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PHYCOLOGICAL SOCIETY OF AMERICA

Pseudo-nitzschia Protoplast

What's in the Protoplast?

The protoplast, literally "living matter," includes organelles and structures that do indeed confer "life" to Pseudo-nitzschia . The most apparent elements of the protoplast will be discussed in the following passages and will include the plasmalemma, plastids, mitochondria, dictyosomes, nucleus, and vacuoles.

Hey! Don't forget that General Information includes a great description of other elements in the protoplast!

Plasmalemma

The plasmalemma is situated very near the cell wall, pressing upon it as well as the silica frustule. With those species of Pseudo-nitzschia that have chambered valves, the plasmalemma may extend into these chambers. This structure is integral in cell function, for it provides attachment sites that aid in valve shaping. For example, the plasmalemma houses attachment sites to the hypocingulum, aiding in proportionate cell division.

Plastids

Whereas centric diatoms tend to have a large amounts of plastids, pennate diatoms, including Pseudo-nitzschia , can have as few as one plastid. Those species in the genus Pseudo-nitzschia have small plastids that are elaborately lobed and divide during cytokinesis. The structure of these plastids consists of four membranes, lamellae (composed of thylakoids) that run parallel throughout the plastid, and girdlelamella covering. DNA can be found within the lamellae, especially towards the sides of the plastid. The stroma of the plastid contain brown carotenoids that disguise the green color of the chlorophyll.

Mitochondria

The proverbial "powerhouse" of the cell has a structure not unlike the mitochondria in other eukaryotes. The inner membrane projects into the stroma as tubular cristae, and they align themselves peripherally. It is thought that perhaps they are involved with transport, as they can be seen within channels of the cell wall where cytoplasm also exists.

Dictyosomes

Dictyosomes are small structures with a rod-like profile. They may be found near the nucleus, at times even at the periphery of the nuclear membrane.

Nucleus

In P seudo-nitzschia , the nucleus may be visualized as either spherical or ellipsoidal, with a double nuclear membrane, one or more nucleoli, and pores within the envelope. During interphase, the nucleus can be found at the center of the cell within a cytoplasmic bridge (usually between two large vacuoles, see below). However, during mitosis, the nucleus of species within Pseudo-nitzschia shifts to the side, assuming a lateral position next to the girdle elements. The centrosome, affiliated with microtubule formation, helps in nuclear positioning. Chromatin can be seen as a condensed ring within the nucleus during metaphase.

Vacuoles

Like most diatoms, species of Pseudo-nitzschia have large vacuoles that contribute to the fairly low metabolism per volume. The vacuoles usually contain colorless sap. It is thought that the choice of ions help regulate buoyancy (lighter ions are able to aid in keeping the diatom more buoyant).

Pseudo-nitzschia 's two large vacuoles (see Figure) are situated at each end of the cell, separated by a cytoplasmic bridge that usually contains the nucleus (except during mitosis). These two vacuoles exist as two separate entities, not even fusing when pushed together. Volutin granules can be found within the main vacuoles and in smaller vacuoles near the nucleus. The location of the volutin granules help differentiate Pseudo-nitzschia from other genera.

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copyright Jennifer Shin 1999.