- Nurse Cells.- Factors Influencing Gametogenesis.- Temperature.- Size of Animals.- Age and Other Factors.- Sex Determination.- Gamete Release.- Fertilization.- Cleavage.- Somatic Cells Associated with Embryos.- Polarity, Micromeres, Macromeres.- Larval Formation.- Larval Structure.- Processes in Larva Formation.- Reports of Asexually Formed Larvae.- Embryo and Larva Release.- Attachment of Larvae to the Substratum.- The Problem of the Polarity of the Larva.- Larval Metamorphosis.- Experimental Embryology.- 8. Buds, Gemmules, Tissues Regression.- Buds.- Fragmentation: A Special Case of Budding.- Budding.- Gemmules.- Occurrence of Gemmule Producing Species.- Life Cycles Involving Gemmules.- The Gemmule Envelope.- The Gemmular Cells-Thesocytes.- Gemmule Formation in Spongillids.- Gemmule Formation in Marine Species.- Determinants of Gemmule Formation.- Gemmule Dormancy.- Gemmule Germination.- Gemmule Hatching.- Tissue Regression.- A Special Case of Tissue Regression: Reduction Bodies.- 9. Growth, Regeneration, Cell Recognition, Perspective on Cell Differentiation.- Growth.- Initiative Growth.- Maturational Growth.- Remodeling.- Growth as an Increase in Size or Volume.- Experimental Regeneration.- Explants.- Transection of Whole Animals.- Tissue Ablation.- Tissue Recognition.- Reaggregation and Reconstitution of Dissociated Cells.- Perspective on Cell Differentiation.- References.

Tracy L. Simpson

Papers

The Cell Biology of Sponges