Introduction
A complex endosymbiotic and euendolithic microbial community that maintains symbiotic interactions in various ways, such as parasitism or mutualism, is found in association with reef-building corals (Peixoto et al. 2017; Sweet y Bulling 2017). Numerous indications suggest that the ecological link between this community and the coral, which together make up the holobiont (Rohwer et al. 2002), is essential for the growth of hosts and their adaptability to the environment (Kelly et al. 2014). Due of their ability to degrade the coral’s calcium carbonate skeleton, certain coral microbes were occasionally regarded as parasites (Tribollet et al. 2009; Wisshak 2012). However, recent studies suggest that nutrients are transferred to the coral and pathogenic infections were prevented (Hambleton et al. 2019; Wijayanti et al. 2018). Actually, they have successfully colonized the coral skeleton as a result of providing their host with energy (del Campo et al. 2017; Sheppard et al. 2017; Weber y Medina 2012). The euendolithic alga Ostreobium is one most prevalent and widespread coral microeroder. More than 85% of the scleractinian coral species contain these algae, which has a wide geographic and bathymetric distribution (Chazottes et al. 2009). These algae belong to Bryopsidales, originating in the Ordovician approximately 500 million year ago-Ma (Del Cortona et al. 2020; Jackson et al. 2018; Verbruggen et al. 2009) and their relationship with scleractinian corals is at least 150 MA according to the fossil record in geological deposits of the Early Cretaceous (Kołodziej et al. 2012; Wisshak et al. 2017). The algae, which forms small channels linked to growth, can be seen as a green or brown ring inside the coral’s calcareous skeleton (Gonzalez-Zapata, Gómez-Osorio, y Sánchez 2018). Up to one kilogram of reef carbonate per square meter per year is lost due to the disintegration of the coral’s skeleton caused by this process (Grange, Rybarczyk, y Tribollet 2015). But given that a carbon transfer to the host occurs when stressed, the interaction between coral andOstreobium appears to be mutualistic (Fine, Steindler, y Loya 2004). Additionally, due to their photosynthetic pigments’ capacity to collect wavelengths close to the infrared, it is able to live in conditions of low light intensity (Marcelino y Verbruggen 2016) and can change the concentration of chlorophyll a and chlorophyll b depending on the depth (Magnusson, Fine, y Kühl 2007).
Ostreobium is thought to be sister to the two main suborders of Bryopsidales, Bryopsidineae and Halimedineae, according to the phylogeny provided by Verbruggen et al. (2009). Ostreobium queketii andO. constrictum were identified as the two species in the Ostreobiaceae based on taxonomic analyses and morphologic traits (Lukas 1974). Later, a study in the Red Sea used DNA analysis to identify seven different clades of the endolithic algae by taking samples from the coral skeletons of two species: Goniastrea perisi andPorites lutea (Gutner-Hoch y Fine 2011). However, each group may represent more than one species. For instance, the mesophotic coral species Agaricia undata provided rbcL gene sequences to distinguish 12 different types of cryptic Ostreobium species, mostly structured along a depth gradient (Gonzalez-Zapata et al. 2018). Understanding the origin and diversification of the biota requires the ability to recognize species (Wallace 2008). However, it can be difficult to identify and delineate species boundaries in taxa that have a uniform morphology and have existed on Earth for hundreds of millions of years.
One of the few genera of coral that may be found in the entire tropical region of the globe is Porites (Link 1807) (Paz-García et al. 2016), which is one of the most numerous groups of scleractinian corals that have existed from the Miocene to the present (Baker, Correa, y Cunning 2017; Grizzle et al. 2016; Klaus et al. 2008; Zhao et al. 2016). In the Triassic, it started to colonize many environments, and in the Jurassic, it displayed a rapid pace of diversification (Flügel y Senowbari-Daryan 2001). Due to its extensive distribution range and intricate patterns of morphological diversity, this type of coral is a great illustration of the ”species problem” (Zlatarski 2010). A consensus about their phylogeny is not established, and the limits of some species are still debated (Prada et al. 2014; Serrano et al. 2016).Porites holobionts have been identified as ocean acidification-tolerant corals (Connell et al. 2013). Also, some studies have evaluated the permanence of symbiont groups corals showing that most of the associations did not experience seasonal or annual variation in the dominant symbiont, indicating strong mutualistic relationships among species (Thornhill et al. 2014). And also, relationships between corals and their holobiont varied geographically, leading to the theory that coral might adapt to various ecological niches by ”changing” its microbial relationships (Buddemeier et al. 2004), suggesting a co-evolving geographic mosaic with widely distributed host populations and more isolated symbiont populations.
Numerous data are available to compare the diversity ofOstreobium around the world (del Campo et al. 2017; Gonzalez-Zapata et al. 2018; Gutner-Hoch y Fine 2011; Massé et al. 2018), allowing us for a comparison of samples from various geographic locations, including those that were divided by the Panama Isthmus, and providing insight into the impact of the closure of the Isthmus on the evolutionary process. Since this geological formation created different oceanographic conditions on either side of Panama, which resulted in processes of diversification and extinction (Bowen et al. 2013; O’Dea et al. 2016), it is a natural experiment that enables us to determine what effects it had on the evolution of the endolithic algae. Run 50 km from coast to coast, certain geological studies claim it closed 15 Ma (Montes et al. 2012) whereas accepted dogma suggests that they stopped existing at 3 Ma (O’Dea et al. 2016). The biological data yield conflicting conclusions because, according to molecular analyses of 68% of the marine species examined (Marko, Eytan, y Knowlton 2015), the ages of separation occurred more recently than 12 Myr, and, according to the paleontological record using the neodymium isotope, the deep-water connection was severed between 12 and 9.2 Ma (Osborne et al. 2014).
We must comprehend the evolutionary biology of coral symbiosis in order to predict how the ecosystem will react to environmental changes like ocean acidification and climate change. The degree of host specificity between different coral species and their symbionts is not entirely undestood (Silverstein, Correa, y Baker 2012). The diversity of coral and algae pairings may be a sign of the association’s plasticity or distinctiveness, the partners’ capacity to adapt to their environmental surroundings, and even the possibility of coevolution (Apprill et al. 2012; LaJeunesse et al. 2010). Because these connections form the basis of many ecosystems, host-symbiont system research is essential for comprehending ecological patterns (Baker et al. 2017). Sadly, we still don’t fully grasp these associations, the impact of holobionts on host development and diversity, and vice versa. Here, we start to fill in this knowledge gap by describing the genetic diversity ofOstreobium spp. using molecular data. The findings deepen our understanding of this symbiont’s biology and the ecology of their relationships, prompting us to assess the biodiversity and molecular ecology of the endolithic algae associated with Porites and consider how different species of Porites ’ reef-building corals from the Atlantic and Pacific differ in their Ostreobium clades. We also inquired about other geographical areas or hosts. We attempted to determine the patterns of Ostreobium ’s diversification and how it relates to Porites species in the Atlantic and Pacific oceans. As shown by data from other marine organisms (O’Dea et al., 2016), it is predicted that the populations of encrusting algae would be significantly impacted by the closing of the Isthmus, dividing into groups according to their geographic locations shortly after the geological event. However, Ostreobium diversification could happen prior to the closure of the Panama Isthmus (Del Cortona et al. 2020; Verbruggen et al. 2009). To resolve these issues, a phylogeny ofOstreobium spp. was inferred for samples collected from within corals of the genus Porites , and comparing with sequences reported previously in NCBI, to make inferences about the genotypic diversity of Ostreobium spp. in the Atlantic and Pacific oceans and compare with studies from the Red Sea (Gutner-Hoch y Fine 2011), Indo-Pacific (O´kelly et al, 2015), and Atlantic oceans (Gonzalez-Zapata et al. 2018).