291
Photoinduced and Microbial Degradation
From the results of photoinduced degradation of DOM (Table 1), several key
phenomena highlighted the DOM photoinduced degradation in natural waters:
First, photoinduced degradation is greatly dependent on the initial concentration of
DOC: high photo mineralization is observed in waters with low DOC concentration, while mineralization greatly decreases with increasing DOC concentration.
Second, photo mineralization of DOM is typically high in source waters (stream
waters) and then decreases in downstream rivers or lakes or seawater. Third, photoinduced degradation is a relatively rapid process for mineralization of DOM
compared to microbial degradation in natural waters, except for rivers that contain
sewage effluents. It is demonstrated that the photoinduced mineralization of DOM
is relatively high during the first day of irradiation in experimental observations,
and then almost gradually decreases during the irradiation period (Table 1).
The results of photoinduced degradation on molecular size fractions of DOM
demonstrate that photo mineralization is approximately 6 % for large molecular
fractions (<0.10 μm) in surface lake waters compared to 9 % in deep waters during 12 days irradiation (Table 1). The results also show that photo mineralization
of molecular fractions <5 kDa is relatively higher in deeper waters (16 %) than in
surface layers (11 %), and it is higher compared to the 0.1 μm fractions of DOM
(Table 1). These results can highlight four important features about the photoinduced degradation of DOM. First, molecular fractions <0.1 μm in surface waters
composed of approximately 35 % of autochthonously produced DOC during the
summer stratification period may not be photolytically susceptible to mineralization (Mostofa et al. 2009a). Second, molecular fraction of <5 kDa are highly
susceptible to photoinduced degradation. Third, DOM in deeper layers is highly
susceptible to photo mineralization. It can be noted that the DOC in Lake Biwa
is autochthounously produced (45 %) during the summer stratification period,
as estimated from higher DOC in summer (135 μM C in August) than in winter
(~93 μM C in January) samples during the vertical mixing period (Mostofa et al.
2005). The low photodegradative nature of surface DOM appears to be caused by
autochthonous production (~45 % in Lake Biwa), thus autochthonous DOM might
be resistant to photoinduced mineralization. This result is in agreement with earlier studies where phytoplankton-exudate DOM, which is the major DOM source
from bacterial production (Azam and Cho 1987), is resistant to photoinduced degradation by natural sunlight (Thomas and Lara 1995).
The highly photo-reactive nature of DOM in deeper waters than in the surface layer appears to be caused by two phenomena. First, major fractions of DOM
in deeper waters belong to low molecular weight substances (Yoshioka et al. 2007),
which may be photolytically mineralized. Second, microbial assimilation or respiration of particulate organic matter (POM: ca. algae) can produce the autchthonous DOM, which are highly photosensitive and photodegradable (Mostofa K
et al., unpublished; Zhang et al. 2009; Johannessen et al. 2007; Zhang et al. 2009).
The experimental study suggests that the algal-derived DOM is photolytically decomposed by natural sunlight, which is a more efficient photoinduced substrate than is the
allochthonous DOM (Mostofa K et al., unpublished; Johannessen et al. 2007; Hulatt
et al. 2009). The autochthonous organic substances in deeper layers are typically
Photoinduced and Microbial Degradation
From the results of photoinduced degradation of DOM (Table 1), several key
phenomena highlighted the DOM photoinduced degradation in natural waters:
First, photoinduced degradation is greatly dependent on the initial concentration of
DOC: high photo mineralization is observed in waters with low DOC concentration, while mineralization greatly decreases with increasing DOC concentration.
Second, photo mineralization of DOM is typically high in source waters (stream
waters) and then decreases in downstream rivers or lakes or seawater. Third, photoinduced degradation is a relatively rapid process for mineralization of DOM
compared to microbial degradation in natural waters, except for rivers that contain
sewage effluents. It is demonstrated that the photoinduced mineralization of DOM
is relatively high during the first day of irradiation in experimental observations,
and then almost gradually decreases during the irradiation period (Table 1).
The results of photoinduced degradation on molecular size fractions of DOM
demonstrate that photo mineralization is approximately 6 % for large molecular
fractions (<0.10 μm) in surface lake waters compared to 9 % in deep waters during 12 days irradiation (Table 1). The results also show that photo mineralization
of molecular fractions <5 kDa is relatively higher in deeper waters (16 %) than in
surface layers (11 %), and it is higher compared to the 0.1 μm fractions of DOM
(Table 1). These results can highlight four important features about the photoinduced degradation of DOM. First, molecular fractions <0.1 μm in surface waters
composed of approximately 35 % of autochthonously produced DOC during the
summer stratification period may not be photolytically susceptible to mineralization (Mostofa et al. 2009a). Second, molecular fraction of <5 kDa are highly
susceptible to photoinduced degradation. Third, DOM in deeper layers is highly
susceptible to photo mineralization. It can be noted that the DOC in Lake Biwa
is autochthounously produced (45 %) during the summer stratification period,
as estimated from higher DOC in summer (135 μM C in August) than in winter
(~93 μM C in January) samples during the vertical mixing period (Mostofa et al.
2005). The low photodegradative nature of surface DOM appears to be caused by
autochthonous production (~45 % in Lake Biwa), thus autochthonous DOM might
be resistant to photoinduced mineralization. This result is in agreement with earlier studies where phytoplankton-exudate DOM, which is the major DOM source
from bacterial production (Azam and Cho 1987), is resistant to photoinduced degradation by natural sunlight (Thomas and Lara 1995).
The highly photo-reactive nature of DOM in deeper waters than in the surface layer appears to be caused by two phenomena. First, major fractions of DOM
in deeper waters belong to low molecular weight substances (Yoshioka et al. 2007),
which may be photolytically mineralized. Second, microbial assimilation or respiration of particulate organic matter (POM: ca. algae) can produce the autchthonous DOM, which are highly photosensitive and photodegradable (Mostofa K
et al., unpublished; Zhang et al. 2009; Johannessen et al. 2007; Zhang et al. 2009).
The experimental study suggests that the algal-derived DOM is photolytically decomposed by natural sunlight, which is a more efficient photoinduced substrate than is the
allochthonous DOM (Mostofa K et al., unpublished; Johannessen et al. 2007; Hulatt
et al. 2009). The autochthonous organic substances in deeper layers are typically