Naviculales

Gyrosigma observed in freshwater sediments in the vicinity of Broome, Western Australia in September 2003. This image was taken using phase contrast optics.   This work was supported by the Australian Biological Resources Study.

Gyrosigma observed in freshwater sediments in the vicinity of Broome, Western Australia in September 2003. This image was taken using phase contrast optics.   This work was supported by the Australian Biological Resources Study.

Gyrosigma observed in freshwater sediments in the vicinity of Broome, Western Australia in September 2003. This image was taken using differential interference contrast optics.   This work was supported by the Australian Biological Resources Study.

Valve view of the cleaned frustule showing the surface tolology of the cell. Differential interference contrast micrograph.

Gyrosigma sp. (pennate diatom) is a benthic alga in Lake Kinneret that at times appears in the plankton, especially near the shore.

Stauroneis (star-owe-neigh-is), pennate (naviculoid) diatom. The siliceous shell (frustule) has axial raphe and arrays of fine holes which give the frustule its lined appearance. Plastids with chlorophylls a and c, hence the yellow brown colour. Phase contrast.

Diatoms are algae which live in little glass boxes or frustules - rather like little petri dishes. Pennate diatoms are motile, and rely on, we believe, the excretion of mucus through a slit in the wall - the slit is the raphe and runs down the flat faces of the petri dishes (frustules in diatom terminology). The frustule is perforated with tiny holes to allow the diatom to excrete waste products, and pick up nutrients, dissolved gases etc. from the surrounding water. The one is seriously asymmetric. Phase contrast.

Amphiprora (am-fee-pro-ra). Pennate diatoms are usually regarded as boat shaped, although some of the boats have very odd shapes. They can usually glide. This particular form is very common in sediments - the form apparently allowing the cell to penetrate through the pores of the sediments. Although enclosed in a siliceous shell, the shell of these rather delicate diatoms is flexible. The plastids contain chlorophylls a and c which gives the yellowy-brown colour. Phase contrast.

Pennate diatom seen in girdle view. The cells are enclosed in siliceous valves. There are typically two valves - a top one and a lower one, and they are joined together with fine bands or girdle strips. This image is a view from the side and shows the girdle strips. Differential interference contrast.

Pennate diatoms. The cells are enclosed in siliceous valves. There are typically two valves - a top one and a lower one, and they are joined together with fine bands or girdle strips. With plastids containing chlorophylls a and c (they are stramenopiles after all). Genera and species distinguished largely by the shape of the organism and the pattern of pores and sculptings of the siliceous shell or frustule. Pennate diatoms are important in intertidal and illuminated subtidal sediments in marine ecosystems and primary producers. Pennate diatoms are capable of movement, relying on the raphe to produce thrust. Movement benefits diatoms which may move towards the light, recover their location after disturbances by overlying water currents, wave actions, animal burial and so on. Several species illustrated. Phase contrast.

Material from the Netherlands.

Valves are rectangular in girdle view and elliptical in valve view. Each cell has four ribbon like and folded chloroplasts, two along each side of the girdle.