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

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 quantity of extracellular polymeric substances (EPS) that contribute significantly to the cohesive properties observed in biofilms. Changes in carbohydrate content and composition and the persistence of diatom EPS were demonstrated with pulse-chase (NaHCO3)-C-13 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 proportion of fixed carbon was initially sequestered into intracellular chrysolaminaran, and labeled carbon was also quickly used for production of the diatom PLFA 20:5 omega 3 and EPS. Saccharides in the EPS fractions were enriched rapidly, followed by labeling of distinct bacterial PLFAs through heterotrophic utilization of EPS within 4 h. Maximal isotopic enrichment of diatom and Gram-negative bacterial PLFAs and hot water- and hot bicarbonate-soluble EPS fractions occurred after 4 h, and C-13-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 assimilated 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 C-13 in polysaccharides and PLFAs directly demonstrated the flow of carbon between biofilm autotrophs and heterotrophic bacteria and the significance of diatom-derived carbohydrates in this exchange.