| Summary: | Benthic microalgae (BMA) living within the surface sediment of salt marshes are
highly productive organisms that provide a significant proportion of organic carbon
inputs into estuarine systems. BMA secrete extracellular carbohydrates in the form of
low molecular weight carbohydrates and extracellular polymeric substances (EPS) as
they migrate within the sediment. EPS plays an important role in the structure and
function of BMA biofilms in shallow-water systems as EPS affects habitat structure,
stabilizes the sediment, reduces sediment erosion, and is a carbon source for organisms.
This study looked at the effect of nutrients and carbohydrate additions on BMA
biomass, bacterial biomass, carbohydrate production, and glycosidase activity in the
surface 5 mm of intertidal sediment in a subtropical salt marsh (Galveston Bay, Texas).
Nitrogen and phosphorus were added to cores collected from the salt marsh and
incubated in the lab over four days. Very little change was seen in the biomass of the
benthic microalgae or in the different carbohydrate fractions with the added nutrients.
The mean chlorophyll a concentration was 13 +/- 5 ug g-1 sediment, the mean saline
extractable carbohydrate concentration was 237 +/- 113 ug g-1 sediment, and the mean EPS concentration was 48 +/- 25 ug g-1 sediment. The chlorophyll a and saline
extractable carbohydrate concentrations initially decreased over the first 24 hours, but
then increased over the rest of the experiment, indicating a possible species
compositional shift in the BMA. With no major response with nutrient additions, it is
likely that a different environmental factor is limiting for the growth of the benthic
microalgae, and therefore the production of sEPS, in this salt marsh.
A series of experiments was conducted in situ by adding glucose, alginic acid,
and phosphorus to sediment within experimental plots. Samples were taken periodically
over three to seven days to determine the biomass of the microbial community, enzyme
activities and kinetics, and changes in the concentrations of several sediment
carbohydrate pools. u-glucosidase activities (15 +/- 3 nmol g-1 h-1) were significantly
higher than u-xylosidase (6 +/- 2 nmol g-1 h-1) and u-galactosidase (8 +/- 2 nmol g-1 h-1)
activities within the sediment, and there was no suppression of u-glucosidase activity
measured with the glucose addition. These data represent the first measurement of u-
xylosidase and u-galactosidase activity in intertidal sediment dominated by BMA.
Although preliminary experiments suggested a possible phosphorus limitation within the
sediment, there was little change in the bacteria abundance or the benthic microalgae
biomass when phosphorus was added in situ.
This study begins to illustrate the dynamics of carbohydrate production and loss
in this salt marsh, and the ability for the microbial community in the salt marshes of
Galveston Bay to adjust to the nutrient and carbohydrate treatments.
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