Studies of Bacterial Ectoenzymes

James Christian and David Karl
University of Hawaii, SOEST, Department of Oceanography
Honolulu, HI 96822
(jamesc@soest.hawaii.edu)

Extensive studies of the bacterial ectoenzymes leucine aminopeptidase and α- and β- glucosidase were undertaken on PD94-12 using the method of fluorimetric substrate analogues (4-methylumbelliferone (4MUF) and β-naphthyamine derivatives) pioneered by Hoppe (1983). In addition to water column sampling in concert with other measurements made routinely on this cruise, we focused particularly on three areas:

1. Ectoenzyme activity associated with ice-algal communities: As on previous cruises, ectoenzyme activities associated with sea ice rich in microalgae ("brown ice") were enriched by several orders of magnitude over water column activities, and showed a greater ratio of proteolytic to glycolytic enzymes than in the water column. Under-ice samples collected by SCUBA divers (S-028 personnel) also showed elevated activities and were sampled for many other parameters such as chlorophyll and ATP so that the relationship of activity to algal biomass can be determined.

2. Regulation of ectoenzymes: The effects of various monomeric organic compounds (amino acids, sugars, nucleobases) as well as sterilized extracts of ice algae on ectoenzyme expression by bacterioplankton were examined on this cruise. It was found that aminopeptidase expression is repressed by certain amino acids normally found at low concentrations in seawater, particularly histidine. High concentrations of ammonium, as well as glycine and other relatively abundant amino acids, do not result in such repression, suggesting that nitrogen availability alone is a poor predictor of aminopeptidase expression.

3. Ectoenzyme specificity: In contrast to results obtained on previous cruises, on PD94-12 there appeared to be little specificity for α- and β-anomers in the enzymes hydrolyzing 4MUF glucosides. An apparently nonspecific α/β glucosidase was observed in the "bottle bloom" experiment where rates of hydrolysis of 4MUF α- and β-glucosides varied significantly but almost perfectly in concert (i.e. with little or no change in the α/β ratio).

Reference

Hoppe, H.-G. 1983. Significance of exoenzymatic activities in the ecology of brackish water: measurements by means of methylumbelliferyl substrates. Marine Ecology Progress Series 11: 299-308.