Page 134 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 117

The phycobiliproteins differ from those in the red algae and cyanobacteria, because they are of
lower molecular weight, and do not aggregate to form discrete phycobilisomes, being localized
within the lumen of the thylakoids. DNA is organized into numerous nucleoids scattered throughout
the chloroplast.

Dinophyta

Chloroplasts may be present or absent in these algae, depending on the nutritional regimen. When
present, they are characterized by triple-membrane envelopes not connected with the endoplasmic
reticulum; thylakoids are usually in group of three, unappressed, and girdle lamellae are generally
absent. Pyrenoids show various types, stalked or embedded within the chloroplast. The pigment
assortment includes chlorophylls a and c 2 , b-carotene, and several xanthophylls, among which is
piridinin. DNA is organized into numerous nodules scattered throughout the chloroplast. This
description corresponds to the most important dinoflagellates plastid, the peridinin-type plastid.
About 50% of dinoflagellates with plastids acquired them from a variety of photosynthetic
eukaryotes by endosymbiosis (cf. Chapter 1). Those containing three-membrane, peridinin-
containing plastids, such as Amphidinium carterae, probably derived from the red lineage by sec-
ondary endosymbiosis, that is, through the uptake of primary plastid-containing endosymbiont.
Other groups of dinoflagellates have plastids derived from a tertiary endosymbiotic event, that is,
the uptake of a secondary plastid-containing endosymbiont. Tertiary plastids are present in the
toxic genus Dinophysis, characterized by two-membrane, cryptophyte-derived plastids, in other
important species (Karenia sp., Gyrodinium aureolum, and Gymnodinium galatheanum) with
fucoxanthin as accessory pigment, which possess haptophyte-derived plastids surrounded by two
or four membranes, and in Kryptoperidinium sp. and close relatives, which have a five-membrane,
heterokontophyte-derived (diatom) plastid that includes a diatom nucleus of unknown complexity.
Another case is that of Gymnodinium acidotum, characterized by a cryptophyte endosymbiont,
which may or may not represent a permanent endosymbiosis, because the endosymbiont may be
acquired as prey and retained for long periods of time, but not kept permanently.
A serial secondary endosymbiosis (the uptake of a new primary plastid-containing endosym-
biont) occurred in Lepidodinium sp. and its close relatives, in which the peridinin plastid has appar-
ently been replaced by a secondary plastid derived probably from a prasinophycean alga
(Chlorophyta) and surrounded by two membranes. This dinoflagellate also has external scales
atypical for its division, but closely resembling those of Prasinophyceae, suggesting that Lepidodi-
nium expresses genes for scale formation acquired from its endosimbiont. Also in Noctiluca sp. a
Prasinophyceae endosymbiont has been observed.

Euglenophyta
As in the Dinophyta the chloroplast envelope consists of three membranes. Chloroplasts are typi-
cally many per cell and show considerable diversity of size, shape, and morphology (Figure 2.80a
and 2.80b).
Five main types can be recognized:

. Discoid chloroplasts with no pyrenoids (Phacus)
. Elongated or shield shaped chloroplasts with a central naked pyrenoid (Trachelomonas)
. Large plated chloroplasts with the so-called double sheeted pyrenoid, that is, a pyrenoid
which carries on both plastid surfaces a watch glass shaped cup of paramylon (Euglena
obtusa)
. Plate chloroplasts having a large pyrenoid projecting from the inner surface, which is
covered by a cylindrical or spherical cup of paramylon (Colacium)
. Chloroplast ribbons radiating from one or three paramylon centers (Eutreptia)