Biology Reference
In-Depth Information
7. CHROMERIDS AS POSSIBLE SYMBIONTS OF CORALS
It is well known that dinoflagellates of the genus
Symbiodinium
form
symbiotic relationships with corals, and this relation appears to be essential
for the survival of coral reefs (
Bourne et al., 2009
). Such symbiosis is well
described and many species of
Symbiodinium
have been isolated so far. In fact,
chromerids were initially obtained as a kind of by-product of a study on sym-
biotic dinoflagellates in Australian stony corals, in particular,
C. velia
from
Plesiastrea versipora
and
V. brassicaformis
from
Leptastrea purpurea
(
Moore
et al., 2008; Oborn´k et al., 2012
). In order to visualize
C. velia
within corals,
a highly specific fluorescence
in situ
hybridization protocol has been recently
developed (
Morin-Adeline et al., 2012
). However, serious doubts have been
expressed concerning the putative symbiotic nature of these algae, since a
vast majority of chromerids have been detected as an epiphytic algae grow-
ing on the surface of corals (
Janouˇkovec et al., 2012a,b
).
Consequently, a new group of so far uncultivated alveolates possessing
plastids has been found, based on the analysis of environmental samples from
corals and their close surroundings (
Janouˇkovec et al., 2012a
). The fact that
most of these alveolates were detected in depths not exceeding 5 m indicates
their possible photosynthetic nature. However, recent histological investi-
gations of larvae of the reef coral
Acropora digifera
and
A. tenuis
by use of
C. velia
-specific probe showed their ability to form a symbiotic relationship
with coral larvae (
Cumbo et al., 2013
). This may suggest
C. velia
as a fac-
ultative symbiont of corals, because a substantial fraction of chromerid algae
lives epiphytically on the coral surface. The latest analysis of chromerid-
related sequences found in the genomic databases identified a novel clade
of chromerids, termed ARL-V, which is likely associated with more than
a dozen symbiotic corals (
Janouˇkovec et al., 2012b
). Indeed, these authors
even observed an enrichment for these sequences in healthy coral tissues, a
finding with potentially very important environmental consequences. It can
be concluded that while some chromerid-related sequences were isolated
from marine environments lacking corals, the association of at least some
chromerids with coral reefs is stronger than ever.
8. CONCLUSIONS
Chromerid algae,
C. velia
and
V. brassicaformis
, isolated from corals in
Australia, are the closest known phototrophic relatives of the apicomplexan
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