Bacterioplankton strategies for leucine and glucose uptake after a cyanobacterial bloom in an eutrophic shallow lake
V. Kisand
a,b,
, H. Tammert
a,b
a
Vo˜rtsja¨rv Limnological Station, Rannu 61101, Tartumaa, Estonia
b
Institute of Zoology and Hydrobiology, University of Tartu, Vanemuise 46 51014, Estonia Accepted 25 June 2000
Abstract
Extracellular enzyme activities and the kinetics of glucose and leucine uptake were measured to study the role of different substrate pools for bacterioplankton in a shallow eutrophic lake. The study took place during the period of cyanobacterial bloom in late summer and its
collapse in the autumn. Leucine aminopeptidase activity LAP, b-glucosidase activity b-Gluc,
3
H-leucine incorporation LI and
14
C- glucose incorporation GI were measured in Lake Vo˜rtsja¨rv during the autumn of 1997 September–October. The kinetic parameters V
max
and K
M
were determined for both enzymes using artificial fluorogenic substrates leucine amino-methylcoumarin and methyl-umbelliferyl b- glucose. Leucine and glucose uptake were measured using radiolabelled compounds. Abundance and production of bacterioplankton were
also measured. Several environmental parameters including temperature, nutrient concentrations, seston content, and phytoplankton char- acteristics such as biomass, chlorophyll a concentration, and primary production were followed. The GI V
max
correlated positively with release of low molecular weight products of primary production, b-Gluc activity was more closely correlated with polymeric substrates
released after breakdown of cyanobacterial bloom. LAP specific activity i.e. activity per cell increased towards the end of the experimental period and correlated more closely with the presence of specific populations than to the total number of bacteria. By the end of the experiment
time, LI switched to a lower affinity system with higher specific V
max
and K
M
. Bacteria preferentially used available carbon as dissolved glucose over carbon sources derived from the exoenzymatic hydrolysis of polymers. However, both sources of leucine dissolved and
hydrolysis products were used equally by bacteria. Phytoplankton dominated by cyanophytes was the main origin of readily available dissolved low molecular weight compounds. After a cyanobacterial bloom some populations of bacteria grew on the dead cell material of
algae, but the total number of heterotrophic bacteria decreased from approximately 7 × 10
6
to 1 :
9 × 10
6
ml
21
: q
2000 Elsevier Science Ltd. All rights reserved.
Keywords: Extracellular hydrolytic enzymes; Glucose and leucine uptake; Bacterioplankton; Shallow eutrophic lake
1. Introduction
Most organic matter in soil and aquatic environments is composed of polymeric compounds and is not directly
permeable through bacterial cell membranes. The activities of extracellular enzymes increase the soluble “available”
carbon sources, which are then taken up and metabolised by bacteria. Glucolytic and proteolytic enzymes associated
with bacterial cells are the most widely studied extracellular enzymes in water bodies Chrost and Riemann, 1994;
Chrost, 1991; Chrost et al., 1986; Mu¨nster, 1991.
Bacterioplankton is considered to be a major pool of organic matter, as many bacteria are able to degrade organic
polymers into small compounds for subsequent rapid uptake and assimilation. In principle, this process has two impor-
tant consequences: 1 bacteria incorporate these products into their biomass; and 2 via the microbial loop, its carbon
becomes available to higher food web organisms Azam et al., 1983. Bacterial extracellular enzymes Ammerman,
1991; Billen, 1991; Chrost, 1991 have been shown to have an important function in the degradation of polymers
in aquatic ecosystems Chrost, 1990.
b -d-Glucosidases b-d-glucoside glucohydrolase; EC
3.2.1.21, b-Gluc and leucine aminopeptidase LAP, EC 3.4.1.1, are responsible for the hydrolysis of organic consti-
tuents in the dissolved DOM and particulate organic matter POM pools. As a result of their activities glucose
and amino acids are released into the environment. These compounds are then available to the uptake systems of the
bacterial cells. It has been suggested that the corresponding
Soil Biology Biochemistry 32 2000 1965–1972
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Corresponding author. Address: Department of Microbiology, Umea˚ University S-90187, Umea˚, Sweden. Fax:
146-90-77-26-30. E-mail address: veljo.kisandmicro.umu.se V. Kisand.
hydrolytic and transport systems should form a closely related unit a ‘complex’ to ensure the efficient utilisation
of substrates. Although the transport of small molecules through
membranes is believed to be not a limiting step for the system Chrost, 1989; Mu¨nster and Chrost, 1990, both
components of a complex can become saturated. Compar- ison of the kinetics of the extracellular enzymeuptake
complex could give an insight into the ecology and substrate utilisation in aquatic and soil systems.
The highest extracellular enzymatic activities are usually found during the period after phytoplankton bloom when
algae are dying and being lysed e.g. Chrost, 1989. During this time bacterial uptake of the products of hydrolysis,
rather than algal exudates as during the bloom are believed to support metabolism and growth. Extracellular enzymes
may not have great importance during the period of active phytoplankton growth as most of the newly produced
organic carbon available to bacteria originates directly from the release of photosynthesis products Chrost and
Overbeck, 1990; Middelboe et al., 1995 and zooplankton grazing of algae Vrba et al., 1992. However, during the
period after the phytoplankton bloom, production of extra- cellular enzymes is the response of bacterioplankton to
carbon sources of different type which become available.
The objectives of the present study were to follow both the uptake of two readily utilisable substrates, glucose and
leucine, and the extracellular enzymatic activities of the b- glucosidase and leucine aminopeptidase, both during and
after the cyanobacterial bloom. Several other bacterioplank- ton parameters, such as biomass and production, phyto-
plankton biomass, chlorophyll concentration, and the physico-chemical properties of water were also followed.
2. Material and methods
