Biology Reference
In-Depth Information
and 14% Gigartinales. The green CO
2
-using macroalgae are different species of
Caulerpa
and
Udotea petiolata
(Raven et al
2002b
;Kevekordesetal.
2006
; Vizzini
and Mazzola
2006
). The
13
C signature of the brown macroalga
Desmarestia anceps
is inconclusive with values ranging from
d
‰
(Fischer and Wiecke
1992
). Inconclusive values are also observed in six red
macroalgae,
Lomentaria articulata, Membranoptera alata, Odonthalia dentata,
Palmaria decipiens, Ptilota gunneri
,and
Trailliella intricata
(Fischer and Wiencke
1992; Kubler and Raven
1994
; Maberly et al.
1992
; Raven et al.
2002b
)andin
the green macroalga
Caulerpa obscura
(Raven et al.
2002b
). On the other side of
the spectrum,
25.3
‰
(Dunton
2001
)to
30.68
13
C signatures were observed in the green
Ulva
intestinalis
(
Enteromorpha intestinalis
), with values ranging from
inconclusive
d
8.81
to
‰
20.30
(Maberly et al.
1992
), and the browns
Adenocystis utricularis
(
8.81
‰
‰
to
20.30
, Dunton
2001
; Raven et al.
2002b
)and
Colpomenia peregrina
(
6.14
‰
‰
to
12.09
, Raven et al.
1995
,
2002b
).
‰
19.4 Calcification
The biogenic formation of calcium carbonate from Ca
2+
and CO
3
2
is termed
calcification. Three polymorphs of calcium carbonate are made by seaweeds:
calcite, aragonite, and high-magnesium calcite, and in the present-day surface
ocean, these are supersaturated so that biogenic precipitation by algae is “thermo-
dynamically favored” (Raven and Giordano
2009
). However, the lower pH
predicted for future oceans causes a reduction in the saturation state of the carbon-
ate ion, making it more difficult for calcifying organisms to calcify (Cao and
Caldeira
2008
). High-magnesium calcite, synthesized by coralline seaweeds, is
the most soluble polymorph, potentially making this group most susceptible to
OA. The site of calcification varies among seaweeds. For example, coralline
seaweeds deposit high magnesium calcite into their cell walls, the brown genus
Padina
deposits a fine layer of aragonite onto its cell surface, and for the tropical
genus
Halimeda
, aragonite mineralizes in the intercellular spaces between the
utricles (Stanley
2008
; Raven and Giordano
2009
; Nelson
2009
; Ries
2009
,
2010
).
There have been few studies (e.g., Borowitzka and Larkum
1976
;Borowitzka
1987
) on the underlying physiological mechanisms of calcification in seaweeds, but
the recent interest in OA has stimulated research in this important area (see Ries
2009
,
2010
). Photosynthesis (and primary production) and calcification are thought to
be intimately linked because rates of calcification are enhanced in the light, but the
mechanisms are not fully understood (Ries
2009
,
2010
). Also affecting calcification
rate is the ratio of magnesium to calcite (Mg/Ca ratio) and the carbonate saturation
state, both of which are influenced by pH (Ries
2009
,
2010
). Over geological
timescales, the world's ocean has switched between “aragonite seas” (today's
ocean) and “calcite seas”. When the green seaweeds
Halimeda
,
Penicillus
,and
Udotea
were grown in seawater with a Mg/Ca ratio
2 (aragonite sea), rates of
growth, calcification, and primary production were greater than when grown in
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