Environmental Engineering Reference
In-Depth Information
•
either by direct photoinduced reaction (H
2
O
2
+
h
υ
→
2HO
) or by photo-Fenton
processes is susceptible to decompose DOM in aqueous solution (Zepp et al.
1992
;
Zellner et al.
1990
; Goldstein and Rabani
2008
). These photoinduced effects are
associated with two impacts on growth of primary production: (i) photoinduced
generation of HO
•
has direct negative effects on bacterial growth and/or indirect
effects, because of the loss of bioavailable DOM associated to ROS mineralization
(Scully et al.
2003a
). Correspondingly, extracellular enzymes (e.g., phosphatase
and glucosidase) can be inactivated in natural waters by secondary photoinduced
processes that can lead to a reduction of the substrate uptake by bacteria (Scully
et al.
2003b
; Ortega-Retuerta et al.
2007
). (ii) Studies of abundance and
growth in the presence of humic substances indicate that bacteria are the
most significant utilizers of allochthonous DOM. This issue is appar-
ently made easier by DOM photolysis under natural sunlight, with pro-
duction of lower molecular weight, and biologically labile organic
products (Miller and Zepp
1995
; Strome and Miller
1978
; Amador
et al.
1989
; Kieber et al.
1989
; Moran and Zepp
1997
). This photoinduced effect can
be supported by the observation that DOM photobleaching is accompanied by bacte-
rial growth in humic lakes with significant amounts of chromophoric DOM (Lindell
et al.
1995
; Reche et al.
1998
; de Lange et al.
2003
). Thus, humic substances in lakes
may serve as a substrate for bacterioplankton and lead to enhanced microbial pro-
duction. Such stimulation of bacterioplankton productivity could influence food
chains in two ways (Jones
1992
): firstly, by providing an alternative base (in addi-
tion to autotrophic primary production) for the energetic and nutritional support of
consumer organisms, of course if bacterial production can be effectively grazed; sec-
ondly, by increasing bacterial demand for limiting nutrients at the expense of phyto-
plankton, thereby depressing autotrophic primary production (Jones
1992
).
A further issue is the dependence of photosynthesis on autochthonous DOM.
Autochthonous DOM or unknown compounds produced by the cyanobacte-
rium
Trichormus doliolum
or filtrates of dinoflagellate
Peridinium aciculiferum
or
Prorocentrum lima
can inhibit the PSII in other cyanobacteria, decreasing the pho-
tosynthetic efficiency (Igarashi et al.
1998
; Rengefors and Legrand
2001
; Sukenik
et al.
2002
; Windust et al.
1996
; von Elert and Juttner
1997
; Sugg and VanDolah
1999
).
Compounds produced by the cyanobacterium
Microcystis
sp. can inhibit carbonic
anhydrase activity of the dinoflagellate
P. gatunense
, leading to CO
2
limitation and
inhibition of photosynthesis (Sukenik et al.
2002
). When tested as a pure compound,
okadaic acid produced by the dinoflagellate
Prorocentrum lima
could inhibit the growth
of three microalgal species (Windust et al.
1996
), possibly because okadaic acid is a
potent phosphatase inhibitor (Bialojan and Takai
1988
). Also microcystins produced by
the cyanobacterium
Microcystis aeruginosa
can inhibit phosphatase (Dawson
1998
).
Microalgal compounds have been shown to damage red blood cell membranes, which
suggest that competing phytoplankton could be similarly affected (Igarashi et al.
1998
).
On the other hand, autochthonous DOM released by phytoplankton can be utilized with
high efficiency by heterotrophic bacteria and can thus stimulate heterotrophic growth
and nutrient cycling (Brussaard et al.
1996
,
2005
,
2007
; Gobler et al.
1997
; Fuhrman
1992
; Bratbak et al.
1998
; Middelboe
2003
).
