Solar UV has been shown to degrade dissolved organic carbon
(DOC) photolytically, most of the DOC being of terrestrial origin and relatively
resistant to other forces (Naganuma et al., 1996). Humic substances are
fairly resistant to bacterial degradation, but after photolytic activity
the products (e.g., formaldehyde, acetaldehyde, glyoxylate, and pyruvate)
are readily taken up by bacterioplankton (Wetzel et al., 1995). Humic substances
strongly absorb ultraviolet radiation. Thus, increased breakdown of DOC
and subsequent consumption by bacteria increases the UV-B penetration into
the water column. Close to the surface, solar UV radiation inhibits bacterioplankton
activity and rapidly photolyzes DOC. However, the uptake of the fragments
is hampered by the inhibition of the bacterial ectoenzymes. Only when both
bacteria and the photolyzed DOC circulate to deeper layers does the uptake
rate increase. Another aspect of DOC photolysis is the generation of photosensitizers,
which upon absorption of UV radiation produce reactive oxygen species or
free radicals. DMS is released from the water at a rate that is closely
correlated with the concentration of DOC (Herndl, 1997). DMSP is considered
an osmoregulator in phytoplankton.
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