Environmental Engineering Reference
In-Depth Information
Planktonic lipids are more susceptible to biodegradation than terrestrial lipids.
Moreover, biodegradation is more intense in sinking particulate organic matter
(POM) than in suspended POM (Rontani et al.
2011
). Simultaneously, there would
be efficient transfer of singlet oxygen from suspended and senescent phytoplankton
cells to associated bacteria, with subsequent inhibition of heterotrophic degradation
(Rontani et al.
2011
). The in vitro enzymatic degradation of Chl
a
in several species
of marine phytoplankton can produce chlorophyllide
a
, pheophorbide
a
, pheophytin
a
, and pyropheophytin
a
(Owens and Falkowskit
1982
). In some species, Chl
a
can
be degraded to products that do not absorb visible light. It has also been observed
that losses of phytol and Mg
2
+
are catalysed by chlorophyllase and by a magnesium-
releasing enzyme, respectively. Both enzymes are activated by cell disintegration
(Owens and Falkowskit
1982
). Phaeophytin
a
, pyrophaeophytin
a
, phaeophorbide
a
, and pyrophaeophorbide
a
are the phaeopigments found in largest amount in both
sediments and water column (Furlong and Carpenter
1988
). Tetrapyrrole derivatives
of chloropigments (phaeopigments) are formed as a result of bacterial or autolytic
cell lysis, and of metazoan grazing activities (Welschmeyer and Lorenzen
1985
;
Sanger and Gorham
1970
; Shuman and Lorenzen
1975
; Bianchi et al.
1988
,
1991
).
Further degradation may produce several colorless organic substances (Brown et al.
1991
; Westrich and Berner
1984
; Henrichs and Doyle
1986
).
From the differences between anoxic and oxic decomposition in incuba-
tion experiments, together with naturally observed concentration profiles, it can
be inferred that Chl
a
in natural sediments can be degraded during the oscillation
between oxic and anoxic conditions caused by physical and biological mixing pro-
cesses (Ming-Yi et al.
1993
). Oscillation experiments (oxic vs. anoxic and anoxic
vs. oxic) also suggest that the activity of aerobic organisms may be an important
factor that affects Chl
a
degradation (Ming-Yi et al.
1993
). Examination of the
effects of meiofauna on Chl
a
degradation under oxic conditions, implies that
microorganisms may play a stronger role in Chl
a
degradation than meiofauna
(Ming-Yi et al.
1993
). The relative temperature independence of anoxic degradation
and temperature dependence of oxic degradation suggest that anoxic degradation
may be largely controlled by chemical factors, while oxic degradation may be more
strongly controlled by biophysical and biochemical processes (Ming-Yi et al.
1993
).
It is shown that the maximum DOM production lags in time relative to Chl
a
concentration in surface waters, whilst Chl
a
concentrations were relatively low
and fluctuated during the summer stratification period in Lake Biwa (Fig.
3
a and
b) (Zhang et al.
2009
; Mostofa KMG et al. unpublished data; Mostofa et al.
2005
;
Sasaki et al.
2005
; Hanamachi et al.
2008
). The summertime fluctuation of Chl
a
is possibly linked to its photoinduced degradation, which can contribute to the
DOC increase in the surface water of Lake Biwa (Fig.
3
a and b) (Mostofa KMG et
al. unpublished data; Mostofa et al.
2005
). The release of DOM from algae or phy-
toplankton might be one of the key causes for the decrease of Chl
a
or of the pri-
mary production in the surface layer, during the summer season. It is shown that
both 'labile' and 'refractory' fractions of DOM are produced during phytoplankton
or algal biomass degradation. However, the 'labile' fraction of organic matter, such
as glucose, is rapidly decomposed within a few days and the 'refractory' fraction
