Sc. Keith et C. Arnosti, Extracellular enzyme activity in a river-bay-shelf transect: variations inpolysaccharide hydrolysis rates with substrate and size class, AQUAT MIC E, 24(3), 2001, pp. 243-253
The microbial remineralization of high molecular weight dissolved organic c
arbon (DOC) begins with extracellular enzymatic hydrolysis, since macromole
cules must be hydrolyzed to produce substrates sufficiently small to cross
microbial membranes. In order to investigate substrate and size-class relat
ed patterns in extracellular enzymatic activity in the water column, potent
ial hydrolysis rates of 4 polysaccharides (xylan, laminarin, pullulan, and
fucoidan) were measured at stations in the Delaware River, Bay, and shelf.
Potential hydrolysis rates of xylan and laminarin varied by station and sea
son, but xylan hydrolysis rates typically exceeded those of laminarin by fa
ctors of 2 to 10. In contrast, pullulan and fucoidan hydrolysis rates were
(with a single exception for fucoidan) essentially zero for all seasons and
stations. Size fractionation experiments showed that most xylan- and lamin
arin-hydrolyzing activity was associated with the >0.45 mum size fraction.
The contribution of free dissolved enzymes (the <0.2 mum fraction), however
, was at times substantial. In September, when potential activities of both
enzymes were at a maximum, the free-enzyme fraction contributed 48 to 69%
of the xylan-hydrolyzing activity at the freshwater and midbay stations, an
d nearly 100% of the laminarin-hydrolyzing activity at the midbay and plume
stations. These spatial and temporal variations in the contributions of fr
ee enzymes to laminarin and xylan hydrolysis may help to explain the decoup
ling of measurements of specific enzyme activities from large-scale measure
ments of total microbial populations. The contrasting behaviors of laminari
n, xylan, pullulan, and fucoidan, all of which are soluble polysaccharides,
suggest that certain types of polysaccharides may resist hydrolysis by pla
nktonic microbial extracellular enzymes and, therefore, are relatively unav
ailable as substrates to the planktonic community.