Geoscience Reference
In-Depth Information
Several perturbation experiments performed in
mesocosms investigated the effect of ocean acidii -
cation on enzymatic activity. Grossart
et al.
( 2006 )
found higher enzyme (
a
- and
b
-glucosidase and
protease) activity in prokaryotes at elevated
p
CO
2
,
with signii cant differences only found for protease
activity. Piontek
et al.
(2010) reported higher rates of
extracellular glucosidase at higher
p
CO
2
, which also
resulted in higher rates of polysaccharide degrada-
tion. This might result in an enhanced processing of
organic matter at higher
p
CO
2
. However, Tanaka
et al.
(2008) reported that there was no effect of
elevated
p
CO
2
on glucose afi nity. In a mesocosm
experiment, there was no effect of a
p
CO
2
level of
1050 μatm on the uptake of phosphate by bacteria,
whereas the uptake of phosphate was stimulated in
the >10 μm fraction (Tanaka
et al.
2008 ). A recent
study using a buffer to adjust pH showed evidence
for a negative effect of elevated
p
CO
2
on enzyme
activity, with a rapid decrease of some enzyme
activities, such as leucine aminopeptidase or lipase,
for a decrease of pH
NBS
from 8.2 to 7.8 (Yamada and
Suzumura 2010). Thus, elevated
p
CO
2
levels seem
to inl uence the activity of some enzymes.
either higher lysis rates of cells, which can be
infected and lysed by this type of virus, or higher
viral production rates of these specii c viruses at
elevated
p
CO
2
levels (e.g. by an increased number
of viruses released per cell).
A single study reported a decrease in the abun-
dance of the
E. huxleyi
virus EhV at elevated (700
and 1050 compared to 350 μatm)
p
CO
2
( Larsen
et al.
2008), but there was no signii cant difference in the
abundance of
E. huxleyi
across treatments (see
Paulino
et al.
2008). This could suggest that a poten-
tially negative effect of elevated
p
CO
2
on the host
was counterbalanced by a negative effect on viral
infection and lysis, thus reducing mortality.
It is known that metabolically active cells are
often infected at higher rates, for example due to a
greater number of receptors on the cell surface
(Weinbauer 2004). Thus, if ocean acidii cation
reduces the metabolism of the host, this could lead
to a decrease in the infection rate. Also, metaboli-
cally active cells often produce more progeny, i.e.
they have a higher burst size (number of viruses
released upon cell lysis), than less active ones
( Parada
et al.
2006). In contrast, one could argue that
a weakening of the defence system by ocean acidii -
cation could increase viral infection as happens in
metazoans. However, there seems to be no evidence
for that in microorganisms.
Viruses, i.e. viral DNA, can also remain within
the host cells, usually integrated as provirus or
prophage into the host DNA (Weinbauer 2004). This
virus-host relationship is called lysogeny for
prokaryotes and latency for eukaryotes. In such
lysogenic cells and cells with latent infections, the
viral DNA replicates along with the host. This
occurs until either so-called prophage curing occurs
spontaneously, which excludes the viral DNA, or
until a factor induces the lytic cycle in the viral
DNA, which results in the formation of viral parti-
cles, the lysis of the cell, and the release of the viral
progeny. Inducing agents can be stress factors such
as DNA damage. It is unknown whether changes in
p
CO
2
levels can act as an inducing agent.
The effect of elevated
p
CO
2
on grazing by micro-
zooplankton (including ciliates) on phytoplankton
such as cyanobacteria has been investigated in
only two studies. One experiment reported higher
prey abundances and grazing rates in an on-board
5.3.5
Viral lysis and grazing
The lysis of cells by viruses as well as the ingestion
and digestion of cells by protistan predators are the
two main factors involved in the mortality of
prokaryotes (see Section 5.2.2). To the best of our
knowledge, no information is available on the effect
of ocean acidii cation on rates of viral lysis. Thus,
the effects of ocean acidii cation on viral lysis can
only be inferred from changes in viral abundance.
In a mesocosm experiment, total viral abundance
did not change between the three
p
CO
2
levels that
were investigated (190, 414, and 714 μatm; Rochelle-
Newall
et al.
2004). In a subsequent study, viral
abundance was followed by l ow cytometry in
mesocosms subject to
p
CO
2
levels of 350, 700, and
1050 μatm (Larsen
et al.
2008 ). Viral abundance and
the abundance of three viral groups detected by
l ow cytometry did not show any signii cant change
between
p
CO
2
levels, whereas for the 'high-
l uorescence viruses' maximum abundances were
higher at a
p
CO
2
of 700 and 1050 μatm than at 350
μatm. Thus, the i ndings published so far suggest
