Stimulation of peptidase activity in nutrient gradients in the Baltic Sea
M. Nausch, G. Nausch
Institut fu¨r Ostseeforschung, Seestrasse 15, D-18119 Rostock-Warnemu¨nde, Germany Accepted 3 May 2000
Abstract
In the Baltic Sea, peptidase and alkaline phosphatase activities always show the same pattern in nutrient gradients: increasing enzyme activities with decreasing dissolved inorganic nitrogen DIN and phosphate. The field experiments reported here suggest that peptidase
activity may be regulated by DIN in a manner similar to that of alkaline phosphatase by phosphate. In nutrient gradients established in mesocosms, peptidase activity increased 5- to 7-fold, whereas the alkaline phosphatase activity increased between 29- and 65-fold. Parallel to
the enzyme activities, phytoplankton biomass measured as chlorophyll a, organic phosphorus and nitrogen increased, but the closest correlation was found between peptidase and alkaline phosphatase activity
r 0
: 90; n 18; p
, 0 :
01 :
The addition of nitrate, and nitrate plus phosphate, at the time where nutrients were depleted and both enzyme activities were high, did not result in a decrease of peptidase
activity. Rather, the peptidase activity was highest after simultaneous treatment with phosphate and nitrate. The addition of 1 mg l
21
alkaline phosphatase caused a 6-fold increase of the peptidase activity after 96 h. Several mechanisms of the stimulation of peptidase activity by
alkaline phosphatase are discussed. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Peptidase; Alkaline phosphatase; Phosphate; Inorganic nitrogen; Extracellular enzyme activities; Aquatic ecosystems
1. Introduction
Studies of extracellular enzyme activities are used to describe the degradation of organic matter in aquatic
ecosystems Hollibaugh and Azam, 1983; Azam and Cho, 1987; Billen, 1991; Boetius and Lochte, 1994, 1996; Karner
et al., 1995; Martinez et al., 1996 and terrestrial soils Asmar et al., 1994; Bishop et al., 1994; Falih and Wain-
wright, 1996; Chander et al., 1997. They are determined by the addition of artificial substrates Darrah and Harris 1986;
Chro´st, 1991; Hoppe, 1993; Shand and Smith 1997. The maximum rate of hydrolysis V
max
of artificial substrates is used to characterise these enzyme activities, and is an indir-
ect measure of the quantity of enzymes in the environment. Due to differences in their regulation, in aquatic environ-
ments some extracellular enzyme activities e.g. alkaline phosphatase are highest during the intensive phase of
phytoplankton growth, whereas others reach highest values during phytoplankton breakdown or at the end of the growth
season Chro´st, 1991; Mu¨nster, 1991; Nausch et al., 1998.
Different abiotic and biotic factors may influence the synthesis and activity of extracellular enzymes, including
temperature, pH Chro´st, 1990; Christian and Karl, 1995, the presence of humic substances Stewart and Wetzel,
1982; Carlsson and Graneli, 1993; Boavida and Wetzel, 1998, oxygen conditions Newman and Reddy, 1993,
hydrogen sulphide Hoppe et al., 1990, or UV-B radiation Garde and Gustavson, 1999. In addition, heavy metals
Yamada et al., 1983; Kandeler et al., 1996 and herbicides Simon and Bergerova, 1984 can influence enzyme activ-
ities in soils. Enzyme stimulation and repression by substrates and end-products are assumed to be the main
regulators Chro´st, 1990. However, the interaction of enzymes with their substrates is more complex. For exam-
ple, the degradation of proteins by peptidases is not only necessary for the supply of nitrogen, but also to provide
carbon. This is how glucose is thought to inhibit peptidase activity Chro´st, 1991; Boetius and Lochte, 1996. Hoppe
and Ullrich 1999 proposed similar mechanisms for the regulation of phosphatase activity in the deep sea, where
phosphate is available and hydrolysis of organic phosphorus compounds is used to meet the carbon demand of bacteria.
In the Baltic Sea, inorganic nutrients are depleted below their detection limits PO
4
, 0.02 mM, NO
2 13
, 0.05 mM during the growth season Nehring et al., 1995. In the
central part of the Baltic Sea, where the input from land sources can be neglected, regeneration from organic matter
is the main source apart from nitrogen fixation of nutrients
Soil Biology Biochemistry 32 2000 1973–1983
0038-071700 - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 0 0 0 0 1 7 3 - 5
www.elsevier.comlocatesoilbio
Corresponding author. Tel.: 149-381-5197-227; fax: 149-381-5197-
440. E-mail address: monika.nauschio-warnemuende.de M. Nausch.
to maintain primary production Wasmund et al., 2000. We investigated both peptidase and alkaline phosphatase activ-
ity in nutrient gradients in the Baltic Sea, and addressed the question of regulatory factors. It is assumed that peptidases
are regulated by the availability of polymeric nitrogen substrates, such as proteins and large polypeptides, and
provide low molecular weight compounds for rapid assim- ilation by bacteria Hollibaugh and Azam, 1983; Billen,
1991; Chro´st 1991. However, the influence of inorganic nitrogen on the peptidase activity of pure bacterial cultures
is shown by Priest 1984 and was shown in the environment only by Chro´st 1991 in lakes. In contrast, the regulation of
alkaline phosphatase in aquatic environments by phosphate availability has been described extensively e.g. Halemejko
and Chro´st, 1984; Gage and Gorham, 1985; Paasche and Erga, 1988; Hernandez et al., 1993. We investigated the
influence of nitrate, phosphate and alkaline phosphatase on peptidase activity in an attempt to explain the observed
behaviour of peptidases in nutrient gradients in marine environments.
2. Material and methods
