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.