Biology Reference
In-Depth Information
results. First of all, both chromerids lack chlorophyll
c
in their plastids,
the pigment hallmark of the red-derived secondary plastids (
Moore
et al., 2008; Oborn´k et al., 2012
). So far, only a single stramenopile group
named Eustigmatophyceae has been shown to lack this pigment (
Sukenik
et al., 1992
). Moreover, in a single chromophyte species (
Xanthonema debile
)
belonging to Xantophyceae, chlorophyll
c
is also absent (
Gardian et al.,
2011
). An absence of this particular pigment from not only the chromerid
algae but also from other independent lineages may suggest that in the
evolution of the SAR group, chlorophyll
c
has been lost multiple times.
However, since the pigment composition of
V. brassicaformis
is virtually
identical to that found in the eustigmatophyte
Nannochloropsis lineata
(
Oborn
´
k et al., 2012
), the scenario postulating the origin of the chromerid
plastid in tertiary endosymbiosis involving an eustigmatophyte alga cannot
be fully rejected. Since the tertiary red plastids of dinoflagellates, the second
group of photosynthetic alveolates, are bound by a four-membrane
envelope, the number of membranes in chromerid and apicomplexan plastid
supports their possible tertiary origin as well. Such a scenario can be
preferred also by the unusual structure of the
C. velia
plastid genome (see
below;
Janouˇkovec et al., 2010
), which resembles that of the tertiary plastid
of the dinoflagellate
Karlodinium veneficum
(
Gabrielsen et al., 2011
). On the
other hand, the presence of canonical highly conserved and compact plastid
genome in
V. brassicaformis
makes this evolutionary pathway much less likely
(
Oborn´k et al., 2012
). Although both chromerid species lack chlorophyll
c
,
the composition of other pigments is again different. While in addition to
chlorophyll
a
, violaxanthin, and
b
-carotene, the
C. velia
plastid also contains
novel isoform of isofucoxanthin (
Moore et al., 2008
),
V. brassicaformis
is
instead pigmented by
the
carotenoid vaucheriaxanthin (
Oborn´k
et al., 2012
).
Together with unusual pigmentation of its plastid,
C. velia
has other spe-
cial characters associated with its photosynthetic activity, which was mea-
sured either under continuous low light (15 mol m
2
s
1
) or high light
(200 mol m
2
s
1
). The alga was able to acclimatize to the particular light
conditions; under the low-light regime, it contains more C and N, shows
higher chlorophyll
a
quotas and the connectivity of photosystem II is
increasing. In contrast, when exposed to the high-light regime, the alga pro-
duced more photoprotective carotenoids, violaxanthin and isofucoxanthin.
This adjustment was accompanied by a significant decrease of chlorophyll
content and consequently led to the increase of the carotenoid:chlorophyll
ratio (
Quigg et al., 2012
). The ability to adjust to different light conditions
Search WWH ::
Custom Search