Growth Rates of Marine Bacterial Isolates on Particulate Organic Substrates Solubilized by Freely Released Extracellular Enzymes Y.A. Vetter, J.W. Deming School of Oceanography Box 357940, University of Washington, Seattle, WA 98195, USA Received: 26 June 1998; Accepted: 20 October 1998 A B S T R A C T Growth rates of marine bacterial isolates on particulate organic substrates were measured using a novel apparatus which restricts bacterial cells to the uptake of hydrolysate produced from particu- late substrates only by enzymes that are actively released from the bacterium into the culture medium. Significant, varying growth rates were measured for four different marine bacteria, using three different, ecologically significant particulate organic substrates (preparations of amylopectin, chitin, and animal hide). Growth rates sometimes approached but were usually lower than rates that have been reported in laboratory experiments using dissolved organic growth substrates. These results are consistent with recent model predictions and have important implications for microbial ecology and material cycling in diverse liquid-bathed environments. Introduction and Background Heterotrophic bacterial feeding on organic materials is a critical part of biogeochemistry and the ecology of myriad organisms throughout liquid-bathed marine and terrestrial environments. Microbial feeding on dissolved organic car- bon (DOC) is studied extensively, by observation and ma- nipulation in diverse laboratory settings and environments, and through modeling of various components of DOC feed- ing. Microbial feeding on particulate organic carbon (POC) is less well studied, despite the fact that POC is often an abundant and important microbial food source. Although DOC may sometimes be the dominant food source, atten- tion is often drawn away from POC feeding instead by tech- nical difficulties or model complexity with two-phase media. Extracellular enzymes (EE) are necessary for bacterial feeding because most utilizable marine DOC and all POC is too large to be taken into bacterial cells without first being hydrolyzed into lower molecular weight compounds [3, 44]. On land, EE activity has been linked to organic matter de- composition and nutrient cycling by observation and models [4, 26, 33]. In the ocean, microbial EE may control organic polymer degradation and utilization [13, 23] and be coupled with bacterial substrate uptake and growth [2, 12,14]. EE are also involved in the etiology of significant human diseases Correspondence to: Y.A. Vetter; Fax: (206) 543-0275; E-mail: yav@u. washington.edu. MICROBIAL ECOLOGY Microb Ecol (1999) 37:86–94 © 1999 Springer-Verlag New York Inc.