Photosynthesis in Nature: A New Look - Photobiogeochemistry of Organic Matter - page 564

produced through photosynthesis. (iv) Plant litter materials or biomass, developed
initially through photosynthesis, represent the largest pool of terrestrial carbon. It
is currently estimated at approximately 1500-2000 Pg of C that are stored in the
world's soils (Schlesinger 1997 ; CAST 2004 ). Upon microbial processing, this
material can produce soil organic matter or allochthonous dissolved organic mat-
ter (DOM), including humic substances (fulvic and humic acids) and inorganic
components such as nutrients and various elements (see also chapter “ Dissolved
Organic Matter in Natural Waters ”) (Mostofa et al. 2009 ; Nakane et al. 1997 ;
Uchida et al. 2000 ; Kögel-Knabner 2002 ; Grandy and Neff 2008 ; Moore et al.
2008 ; Braakhekke et al. 2011 ; Spence et al. 2011 ; Tu et al. 2011 ). These chemical
components are ultimately released into the water ecosystem and undergo photoin-
duced and microbial degradation. Their end-products are CO 2 , H 2 O 2 and dissolved
inorganic carbon (DIC: generally defined as dissolved CO 2 , H 2 CO 3 , HCO 3

−

,
and CO 3 2 − ), which can fuel/accelerate the primary production (see also chapter
“ Photoinduced and Microbial Degradation of Dissolved Organic Matter in Natural
Waters ” and “ Impacts of Global Warming on Biogeochemical Cycles in Natural
Waters ”) (Mostofa et al. 2009 ; Jones 1992 ; Hessen and Tranvik 1998 ; Jansson et
al. 2000 ; Meili et al. 2000 ; Grey et al. 2001 ; Hernes and Benner 2003 ; Tranvik
et al. 2009 ; Ballaré et al. 2011 ; Zepp et al. 2011 ). (v) Photosynthesis is the key
process for primary and secondary production and uses natural sunlight in aquatic
ecosystems. Aquatic microorganisms that are key components of the Earth's bio-
sphere can produce more than 50 % of the biomass of our planet through photosyn-
thesis, using allochthonous DOM and nutrients. Therefore, aquatic environments
can incorporate at least the same amount of atmospheric carbon dioxide (CO 2 ) as
terrestrial ecosystems (de Haan 1974 , 1977 ; Tranvik 1988 ; Häder et al. 2003 ; Zepp
et al. 2007 ). Life is mostly composed of the elements carbon, hydrogen, nitrogen,
oxygen, sulfur and phosphorus, which make up nucleic acids (e.g. DNA), pro-
teins and lipids and can thus form the bulk of living matter (Wolfe-Simon et al.
2011 ). (vi) Aquatic microorganisms (e.g. algae or phytoplankton cells) can produce
autochthonous DOM, including autochthonous fulvic acids, CO 2 and nutrients
under both photoinduced and microbial respiration or assimilation (see also chap-
ters “ Dissolved Organic Matter in Natural Waters ” , “ Photoinduced and Microbial
Degradation of Dissolved Organic Matter in Natural Waters ”, “ Fluorescent
Dissolved Organic Matter in Natural Waters ”, and “ Impacts of Global Warming on
Biogeochemical Cycles in Natural Waters ”) (Mostofa et al. 2009 ; Mostofa et al.
2009 ; Zhang et al. 2009 ; Tranvik et al. 2009 ; Biddanda and Benner 1997 ; Gobler et
al. 1997 ; Kritzberg et al. 2006 ; Mostofa et al. 2011 ). These compounds can be used
by aquatic microorganisms for their further photosynthetic activity and can, there-
fore, promote the primary production (see also chapters “ Dissolved Organic Matter
in Natural Waters ” , and “ Impacts of Global Warming on Biogeochemical Cycles in
Natural Waters ” ) (Hessen and Tranvik 1998 ; Cole et al. 1982 ). (vii) Photosynthesis
is the dominant energy mobilization process for secondary production in natural
waters, where organic carbon fixed by primary producers is consumed directly
by grazing or is recycled via the microbial loop (Wetzel 2001 ). (viii) The primary
producers in freshwater and marine ecosystems can constitute the basis of the