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Prochlorophytes*

The prochlorophytes are now considered part of the Cyanophyta; the three genera of "Prochlorophyta" are probably not closely related. Link.

Characteristics that seemed to warrant a separate Division

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Prokaryotic structure (like Cyanophyta: no mitochondria, chloroplasts, nucleus, etc.) but with chlorophylls and and b, and no phycobilins (as in Chlorophyta).
Thylakoids are not single and equidistant (as in the Cyanophyta), but grouped into stacks (lamellae) containing two to several thylakoids, as in the Chlorophyta and higher plants.
The reserve polysaccharide is starch-like. No cyanophycin (contrast Cyanophyta);  polyhedral bodies (carboxysomes) yes.
DNA has a diffuse distributionnot concentrated in the center of the cell as it usually is in the Cyanophyta.
The cell wall is similar to that of the Cyanophyta and contains a peptidoglycan (murein) layer.


Didemnum molle, one of the ascidians (tunicates) that harbors Prochloron.


Lissoclinum bistratum, another host.



Vertical section through  part of a colony of Diplosoma similis showing algal cells lining 
the common cloacal canals.   Drawing and two photos of colonies from Patricia Kott, 1982:
Micronesica 18: 95-127. Used with permission of the journal.

 

Notes on argument for including in Cyanophyta

*The proposed Division Prochlorophyta now appears to be artificial (see van den Hoek et al. 1995: 44 and http://hypnea.botany.uwc.ac.za/phylogeny/classif/cyan6.htm), and when P. didemni was formally named, Florenzano et al. (1986) placed it in Photobacteria (=Cyanophyta). [Reference] This species occurs as a consistent symbiont in six didemnid ascidians (tunicates), including Didemnum molle (Herdman, 1886) (Kott 1982; Lambert 2002).

Urbach, E.; Robertson, D.L. 1992. Multiple evolutionary origins of prochlorophytes within the cyanobacterial radiation. Nature 355: 267-271. Presents results from 16S ribosomal RNA sequence comparisons which indicate the prochlorophytes are polyphyletic within the cyanobacterial radiation and suggest that none of the known species is specifically related to chlorplasts. Use of new sequences from the marine picoplankter Prochlorococcus marinus.  [Abstract via EBSCO]

Chris Lobban. Revised 8/23/02