Characteristic for Green algae are the chloroplasts that are green due to the dominating presence of Chlorophyl a and b (besides other pigments). Green algae can be found in sea water, fresh water and in humid terrestial environments. In the phylogenetic hypothesis shown in the study book (see life cycles) the group of the Green algae consists of the Chlorophyta together with the Charophyceae. Well-known representants of the Charophyceae are Spirogyra and Desmides. Common representants of the Chlorophytae are Chlamydomonas, Volvox and Ulva. Characteristics of these Green algae are further described here below.
Chlamydomonas is an unicellular Green alga. Besides a nucleus and a true cell wall with two or more layers of cellulose and pectine, one can find in the cytoplasm of those organisms mitochondria, Golgi, and one or more chloroplasts. These chloroplasts have a variable shape, but they always contain a protein-rich center involved in the production of starch: a pyrenoid. Besides, in Chlamydomonas two contractive vacuoles are found, which play a role in expelling the excess of water that penetrates into the cell through osmosis. Two flagellae allow active displacement. One eyespot or ocellus is able to sense light; the flagelated alga can respond adequately to light intensity and direction by swimming towards the light or away from it (resp. positive and negative phototaxis).
Chlamydomonas: life cycle
Life cycle of Chlamydomonas
Chlamydomonas is a haplont. After fertilization the zygote soon goes through meiosis and produces four haploid flagellated spores (zoo-meiospores), which develop to haploid unicellular individuals. Sometimes, spores are formed from the meiotic products through an additional mitotic division (formation of zoomitospores). This is a type of vegetative propagation. Further, the haploid individuals can go through a mitotic step and become gametes. Gametes fuse (syngamie) together on their turn to form a diploid zygote. The zygote can go to quiescence; it is then called a cystozygote.
Within the genus Chlamydomonas differentiation is observed in the gametes. In some species the gametes are exactly similar to each other (isogamy), but in other species the one gamete is clearly larger than the other (anisogamie) and sometimes the flagella are absent in these large gametes (oogamie). One speaks then of "female" (the large one) and "male" (the smaller motile one) gametes.
Anatomy of Chlamydomonas
Schemes of the anatomy of Chlamydomonas
Schetsen of variations in the shape of chloroplasts in Chlamydomonas
Microscopical views of stained preparations of Chlamydomonas
Volvox: unicellular appearance and colony
Volvox is a green alga that can be considered as an aggregation of unicellular Chlamydomonas-like green algae: a colony. The daughter cells of such unicellular algae doe not separate, but they are kept together by a gel mass. The colony which consists of merely 1 n cells, becomes larger by growth of individual cells and by -haploid- mitotic divisions. The daughter cells on their turn are kept in place by connecting arrays of plasma. This is how in Volvox hollow spheres of 500 up to 60 000 uniform cells can arise! The outwards oriented flagella generate a rotatory movement.
Video of the life cycle of Volvox
Confocal laser scanning microscopy of Volvox. Anaglyph stereo projection (depth visible with red -green stereo glasses). See a larger view
Volvox: life cycles with vegetative and sexual reproduction
In Volvox both asexual and sexual propagation occurs. In large colonies some kind of differentiation occurs between vegetative and not vegetative cells, although the cells are all haploid.
Vegetative (asexual) and sexual reproduction in Volvox
Vegetative propagation is achieved by repeated cellular division leading to invagination of a group of cells. There is however a problem: the flagellae of this new formation are oriented inwards. By an unusual process, called inversion, the layer of newly-formed cells turns inside out (see scheme) so that eventually the flagellae face up the outside of the little ball. Next, the daughter colony detaches from the motherwall; the young colony floats inside the large mother sphere. The whole process of invagination, inversion, detachment is repeated (and colonies can be arranged like painted Russian dolls). Eventually, the old colony desintegrates and the young one is released.
In preparation to sexual reproduction some vegetative cells enlarge to form egg cells, while other divide to form antherozoids. These sperm cells reach the egg cell by swim and fertilization can occur. The zygote forms a thick and hard wall and remains in the cavity of the colony until the later dies. After being liberated from its "shell", the naked zygote goes into meiosis. The products of meiosis can build a new colony.
Zygotes in Volvox
Zygotes: overview and detail
Confocal laser scanning microscopy of Volvox. Anaglyph stereo projection of a zygote incapsulated into a thick cell wall (depth visible with red-green stereo glasses; zoom)