Nostocales

Tolypothrix (toll-eep-o-thricks) is a filamentous blue-green alga (cyanobacterium) which expresses false branching, i.e., the cells divide in one plane only, but a heterocyst may cause cells to break free and form an apparent branch. Heterocysts are differentiated thick-walled cells that provide the site for nitrogen fixation. In this image, the heterocyst is terminal and has a hemispherical shape. Differential interference microscopy. data on this strain.

Aphanizomenon is a cyanobacterium that often occurs in large aggregates. This was isolated on 4th April in a 20 micron plankton net haul at Sällvik (sample 4). Phase contrast micrograph.

Aphanizomenon is a cyanobacterium that often occurs in large aggregates. This was isolated on 4th April in a 20 micron plankton net haul at Sällvik (sample 4). Phase contrast micrograph.

Aphanizomenon is a cyanobacterium that often occurs in large aggregates. This was isolated on 4th April in a 20 micron plankton net haul at Sällvik (sample 4). These DIC images show the vacuolate character of the cell contents.

Cylindrospermum has many cells joined end to end to form long filaments. Not all the cells are identical and probably have different functions.

The cells at the end of the filaments are heterocysts. Some long rod shaped bacteria have colonized most heterocysts suggesting that there is some kind of mutualistic relationship. Phase contrast microscopy.

Under conditions of nitrogen deficiency A. ovalisporum develops heterocytes (central round and empty-looking cell in this filament), in which N2 fixation by nitrogenase enzymes takes place.

Aphanizomenon ovalisporum (Cyanobacteria, Nostocales), showing atypical long cells in a filament grown in culture.

The specialized cells of Aphanizomenon ovalisporum: an akinete (large, thick-walled, egg-shaped cell in the upper filament) and a hetetocyte (egg-shaped, transparent cells in the middle and bottom filaments).   The akinetes are much larger than regular cells and can be mid-filament or terminal, at times a few in a single filament, sometimes several in a row. Akinetes function as asexual resting stage and are resistant to harsh conditions. Akinete formation is induced by P-limitation. The akinetes separate from the filament and sink to the sediments, where they remain until conditions are suitable for germination.

Aphanizomenon ovalisporum (Cyanobacteria, Nostocales) was the first N2-fixing filamentous cyanobacterium that ever bloomed in Lake Kinneret. It happened in 1994 when this species formed an unprecedented autumn bloom. Since then Cylindrospermosis cuspis, another N2 fixer, invaded the lake and now dominates the summer-fall phytoplankton assemblages. A. ovalisporum produces the toxin cylindrospermopsin, which makes it an undesirable species in this important source of drinking water. Aphanizomenon ovalisporum is abundant during summer and fall, being evenly distributed with depth throughout the epilimnion. The filaments are 2 – 5 µm thick and 20 – 500 µm long. Heterocytes (seen as 3 distinct oval cells dispersed along the filament) appear when N-deficient conditions prevail. Akinete formation is induced by P-deficiency. The akinetes (not seen here) are much larger than regular cells and can be mid-filament or terminal, at times a few in a single filament, sometimes several in a row. This specimen was sampled from shallow water near the Kinneret Limnological Laboratory in June 2006.

This photo shows the typical terminal cells of Aphanizomenon ovalisproum from Lake Kinneret. The filaments, 2 – 5 µm thick and 20 – 500 µm long, are terminated by an elongated hyaline cell, as seen in this picture.