Coscinodiscophyceae

G. delicatula cells are cylindrica, with flat valves. Each cell as a spine extending from the valve margin. This spine fits into a depression on the adjacent cell to form chains. Few but large chromatophores are present usually at the cell periphery. This species can form large blooms in the spring and early summer

D. fragilissimus forms loosely connected chains. The cells bear processes on the valve end which fit into depressions on the adjacent cell. It is often found together with Leptocylindrus danicus and Guinardia delicatula

This is a typical filament of Aulacoseira (formerly Melosira) granulata (Bacillariophyta, Centrales)that was resuspended from the sediments not long before sampling. Its linking spines are of different lengths, possibly indicative of the filament having been broken at this place. In contrast, spines of equal length as in the other photo of this species are indicative of active growth. The chloroplasts here are partly compacted and do not fill the entire cell volume, indicative of the origin of this particular filament from the sediments. The other photo of this species shows actively growing, planktonic cells with chloroplasts filling the entire cell. This specimen was sampled from the shore of the lake in June 2006.

Aulacoseira (formerly Melosira) granulata (Bacillariophyta, Centrales) is one of the most common diatoms of Lake Kinneret, and certainly the major phytoplankton biomass contributor in winter (Dec – Feb), when the water column is homothermal. Typically, it occupies the entire 43 m water column. At the end of the winter bloom in early March the filaments sink and remain in the sediments in a dormant form with compressed chloroplasts till they are resuspended when the lake destratifies the following fall. It is a relatively large filamentous diatom, with cell diameter of 9 – 15 μm (median: 12.4 μm), cell height of 27-37 μm (median: 31 μm), and mean cell volume of 3700 μm3. The Kinneret Aulacoseira granulata filaments are straight, typically with 8 - 24 cells per filament. The picture shows the typical equal length marginal spines at the perimeter of the end-cell, these are “linking spines” which hold adjacent cells together. The chloroplasts fill the entire cells. This specimen was sampled from the shore of the lake in June 2006.   Aulacoseira granulata is a widespread centric diatom in the phytoplankton of lakes, reservoirs and rivers world-wide but particularly in African lakes and rivers, including the Nile River, L Naivasha, Kenya. It is typical of carbonate-rich, moderately eutrophic to eutrophic waters. It is used as indicator species of water with relatively low concentrations of salts, pH less than 9, and high silica concentrations. 

Filaments of different thickness typical of the winter (Jan-Feb) A. granulata bloom development season in Lake Kinneret

Cells at two focal levels. Material from a plankton tow off Martha's Vineyard, Massachusetts. Image by Jeff Cole.

Melosira (mell-o-sigh-ra) is a centric diatom. The cells are like old-style hat boxes, or old-style pill boxes, or like petri-dishes. In Melosira, many cells are joined end to end to create a filament. The less substantial rings on the lower image are where the two halves of the frustule are joined together by the girdle bands, the more visible connections are where two cells are joined together. Each cell has a radial symmetry. As with other diatoms, plastids have chlorophylls a and c and so have a yellow brown colour. The lower picture reveals the individual disc-shaped plastids. Phase contrast.

Melosira (mell-o-sire-a) is a centric diatom. The cells are like old-style hat boxes, or old-style pill boxes, or like petri-dishes. In Melosira, many cells are joined end to end to create a filament. The less substantial rings on the lower image are where the two halves of the frustule are joined together by the girdle bands, the more visible connections are where two cells are joined together. Each cell has a radial symmetry. As with other diatoms, plastids have chlorophylls a and c and so have a yellow brown colour. Differential interference contrast.

The surface of the siliceous valve of a Melosira cell. Phase contrast microscopy.

Stephanopyxis sp. isolated from a plankton net sample taken from the waters near Martha's Vineyard. Phase contrast image taken by Andrew Schurko.

Phase contrast image of Stephanopyxis sp. (after being cleared with bleach) isolated from the waters near Martha's Vineyard . Photo courtesy of Andrew Schurko.

SEM image of a Stephanopyxis sp. valve. Original sample isolated from the waters near Martha's Vineyard as part of the 2005 ATOL Protistology Workshop. Image courtesy of Shauna Murray and Andrew Schurko.

SEM image showing the valves of Stephanopyxis sp. Original sample isolated from the waters near Martha's Vineyard as part of the 2005 ATOL Protistology Workshop. SEM image courtesy of Shauna Murray and Andrew Schurko.

Centric diatom, seen from valve view. This is an empty frustule of a large marine species. The pattern of pores in the frustule is used in identification. Marine. Phase contrast.