The decomposition process is important for understanding the
ecological role of marine DOM (Carlson, 2002), and a previous
study indicated that macroalgal DOM has a refractory property of
microbial decomposition in a dark incubation experiment (Wada
et al., 2008). In the present study, we evaluated another process of
DOM decomposition, photodecomposition, on the basis of changes
in DOC concentrations and spectral properties under artificial sunlight.
DOC concentrations decreased to 72%–77% of the initial
values, showing that a part of macroalgal DOM was directly mineralized.
Since DOMs from other marine organisms, such as phytoplankton
and bacteria, are not mineralized by sunlight, we
considered that macroalgal DOM has a relatively photo-labile
property. The spectral analysis suggested the generation of small
molecules with low aromaticity from original compounds of the
macroalgal DOM. On the basis of a previous study on the relationship
between aromatic ring content and microbial availability,
we believe that bacterial utilization of macroalgal DOM may be
activated by a decrease in aromaticity. Both results for DOC concentration
and spectral properties suggested that solar radiation
accelerates the decomposition of macroalgal DOM. Therefore, sunlight
exposure for macroalgal DOM will cause a decline in its ecological
functions of transport, sequestration, and reservation of organic
carbon and enhance the energy supply for microbial food
webs.