Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 277

emission, atmospheric chemistry, and cloud formation was suggested to constitute a
feedback loop which could affect climate via regulation of the amount of DMSP
produced by phytoplankton and thence DMS emissions [ 9 ]. Recent modelling of
the effect of DMS emissions into the atmosphere, however, suggests that the overall
effect of DMS on climate may not be as big as previously thought, in comparison to
other marine aerosol generation, e.g., from complex organic matter [ 10 ].
The main source of DMS emitted to the atmosphere from the oceans is DMSP, a
sulfonium compound that is thought to act as a compatible solute, antioxidant,
cryoprotectant and/or metabolic overflow mechanism [ 11 - 13 ] in diverse marine
algae and phytoplankton [ 14 ] (Figure 1 ). Some 10 9 t of DMSP are thought to be
produced in the oceans per annum. Upon release of DMSP into the water column, it
is subject to microbial degradation that leads to about 30 % of the dissolved DMSP
pool to be degraded to DMS by enzymes referred to as DMSP-lyases. Recent
investigation of the genes and enzymes responsible for DMS production from
DMSP have demonstrated that a wide range of different enzymes that belong to
entirely different protein families are able to produce DMS in a DMSP-dependent
manner (reviewed in [ 14 ]).
Typical concentrations of DMS in surface seawater are in the range of 1-7 nM
[ 15 ]. Microbial utilization of DMS as carbon and/or energy source leads to degrada-
tion of the majority of DMS produced from DMSP. Degradation of DMS by
methylotrophic microorganisms leads to assimilation into biomass and degradation
of DMS to CO 2 and inorganic sulfur compounds including sulfate [ 16 ], thiosulfate
[ 17 ], and tetrathionate [ 18 ]. DMS can also be utilized as an auxiliary energy source by
heterotrophic bacteria [ 19 , 20 ]. Utilization of DMS as a sulfur source has been
documented in soil bacteria [ 21 ] and in a strain of Marinobacter in which DMS
assimilation was dependent on light [ 22 ]. Only a fraction of the DMS produced from
DMSP in the oceans is emitted into the atmosphere, while the remainder is subject to
microbial degradation as outlined above [ 23 ].
While DMSP degradation is the main source of DMS in oxic marine environ-
ments, other pathways of DMS formation have been identified as sources of DMS in
anoxic environments. The underlying processes include (i) methylation of sulfide
and methanethiol (MT) [ 24 ], (ii) anaerobic degradation of sulfur containing amino
acids [ 25 , 26 ], and (iii) reduction of DMSO in anaerobic respiration [ 27 ].
Measurements of volatile organic sulfur compounds in anoxic freshwater
sediments and overlying water columns detected MT and DMS concentrations in
the nanomolar range (up to 76 nM for MT; up to 44 nM for DMS) [ 24 ]. It is thought
that in these environments, DMS production is linked to the degradation of complex
organic matter such as lignin, one of the most abundant pools of carbon in the
biosphere, which generates methoxylated aromatic compounds that may be
o -demethylated under anoxic conditions by anaerobic bacteria through methylation
of sulfide [ 28 - 30 ].
Another process responsible for DMS production in anoxic environments is the
respiratory reduction of DMSO [ 31 ], the enzymology of which is discussed in detail
below.