Significance of diatom-derived polymers in carbon flow dynamics within estuarine biofilms determined through isotopic enrichment

Abstract

Pennate diatoms are the main group of primary producers in transient microbial biofilms at the sediment surface in muddy intertidal systems. These microalgae produce a substantial quan- tity of extracellular polymeric substances (EPS) that contribute significantly to the cohesive proper- ties observed in biofilms. Changes in carbohydrate content and composition and the persistence of diatom EPS were demonstrated with pulse-chase NaH 13 CO3 labeling and simultaneous detection of isotopic enrichment in polysaccharides and phospholipid fatty acids (PLFAs) in Colne Estuary, UK, biofilms over a 48 h period. Significant isotopic enrichment of glucan indicated that a large propor- tion of fixed carbon was initially sequestered into intracellular chrysolaminaran, and labeled carbon was also quickly used for production of the diatom PLFA 20:5ω3 and EPS. Saccharides in the EPS fractions were enriched rapidly, followed by labeling of distinct bacterial PLFAs through hetero- trophic utilization of EPS within 4 h. Maximal isotopic enrichment of diatom and Gram-negative bac- terial PLFAs and hot water- and hot bicarbonate-soluble EPS fractions occurred after 4 h, and 13 C- labeling increased in Gram-positive bacterial PLFAs throughout the study period. After 48 h, PLFAs remained highly labeled relative to diatom-derived polysaccharides, indicating persistence of assim- ilated C in organisms within the biofilms through time and rapid turnover of polysaccharide pools within the biofilms. Coupled pulse-chase isotopic labeling and simultaneous tracking of 13 C in poly- saccharides and PLFAs directly demonstrated the flow of carbon between biofilm autotrophs and het- erotrophic bacteria and the significance of diatom-derived carbohydrates in this exchange.