Geoscience Reference
In-Depth Information
there is the possibility that a rise in
p
CO
2
will affect
ecosystem functions and biogeochemical cycles by
altering the relative abundance of species.
Alternatively, changes in the bacterial community
composition of species with similar functions could
be a mechanism of adaptation. The net outcome of
ocean acidii cation will depend on whether rare spe-
cies will serve as a seed bank of species or whether
they are the target of local extinctions.
It is known that the degradation of organic mat-
ter by bacteria in diatom frustules releases silica
and could thus control silica regeneration and dia-
tom production (Nagata 2008). This mechanism
could be enhanced by the increased bacterial pro-
duction in a high-CO
2
ocean. In marine snow, there
are also inorganic particles, e.g. calcii ed cell parts
of phytoplankton (Nagata 2008). Since ocean acidi-
i cation enhances the dissolution of CaCO
3
(see
Chapter 3), it is also likely that the access of
prokaryotes to the organic matter matrix should be
facilitated and result in faster degradation rates,
and a decrease of export to the deep sea.
One of the few relatively consistent i ndings is
that TEP formation is often stimulated at higher
p
CO
2
levels (Table 5.1). One can therefore speculate
that ocean acidii cation directly increases TEP for-
mation (or changes its spatial structure). It has been
argued that this will increase the downward l ux of
organic matter (Riebesell
et al.
2007 ; Schulz
et al.
2008). The reason for postulating this process is
inferred from the imbalance between the build-up
of organic matter and the drawdown of inorganic
nutrients. Consequently, the biological pump and
thus carbon export to the deep sea could be
enhanced. The pathways of this potential scenario
are shown by hatched arrows in Fig. 5.2. This could
also stimulate remineralization in the dark ocean
and increase the occurrence of oxygen minimum
zones (Riebesell
et al.
2007 ).
An additional and not exclusive scenario is pos-
sible based on the i nding of increased TEP forma-
tion (Table 5.1). One can also speculate that this
could stimulate bacterial production and enzymatic
activity, since it is well known that aggregation
stimulates these processes (Simon
et al.
2002 ).
Indeed, in ocean acidii cation experiments only
neutral and positive effects on bacterial production
and enzyme activity were found whereas no nega-
tive effects were reported (Table 5.1). Particles are
hot spots of microbial activity and biogeochemical
transformation (Azam and Long 2001; Kiørboe and
Jackson 2001 ; Azam and Malfatti 2007 ). Ocean
acidii cation could therefore enhance this transfor-
mation and lead to a faster recycling of elements
and increased respiration. If this occurs, then the
export of carbon by the biological pump or the
microbial carbon pump could be reduced. Mari
(2008) has shown that the buoyancy of aggregates
may increase at lower pH due to lowering of the
sticking properties of TEP (i.e. a lower ability to
aggregate dense particles and, thus, to form fast-
sinking aggregates) and concluded that marine
aggregates may ascend to the sea surface under
elevated
p
CO
2
. This could reduce the efi ciency of
the biological pump by partly inverting the carbon
l ow to the deep ocean. Since a signii cant number
of bacteria, viruses, and protists are attached to TEP
( Simon
et al.
2002 ; Weinbauer
et al.
2009 ), ocean
acidii cation might play a role as a microbial eleva-
tor. In the surface microlayer, TEP and associated
microorganisms could become even more concen-
trated and eventually be aerosolized (Wurl and
Holmes 2008). As a result, one can propose a scheme
in which ocean acidii cation will increase the
ascent of organic material and aggregate-attached
microorganisms. The pathways of this potential
scenario are shown by open arrows in Fig. 5.2. This
could enhance remineralization in the euphotic
layer (the surface ocean). Since microorganisms, cell
debris, and small particles are transported by spray
from the surface microlayer into the atmosphere
( Kuznetsova
et al.
2005) and can serve as nuclei for
cloud formation (Leck and Bigg 2005), there could
be an impact on climate. Both scenarios are likely to
operate in parallel; the net outcome of ocean
acidii cation, i.e. whether it primes or short-circuits
the biological pump, remains unknown.
Ocean acidii cation reduces the calcii cation rate
of many coral species (see Chapter 7). This could be
even more important for deep-water coral commu-
nities, which can be found at depths close to the
aragonite saturation level, which will become
increasingly shallow due to ocean acidii cation (see
Chapter 3). Such a decrease in skeleton formation
rate could have other consequences which have not
been investigated yet. According to the holobiont
