Abstract
Endolithic, red unicells residing in the interior of Lithothamnion rhodoliths, collected offshore the NW Gulf of Mexico in mesophotic rhodolith beds at ~54-55 m depth and maintained in closed microcosms, were used to establish cultures following their isolation. These endolithic unicells subsequently developed into amorphous blobs of palmelloid cell colonies. Each cell contains unstacked, 2-5 lobed parietal chloroplasts, one prominent central pyrenoid, and have a thin or thick cell wall. Single cells, or cell clusters (in pairs, tetrads, or up to 12) are embedded inside an extracellular matrix whose boundaries remain closely appressed to neighboring cell clusters. Cell division by concavo-convex division resulted in hemispherical cells subsequently expanding in size. Plastid tufA, psbA and 16S rDNA sequence analyses confirmed that the colonies are Rhodosorus marinus Geitler. This is the first report of a red alga spending part of its life history endolithically inside biogenic rhodoliths.
Highlights
The unicellular marine coccoid red algal genus Rhodosorus was described by Geitler (1930, p. 633, Figure 15) from seawater cultures originating from Las Palmas, Canary Islands
Whereas R. marinus and R. magnei typically grow epiphytically on the surface of seaweeds (Fresnel and Billard, 1995), we recently found a member of Rhodosorus growing inside calcified Lithothamnion (Hapalidiaceae, Hapalidiales) rhodoliths collected in mesophotic rhodolith beds in the northwestern Gulf of Mexico
Each of the cultured cells originating from endolithic cells inhabiting NW Gulf of Mexico Lithothamnion rhodoliths conforms to the concept of R. marinus Geitler, a species that has fewer chloroplast lobes (∼2–5) than R. magnei (8–11) (Fresnel and Billard, 1995); it should be noted that these characteristics were found to be quite variable and dependent on culture conditions and that they cannot be used to differentiate these two species (Wilson et al, 2002)
Summary
The unicellular marine coccoid red algal genus Rhodosorus was described by Geitler (1930, p. 633, Figure 15) from seawater cultures originating from Las Palmas, Canary Islands. The unicellular marine coccoid red algal genus Rhodosorus was described by Geitler Rhodosorus marinus Geitler was the only species in the genus until Fresnel and Billard (1995) described a second species, Rhodosorus magnei, isolated from the French West Indies (Isle de St Barthélemy). The current distribution of Rhodosorus (Guiry and Guiry, 2019) indicates that these unicellular red algae predominantly inhabit warm and coastal waters worldwide (West and Calumpong, 1990; Fresnel and Billard, 1995; Zuccarello et al, 2008). PickettHeaps et al (2001) reported that the four strains of Rhodosorus investigated displayed continuous cytoplasmic rotation within the wall, and Wilson et al (2002) documented chloroplast rotation and morphological plasticity in R. marinus. Placed in the Porphyridiales (e.g., West and Calumpong, 1990), the genus currently belongs in the family Stylonemataceae, order Stylonematales, in the class Stylonematophyceae (Yoon et al, 2006, 2010; Yang et al, 2010, 2016)
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