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This study reports a hitherto undescribed endophytic diatom that forms dense unialgal colonies in the mucilage that fills the internodes of a red alga, Coelarthrum opuntia. The diatom is very similar to Gyrosigma tenuissimum, in that the valve outline is delicate and linear, although it has a central raphe with externally unusual, overlapping central fissures. Based on morphological features and its unusual habitat, this endophyte is described as a new species: G. coelophilum Okamoto & Nagumo sp. nov. Gyrosigma coelophilum was found only in the internodes of C. opuntia, not on the thallus surface, indicating that G. coelophilum is an endophyte and not an epiphyte; this situation was true of the thalli collected from several different localities. Furthermore, we measured the light transmittance of the cortex of C. opuntia to estimate the light condition inside the internodes where G. coelophilum lives as an endophyte. It was shown that more than half the light that G. coelophilum can potentially exploit was absorbed by C. opuntia, so that we suspect that the diatom may be partially heterotrophic.
INTRODUCTION
Diatoms are one of the most successful groups of algae, inhabiting various environments, from aquatic to terrestrial, in equatorial to polar regions. They can also live in extreme conditions, such as hot springs, saline lakes, etc. (Round 1981; Round et al. 1990), and much research has been done on the ecology of those species living in the plankton or benthos of marine and freshwater habitats. However, endozoic and endophytic diatoms are rather unusual and our knowledge of these is still quite fragmentary (Round 1990). There have been only a few reports of endophytes (Baardseth 1966; Hasle 1968), including two species, Cocconeis scutellum Ehrenberg and Navicula endophytica Hasle, which inhabit the receptacles and growing tissues of Ascophyllum nodosum (Linnacus) Le Jolis along Norwegian coasts.
In this study, we report for the first time an endophytic diatom living and forming colonies inside the thallus of a red alga, Coelarthrum opuntia (Endlicher) Borgesen. The thallus of C. opuntia is branched and segmented; each contains a cavity filled with mucilage of unknown chemical composition. We found that some (but not all) internodes of many individuals of C. opuntia collected from Izu district, Japan, contained needle-shaped diatoms in the mucilage. These diatoms formed...