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.