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(A)
30
(B)
2
Methyl iodide CH
3
I
Ethyl iodide C
2
H
5
I
25
1.6
20
1.2
15
0.8
10
0.4
5
0
0
2468 0 2 4 6 8 0 2 4
Date in May 2006
2468 0 2 4 6 8 0 2 4
Date in May 2006
(C)
(D)
800
700
Diiodomethane CH
2
I
2
Chloroiodomethane CH
2
ClI
700
600
600
500
500
400
400
300
300
200
200
100
100
0
0
2468 0 2 4 6 8 0 2 4
2468 0 2 4 6 8 0 2 4
Date in May 2006
Date in May 2006
M1
M2
M3
M4
M5
M6
Figure 11.4.
Concentrations of methyl iodide (CH
3
I) (A), ethyl iodide (C
2
H
5
I) (B), diiodomethane (CH
2
I
2
) (C) and chloroiodomethane (CH
2
ClI) (D) during
nutrient-induced blooms of phytoplankton in mesocosm enclosures in Raunefjord, Bergen, Norway, in May 2006. The nominal CO
2
treatments at
the beginning of the experiment were as follows (M = mesocosm): ~750 μatm in M1, M2 and M3; ~300 μatm in M4, M5 and M6. Data from
Hopkins et al . ( 2010 ).
CH
3
I is often referred to as 'biogenic' as it is
thought to be produced both directly by macroalgae,
phytoplankton, and bacteria, and indirectly through
photochemical reactions with organic matter (Moore
and Zai riou 1994 ; Archer
et al.
2007 ). Less informa-
tion is available on C
2
H
5
I; however, a number of
studies, including Hopkins
et al.
( 2010 ), have found
signii cant correlations between C
2
H
5
I and CH
3
I,
suggesting similar production and removal mecha-
nisms (see also Makela
et al.
2002 ; Richter and
Wallace 2004 ; Archer
et al.
2007 ).
CH
2
I
2
is considered to have a primarily biogenic
source (Moore and Zai riou 1994). It is subject to
rapid photolysis in surface seawater (photolytic
lifetime of around 12 min), with strong evidence
that this reaction is an important source of CH
2
ClI
( Martino
et al.
2005). In the productive mesocosm
environment described by Hopkins
et al
. ( 2010 ), the
processes controlling net iodocarbon production
were susceptible to lowered pH, and were likely to
be a consequence of plankton community shifts in
response to elevated
p
CO
2
conditions. The authors
reported that small picoeukaryotes, cryptophytes,
and
Synechococcus
were all signii cantly lower
under high CO
2
, and similarly to the iodocarbons,
the differences between treatments were most pro-
nounced during the post-bloom phase. If such
organisms are involved in iodocarbon production
or consumption, changes in their abundance may
have had a direct impact on seawater concentra-
tions during this experiment. Further evidence of a
reduction in iodocarbon production in response to
