We studied patterns of production and loss of four different extracellular polymeric substance (EPS) fractionscolloidal carbohydrates, colloidal EPS (cEPS), hot water (HW)-extracted and hot bicarbonate (HB)-extracted fractions-and community profiles of active (RNA) bacterial communities by use of Terminal-Restriction Fragment Length Polymorphism (T-RFLP) analysis of reverse transcription-polymerase chain reaction amplified 16S rRNA in mudflats in the Colne Estuary, United Kingdom, over two tidal emersion-immersion cycles. Colloidal carbohydrates and intracellular storage carbohydrate (HW) increased during tidal emersion and declined during tidal cover. The dynamics of cEPS and uronic acid content were closely coupled, as were the HB fraction and HB uronic acids. Changes in monosaccharide profiles of HW and HB fractions occurred during the diel period, with some similarity between cEPS and HB fractions. Increasing enzymatic release rates of reducing sugars and increased reducing sugar content were correlated with increased concentrations of colloidal carbohydrate and cEPS during the illuminated emersion period, and with the amount of HB-extracted uronic acids (the most refractory EPS fraction measured). Loss of reducing sugars was high, with sediment concentrations far below those predicted by the measured in situ release rates. T-RFLP analysis revealed no significant shifts in the overall taxonomic composition of the active bacterial community. However, 12 of the 59 terminal restriction fragments identified showed significant changes in relative abundance during the tidal cycle. Changes in the relative abundance of three particular terminal restriction fragments (bacterial taxa) were positively correlated to the rate of extracellular hydrolysis. Losses of chlorophyll a and colloidal and cEPS (up to 50-60%) occurred mainly in the first 30 min after tidal cover. About half of this may be owing to in situ degradation, with ''wash away'' into the water column accounting for the remainder.Microphytobenthic biofilms in intertidal sediments play an important role in the ecology of estuarine systems (Under-1 Corresponding author (gjcu@essex.ac.uk).

Microphytobenthic biofilms in estuaries, dominated by epipelic diatoms, are sites of high primary productivity. These diatoms exude large quantities of extracellular polymeric substances (EPS) comprising polysaccharides and glycoproteins, providing a substantial pool of organic carbon available to heterotrophs within the sediment. In this study, sediment slurry microcosms were enriched with either colloidal carbohydrates or colloidal EPS (cEPS) or left unamended. Over 10 days, the fate of these carbohydrates and changes in ␤-glucosidase activity were monitored. Terminal restriction fragment length polymorphism (T-RFLP), DNA sequencing, and quantitative PCR (Q-PCR) analysis of 16S rRNA sequences were used to determine whether sediment bacterial communities exhibited compositional shifts in response to the different available carbon sources. Initial heterotrophic activity led to reductions in carbohydrate concentrations in all three microcosms from day 0 to day 2, with some increases in ␤-glucosidase activity. During this period, treatment-specific shifts in bacterial community composition were not observed. However, by days 4 and 10, the bacterial community in the cEPS-enriched sediment diverged from those in colloid-enriched and unamended sediments, with Q-PCR analysis showing elevated bacterial numbers in the cEPS-enriched sediment at day 4. Community shifts were attributed to changes in cEPS concentrations and increased ␤-glucosidase activity. T-RFLP and sequencing analyses suggested that this shift was not due to a total community response but rather to large increases in the relative abundance of members of the ␥-proteobacteria, particularly Acinetobacter-related bacteria. These experiments suggest that taxon-and substrate-specific responses within the bacterial community are involved in the degradation of diatom-derived extracellular carbohydrates.Intertidal estuarine mudflats are highly productive systems (24, 69) that can support abundant microphytobenthic (MPB) biofilms on the sediment surface. In sediments mainly comprised of fine silts and clays (Ͻ63 m in size), these biofilms tend to be dominated by populations of motile, epipelic pennate benthic diatoms (2, 69). These photoautotrophs migrate vertically through the sediment, in response to tidal and diel cycles (14) and form conspicuous biofilms on the sediment surface during low tide exposure. During cycles of vertical migration, epipelic diatoms exude extracellular polymeric substance (EPS), primarily composed of a complex mixture of polysaccharides and glycoproteins (10) which, together with other extracellular carbohydrates excreted during photosynthesis, contributes to the mucilage component of intertidal biofilms (5,16,30). The pathway of EPS production, from photosynthetic carbon incorporation to EPS excretion, takes between 0.5 and 2 h (55,66,74) and is subject to variation in both rates of production and biochemical composition, in response to a variety of environmental factors including irradiance, temperature (75), and nutrient limitation (...