Iodine in Seaweed: Two Centuries of Research 23.0
593
Part D | 23
Sea water
Atmosphere
Particle information
IO•
Iodine
uptake
V-haloperoxidase
I
I•
I
–
I
–
H 2 O 2 /ROS
I 2
I
–
I
–
I
–
O 3
hv
Fig. 23.1 Known pathways of iodine between algal metabolism and the environment in the brown alga Laminaria digitata, the strongest accumulator of this element among all living systems. Most of the time, kelp forests are covered by
water (left). In this largely unstressed steady state (turquoise), iodide from sea water is taken up by a mechanism driven
by low micromolar levels of hydrogen peroxide and vanadium haloperoxidases [23.26] and stored in the apoplast of cortical cells [23.27]. Iodide is the accumulated species. Upon oxidative stress (red) such as an oxidative burst [23.28], this
accumulated iodide is mobilized and scavenges hydrogen peroxide by a pseudocatalase-like reaction sequence known
as iodide-(halide-)assisted disproportionation of hydrogen peroxide (iodide acts as catalyst, i. e., it is regenerated at the
end of a reaction cycle – there is no accumulation of oxidized or organic iodine species [23.29]). When kelp forests are
partially exposed (right), e.g., during spring tides, significant oxidative stress from various causes can occur, including
both atmospheric oxidants such as ozone but also hydrogen peroxide from the photosynthetic Mehler reaction accumulating in the liquid microlayer on the wet algal surface. Ozone reacts directly with iodide, resulting in the release of
volatile, molecular iodine (I 2 ) at flux rates so high that it can easily be detected in the marine boundary layer of the atmosphere photometrically, using starch paper or even by the human nose. Molecular iodine becomes photolyzed rapidly and
rapidly reacts further with ozone, forming ultrafine particles consisting of iodine oxides. These are highly hygroscopic
and thus provide cloud condensation nuclei [23.30]. In contrast to ozone, hydrogen peroxide from the Mehler reaction is
scavenged by iodide-assisted disproportionation as described above
general. This work also established the involvement of
vanadium haloperoxidase in iodine uptake: Only the
macroscopic Laminaria sporophytes take up significant
levels of iodine, the haploid, filamentous gametophytes
do not. The former have high levels of haloperoxidases, the latter do not. However, iodine uptake can be
induced in gametophytes upon addition of exogenous
H 2 O 2 and haloperoxidase. Subsequently, the Roscoff
group identified Laminaria iodoperoxidase, a novel
subclass of the vanadium haloperoxidases [23.33, 34]
which may explain the selectivity for iodide uptake in
Laminaria.
We joined forces with the Norwich group to investigate iodocarbon emission rates from a range of
ecologically relevant seaweed species from the European Atlantic cost [23.35]. These studies went in
parallel with a survey of natural Laminaria populations
on different parts of the European coast. Oral traditions of Breton seaweed gatherers (goémoniers) had
suggested that kelp contained more iodine in the winter than in the summer, and also that seaweeds from
northerly locations in Europe had more of the precious
commodity than in the south (Kloareg, personal communication). Interestingly, this could be confirmed; at
about 1% dry weight total iodine, fully-grown Laminaria thalli contain about twice as much iodine in the
winter as in the summer [23.36]. Recently, Verhaeghe
and the Roscoff group demonstrated that iodine accumulation occurs in the apoplast of cortical Laminaria
cells [23.27].
Ultimately, we turned our attention to what we considered the core question, which had remained hitherto
unanswered, despite almost two centuries of research
on iodine in seaweeds: why do these algae actually
593
Part D | 23
Sea water
Atmosphere
Particle information
IO•
Iodine
uptake
V-haloperoxidase
I
I•
I
–
I
–
H 2 O 2 /ROS
I 2
I
–
I
–
I
–
O 3
hv
Fig. 23.1 Known pathways of iodine between algal metabolism and the environment in the brown alga Laminaria digitata, the strongest accumulator of this element among all living systems. Most of the time, kelp forests are covered by
water (left). In this largely unstressed steady state (turquoise), iodide from sea water is taken up by a mechanism driven
by low micromolar levels of hydrogen peroxide and vanadium haloperoxidases [23.26] and stored in the apoplast of cortical cells [23.27]. Iodide is the accumulated species. Upon oxidative stress (red) such as an oxidative burst [23.28], this
accumulated iodide is mobilized and scavenges hydrogen peroxide by a pseudocatalase-like reaction sequence known
as iodide-(halide-)assisted disproportionation of hydrogen peroxide (iodide acts as catalyst, i. e., it is regenerated at the
end of a reaction cycle – there is no accumulation of oxidized or organic iodine species [23.29]). When kelp forests are
partially exposed (right), e.g., during spring tides, significant oxidative stress from various causes can occur, including
both atmospheric oxidants such as ozone but also hydrogen peroxide from the photosynthetic Mehler reaction accumulating in the liquid microlayer on the wet algal surface. Ozone reacts directly with iodide, resulting in the release of
volatile, molecular iodine (I 2 ) at flux rates so high that it can easily be detected in the marine boundary layer of the atmosphere photometrically, using starch paper or even by the human nose. Molecular iodine becomes photolyzed rapidly and
rapidly reacts further with ozone, forming ultrafine particles consisting of iodine oxides. These are highly hygroscopic
and thus provide cloud condensation nuclei [23.30]. In contrast to ozone, hydrogen peroxide from the Mehler reaction is
scavenged by iodide-assisted disproportionation as described above
general. This work also established the involvement of
vanadium haloperoxidase in iodine uptake: Only the
macroscopic Laminaria sporophytes take up significant
levels of iodine, the haploid, filamentous gametophytes
do not. The former have high levels of haloperoxidases, the latter do not. However, iodine uptake can be
induced in gametophytes upon addition of exogenous
H 2 O 2 and haloperoxidase. Subsequently, the Roscoff
group identified Laminaria iodoperoxidase, a novel
subclass of the vanadium haloperoxidases [23.33, 34]
which may explain the selectivity for iodide uptake in
Laminaria.
We joined forces with the Norwich group to investigate iodocarbon emission rates from a range of
ecologically relevant seaweed species from the European Atlantic cost [23.35]. These studies went in
parallel with a survey of natural Laminaria populations
on different parts of the European coast. Oral traditions of Breton seaweed gatherers (goémoniers) had
suggested that kelp contained more iodine in the winter than in the summer, and also that seaweeds from
northerly locations in Europe had more of the precious
commodity than in the south (Kloareg, personal communication). Interestingly, this could be confirmed; at
about 1% dry weight total iodine, fully-grown Laminaria thalli contain about twice as much iodine in the
winter as in the summer [23.36]. Recently, Verhaeghe
and the Roscoff group demonstrated that iodine accumulation occurs in the apoplast of cortical Laminaria
cells [23.27].
Ultimately, we turned our attention to what we considered the core question, which had remained hitherto
unanswered, despite almost two centuries of research
on iodine in seaweeds: why do these algae actually