Exopolymer production by intertidal epipelic diatoms

Abstract

Epipelic diatoms are motile, the locomotive force being generated by the extrusion of carbohydrate‐rich exopolymers (EPS). Production rates of EPS, colloidal (water‐soluble) carbohydrates, and glucan (a photosynthetic storage product of diatoms) were investigated in axenic cultures, in sediments under laboratory conditions, and in the field. In stationary‐phase axenic monocultures of Cylindrotheca closterium, Navicula perminuta, and Nitzschia sigma, EPS concentrations significantly increased during darkness. EPS concentrations in darkness corresponded to decreases in glucan concentration. Glucan concentrations increased significantly in illuminated cultures, Identical patterns of concentration changes were observed in intact diatom‐dominated sediments. Inorganic 14C incorporation was used to determine rates of production and flux of photoassimilates through the different carbohydrate fractions in intact sediments. Biofilms exhibited endogenous rhythms of primary and carbohydrate production under laboratory conditions in the absence of tidal influence and under constant illumination (200 µmol m−2 s−1). Maximum production (27.7 mg C m−2 h−1) occurred during periods when sediments would have been emersed had they remained in the field, with 8.0, 3.0, and 0.8% of photoassimilates being incorporated into colloidal‐S, glucan, and EPS, respectively. Higher rates of primary production were measured under natural illumination (617 µmol m−2 s−1) in the field (82.7 mg C m−2 h−1), but the maximum incorporation into colloidal‐S, glucan, and EPS was similar (10.9, 2.8, and 1.0%, respectively). Pulse experiments revealed that 14C initially incorporated into glucan was subsequently reallocated into EPS. Increases in labeled EPS occurred up to 4 h after 14C incorporation, coinciding with periods of migratory activity linked to impending tidal immersion. Production of EPS by epipelic diatoms therefore appears to be closely linked to migratory rhythms, During periods of photosynthesis, assimilated carbon is stored as intracellular glucan, as well as being utilized to produce EPS. A number of stimuli increase the production of EPS: dim light, darkness, and migration linked to tidal immersion. Such increases in EPS production can be met either by increasing the allocation of photoassimilated carbon from current photosynthesis or by reallocating an internal carbon source. This short‐term variability in EPS production has implications for our understanding of estuarine carbon budgets and biogenic stabilization of sediments.