Valve morphogenesis in the diatom genus Pleurosigma W. Smith (Bacillariophyceae): Nature's alternative sandwich

The loculate (chambered) valve structure of centric diatoms like Triceratium favus Ehrenberg has been mentioned time and again in the nanostructure literature. Here we draw attention to the fundamentally different alternative sandwich model nature developed in the genus Pleurosigma, where it is nonloculate. This has so far been overlooked in nanostructural studies. We suggest some mechanical aspects that would offer interesting avenues for experimental testing. The first description of the natural fabrication process (morphogenesis) is presented. This begins with the development of the raphe sternum, which then acts as a rigid backbone. The inner layer of the sandwich-structured valve develops next, with relatively large round single internal foramina not yet closed by a sieve membrane, in offset arrangement. This serves as a substrate for rows of stubby hollow pillars, also in offset arrangement. Then the outer layer of the sandwich develops and two different patterns (coarse-mesh and stellate bridges) have been observed. At first, the external areolar foramina are relatively large and +/- oval, gradually filling up until the tiny slits characteristic of the genus remain. In Pleurosigma species with double internal areolar foramina, small bridges grow from the opposite margins of the single foramina until they fuse. The sieve-membranes then close the internal areolar foramina. The finished valve is a lightweight structure expected to offer excellent strength with parsimonious expenditure of the raw material-silica.