Discussion
We found a complex of haplotypes belonging to the endolytic algaeOstreobium dispersed across hosts and geographical regions. The
results showed an organism that has been on earth for 500 Mya and that
is distributed in more than 50 taxonomic units, agreeing with what was
proposed by (Marcelino y Verbruggen 2016) who had similar unexpected
results in other green algae phylogenies. Ostreobium for some
years was thought to be green algae with little species diversity and
whose main function was to erode corals. An intriguing result of the
study was the diversity of Ostreobium lineages among the several
coral host species. Previous findings (del Campo et al., 2017;
Gonzalez-Zapata et al., 2018; Gutner-Hoch & Fine, 2011) predicted that
the analysis would produce a result with a number less than or equal to
12 lineages. However, the results of GMYC grouped 46 categories and
those of mPTP 56, including 18 groups in common; many of the categories
were represented by individual items. In fact, the group with the
highest number of sequences was number 3 in figure 2A, which is where
the non-coral substrates are grouped. The research based on the
sequences of three genes (16s, rbcL, and 18S: del Campo et al. 2017)
indicated the greatest variety of Ostreobium in the Atlantic but
three main clades worldwide. Our findings corroborate these findings,
which depicts polyphyletic groups of Ostreobium that originate
from various hosts and geographical locations. The sequences from the
Atlantic locality are consistent with sequences from the Red Sea, the
Pacific, and the Indo-Pacific.
It is not surprising that haplotype grouping analyses show a pattern of
groups that include various regions and Ostreobium hosts because
of the widespread distribution of endolytic algae and the basal origin
(480Ma) in the Bryopsidales phylogeny previously proposed by Jackson et
al. (2018) and Verbruggen et al. (2009). In fact, it is long enough to
go through thousands of biological pressures, making host species not a
fundamental factor for their diversification. Microendolytic species
employ every calcareous substrate accessible for colonization, including
calcite crystals, foraminifera, bivalve and brachiopod shells, and coral
skeletons, at least in recent geological times. Only its abundance
varies in different types of substrates, as observed and measured in
modern samples (Vogel y Brett 2009). On the other hand, the algae went
through an invertebrate diversification event that happened in the
Ordovician (Aberhan, Nürnberg, y Kiessling 2012) and that according to
the fossil record shows how the amount of bioeroders increased
considerably, at least considering the number of tunnels per substrate
area, there is an increase through the Ordovician (reaching a peak in
the Late Ordovician to the Early Silurian (Wilson y Palmer 2006), which
favors the diversification of the species by expanding the number of
available niches. This radiation event also promoted a greater intensity
of symbiotic interactions, both skeletal carbonate and life within the
host could have been advantageous, since the new life sites also offered
protection against herbivores, against environmental variations such as
temperature, salinity, etc. physical stress or desiccation during
intertidal periods. Despite this, it seems that endolytic algae only
occupy those environments in which they could best survive and gradually
their way of life within almost any element with calcium carbonate made
them fundamental in the ecosystem for hundreds of millions of years.
The results also provide evidence of multiple Ostreobium lineages
that survived the Permian mass extinction and diversified after the
Triassic (Fig. 2), along with the emergence of scleractinian reefs
(Stanley 2003). Later it also diversified in the Mesozoic, especially
among the sequenced species of the Atlantic, probably associated with
phenomena such as temperature changes associated with
CO2 concentrations that considerably changed sea level.
We can speculate about the Ostreobium endurance at current
temperature regimes and its function as a support for coral when its
main symbiont, Symbiodinum , during bleaching events. Finally, we
see during the Cretaceous (66Mya) a diversification of lineages
associated with the Pacific, which contrasts with what is found in
corals where there is a diversification of Atlantic species during the
Cenozoic (Johnson y Kauffman 2001). Despite the significance of the
symbiotic relationship between the algae and coral for both organisms,
Ostreobium spp. and some coral species have no particular relationship,
so it is not surprising that their evolutionary events—radiation or
extinction—do not exactly coincide. Ostreobium encountered
adaptation challenges because of the coral skeleton’s low light
exposure, daily pH and oxygen level variations brought on by
photosynthesis, and holobiont respiration (Tribollet et al. 2009). The
niche inside the coral also varies greatly from species to species,
which suggests that the groups identified by the analyses were not
evenly distributed among their hosts or areas. In fact, as suggested in
the study of the Red Sea developed by Gutner-Hotch (2011), there is
greater specificity at the depth level than at the species level. During
the last 500 Mya, endolithic algae have surely been exposed to depth
variations that exceed 1000 meters in some geographical regions (Parsons
y Sclater 1977), prompting it to develop adaptations associated with the
lack of light rather than the search for a host.
To summarize, it is important to remember that future research must
strengthen all of the inferences proposed, including a larger number of
genes and developing higher resolution sequencing methods; however, this
research provides interesting results to the biology of a fundamental,
but little known, symbiont. Both the species delimitation and the
phylogeny show that, in addition to the wide diversity of clades found
in Ostreobium , there is no clear pattern of differentiation
between geographic region or coral hosts. This low specificity may be a
response to an opportunistic lineage that has been on Earth for more
than 500 Mya and has, of course, undergone many evolutionary pressures.
In fact, the presence of endolithic algae in non-coral samples and the
relationship between some groups provides yet another hint concerning a
generalist endolytic algae at the host level. It should be highlighted
that the lack of a specificity relationship does not diminish the
significance of the symbiotic interaction between endolithic algae and
corals. Finally, Ostreobium spp. appears to be traveling through
a dispersion of niches by depth, thus genotypes on diverse substrates
(because there is no specificity) across large depth gradients would be
worthwhile to study.