Significance of Small Organic Chelators in Laboratory Cultures of Ephydatia fluviatilis (Porifera: Spongillidae)

Inland waters and coastal marine waters contain a spectrum of complex organic materials, called humic substances, that are formed primarily in soils and reach rivers and lakes by way of runoff. Traditionally, humic substances are separated into three fractions, fulvic acids, humic acids, and humin, which have certain structural similarities but differ in molecular weight and functional group content. Fulvic acid is the least polymerized of the fractions, is highly oxidized, stable, and water-soluble. Humin consists largely of high-molecular-weight colloidal particles. Nonhumic substances are a class of low-molecular-weight organic substances that are formed primarily in water rather than in soils and are derived from degradation of larger humic compounds, from secretion by phytoplankton, and from autolysis of microorganisms. Both humic and nonhumic compounds of less than 700 molecular weight realize 2-6 times the chelation efficacy of the heavier weight fractions. They are more soluble than heavier weight fractions, tend to remain in solution, and contribute much of the yellow stain in waters. We report the results of culture experiments with Ephydatia fluviatilis wherein low-molecular-weight, naturally occurring organic molecules were used as chelators in an effort to establish a culture medium in which chelation was representative of natural waters. Experimental results indicate that various chelators have differential effects on sponge growth rate, and that the total concentration of chelators influences sponge growth rate.