Geoscience Reference
In-Depth Information
CHAPTER 11
Frances Hopkins, Philip Nightingale, and Peter Liss
11.1 Introduction
11.1.1 DMS
The oceans are a major source of sulphur (S), an ele-
ment essential to all life, and marine emissions of
the gas DMS (chemical formula (CH
3
)
2
S) represent a
key pathway in the global biogeochemical sulphur
cycle. The surface oceans are supersaturated with
DMS relative to the atmosphere, resulting in a one-
way l ux from sea to air (Lovelock
et al.
1972 ; Watson
and Liss 1998 ).
DMS is a breakdown product of the biogenically
produced dimethyl sulphoniopropionate (DMSP):
A wide range of trace gases, including dimethyl
sulphide (DMS) and organohalogens, are formed
in the surface oceans via biological and/or photo-
chemical processes. Consequently, these gases
become supersaturated in seawater relative to the
overlying marine air, leading to a net l ux to the
atmosphere. Upon entering the atmosphere, they
are subject to rapid oxidation or radical attack to
produce highly reactive radical species which are
involved in a number of important atmospheric
and climatic processes. Organohalogens can affect
the oxidizing capacity of the atmosphere by inter-
acting with ozone, with implications for air qual-
ity, stratospheric ozone levels, and global radiative
forcing. DMS and iodine-containing organohalo-
gens (iodocarbons) can both contribute to direct
and indirect impacts of aerosols on climate
through the production of new particles and cloud
condensation nuclei (CCN) in the clean marine
atmosphere. Therefore, marine trace gases are
considered a vital component of the earth's cli-
mate system, and changes in the net production
rate and subsequent sea-to-air l ux could have an
impact on globally important processes. In recent
years, attention has turned to the impact that
future ocean acidii cation may have on the pro-
duction of such gases, with the greatest focus on
DMS and organohalogens. In this chapter, the cur-
rent state-of-the-art in this growing area of
research is outlined.
+
−
(CH)SCHCHCOO
(CH ) S
→
32
2
2
(11.1)
+
CH CHCOOH
(acrylic acid)
32
2
Single-celled marine phytoplankton are the chief
producers of DMSP, and this reaction is catalysed
intra- and extracellularly by the enzyme DMSP-lyase
( Malin
et al.
1992 ; Liss
et al.
1997 ). The capacity of
phytoplankton to produce DMSP varies between
species, with prymnesiophytes considered to be the
most prolii c (Malin
et al.
1992 ; Liss
et al.
1997 ; Watson
and Liss 1998). The production of DMSP is thought
to fuli l a number of roles within the algal cell, includ-
ing osmoregulation (Vairavamurthy
et al.
1985 ), cry-
oprotection (Malin
et al.
1992 ; Liss
et al.
1997 ), and
protection against grazing (Wolfe
et al.
1997 ) and oxi-
dative stress (Sunda
et al.
2002 ). Production is often
associated with senescence, grazing, and viral lysis,
when cells begin to break down enabling DMSP to
210
