Metabolic Activation and Scaling in Two Species of Colonial Cnidarians

Morphological, Genetic, and Physiological Effects of Nutrient Manipulation on a Colonial Marine Hydroid

AbstractThe availability of environmental nutrients is an existential constraint for heterotrophic organisms and is thus expected to impact numerous biochemical and physiological features. The continuously proliferative polyp stage of colonial hydroids provides a useful model to study these features, allowing genetically identical replicates to be compared. Two groups of colonies of Eirene sp., defined by different feeding treatments, were grown by explanting the same founder colony onto cover glass. Colonies of both treatments were allowed to grow continuously by explanting them onto new cover glass as they reached the edge of the existing surface. The nutrient-abundant polyps grew faster and produced more clumped or "sheet-like" colonies. Compared to the founder colony, the nutrient-abundant colonies exhibited more mutations (i.e., single-nucleotide polymorphisms) than the nutrient-scarce colonies. Nevertheless, these differences were not commensurate with the differences in growth. Using a polarographic electrode, we found that the nutrient-abundant colonies exhibited lower rates of oxygen uptake relative to total protein. The probe 2',7'-dichlorodihydrofluorescein diacetate and fluorescent microscopy allowed visualization of the mitochondrion-rich cells at the base of the polyps and showed that the nutrient-abundant colonies exhibited greater amounts of reactive oxygen species than the nutrient-scarce colonies. Parallels to the Warburg effect-aerobic glycolysis, diminished oxygen uptake, and lactate secretion-found in human cancers and other proliferative cells may be suggested. However, little is known about anaerobic metabolism in cnidarians. Examination of oxygen uptake suggests an anaerobic threshold at a roughly 1-mg/L oxygen concentration. Nutrient-abundant colonies may respond more dramatically to this threshold than nutrient-scarce colonies.

Can natural selection and druggable targets synergize? Of nutrient scarcity, cancer, and the evolution of cooperation

Since the dawn of molecular biology, cancer therapy has focused on druggable targets. Despite some remarkable successes, cell-level evolution remains a potent antagonist to this approach. We suggest that a deeper understanding of the breakdown of cooperation can synergize the evolutionary and druggable-targets approaches. Complexity requires cooperation, whether between cells of different species (symbiosis) or between cells of the same organism (multicellularity). Both forms of cooperation may be associated with nutrient scarcity, which in turn may be associated with a chemiosmotic metabolism. A variety of examples from modern organisms supports these generalities. Indeed, mammalian cancers-unicellular, glycolytic, and fast-replicating-parallel these examples. Nutrient scarcity, chemiosmosis, and associated signaling may favor cooperation, while under conditions of nutrient abundance a fermentative metabolism may signal the breakdown of cooperation. Manipulating this metabolic milieu may potentiate the effects of targeted therapeutics. Specific opportunities are discussed in this regard, including avicins, a novel plant product.

The colonial cnidarian Hydractinia

Hydractinia, a genus of colonial marine cnidarians, has been used as a model organism for developmental biology and comparative immunology for over a century. It was this animal where stem cells and germ cells were first studied. However, protocols for efficient genetic engineering have only recently been established by a small but interactive community of researchers. The animal grows well in the lab, spawns daily, and its relatively short life cycle allows genetic studies. The availability of genomic tools and resources opens further opportunities for research using this animal. Its accessibility to experimental manipulation, growth- and cellular-plasticity, regenerative ability, and resistance to aging and cancer place Hydractinia as an emerging model for research in many biological and environmental disciplines.