Here, the Protista kingdom of Eukarya domain have been shortly introduced at first to get an idea over general characteristics of one of Protista’s members, the marine flora (mainly algae and seaweeds).
(To know about domains of life, click here: Three Domain System.)
Coming from the Greek words “eu“, which means “true“, and “karyon” which means, “nut“, the domain Eukarya is composed of organisms having “true nucleus“. Besides, every organelle in them is membrane bound.
Biologists used to refer to algae as plants. Many of the unicellular algae, however, show animal-like characteristics. Some swim by moving their flagella, and distinguishing these free-swimming algae from some of the structurally simpler animals can be difficult at first glance.
Some species carry out photosynthesis as plants do, and very similar species move and eat food particles as animals do. Some species do both and are claimed by both botanists (plant biologists) and zoologists (animal biologists).
These unicellular organisms are collectively called protists under the kingdom Protista for convenience, even if the various groups of protists have different evolutionary histories. Multicellular seaweeds are also considered to be protists, mostly because they lack the specialized tissues of plants.
- Includes algae, Paramecium, Protozoa and slime mold.
- Unicellular and multicellular.
- Nucleus and cell organelles in cytoplasm.
- Reproduced by mitosis.
- Autotrophs, heterotrophs.
The variety of marine plants that grow in the sea range from unicellular to multi-cellular macro-algae, to flowering plants, including sea-grasses, salt marsh plants, and mangroves.
Algae (singular, alga) is an informal term for a very diverse group of simple, mostly aquatic, photosynthetic organisms.
- Being eukaryotic, their cells contain a nucleus and other organelles enclosed by membranes.
- Photosynthesis takes place in chloroplasts—green, brown, or red organelles with layers of internal membranes that contain photosynthetic pigments. The color of algae is a result of the pigments and their concentration.
- They are polyphyletic.
- Range from unicellular genera to multi-cellular forms.
- Most are aquatic and autotrophic.
- Algae exhibit a wide range of vegetative and reproductive structures. They have relatively simple reproductive structures, and their non-reproductive cells are also mostly simple and unspecialized.
- In contrast to the land plants with which we are familiar, algae lack true leaves, stems, roots, and flowers.
Phylogenetically or evolutionarily speaking, unicellular green algae may be the ancestor of other green algae and higher plants.
Green lineages relationships or,
phylogenetic relationships among the main lineages of green plants
The green lineage comprises all the green algae and their descendants, the land plants. It is one of the major groups of oxygenic photosynthetic eukaryotes.
(This group i.e. the green lineage in phylogenetic studies have been termed differently by different scientists. Such as: “Viridiplantae” or “Viridaeplantae” (Cavalier-Smith, 1981, 1998), “Chlorobiota” or “Chlorobionta” (Jeffrey, 1971, 1982), “Chloroplastida” (Adl et al. 2005), or simply “green plants” (Sluiman et al., 1983) or “green lineage.” So, any of the above terms mentioned in any paper or article could be traced as about the green lineage.)
Diversification of Green Lineage
Current hypotheses on how green algae evolved posit the early divergence of two discrete lineages: the Chlorophyta and Streptophyta.
- The Chlorophyta includes the majority of described species of green algae.
- The Streptophyta are comprised of the charophytes, a paraphyletic assemblage of freshwater algae, and the land plants.
Origin of Green Lineage: Why Is It Difficult?
The green lineage is ancient, and dating its origin has been a difficult task because of the sparse fossil record of the group. The earliest fossils attributed to green algae date from the Precambrian (ca. 1200 mya) (Tappan, 1980; Knoll, 2003). The nature of these early fossils, however, remains controversial (e.g., Cavalier-Smith, 2006).
On the other hand, the resistant outer walls of prasinophyte cysts (phycomata) are well preserved in fossil deposits and especially abundant and diverse in the Paleozoic era (ca. 250– 540 mya) (Parke et al., 1978; Tappan, 1980; Colbath, 1983). So, to understand the origin of green lineage and also the inception story of chloroplasts, the phycomata is of prime importance.
Origin of Green Lineage: The Chloroplast Evolution
It is supposed that the green lineage originated following an endosymbiotic event in which a heterotrophic eukaryotic host cell captured a cyanobacterium. The cyanobacterium became stably integrated and ultimately turned into a plastid in the host cell (Archibald, 2009; Keeling, 2010). This primary endosymbiosis, which likely happened between 1 and 1.5 billion years ago (Hedges et al., 2004; Yoon et al., 2004), marked the origin of the earliest oxygenic photosynthetic eukaryotes.
But who were those phagotrophs who ate the cyanobacterium?
It is generally accepted that that the ancestral green plants were unicellular green algae with flagella and organic body scales. So, this ancestral green algae from whom the whole green lineage was originated, sometimes, are termed as the Ancestral Green Flagellate (AGF). As these characters of AGF are similar with most extant prasinophytes, the AGF is considered as a Prasinophyte-like-organism or, the ancestor of Prasinophytes.
Though the nature of this hypothetical AGF has been a matter of debate by many biologists, it is said that ancestor e.g. Pyramimonas or other prasinophytes would have swallowed a cyanobacterium and chosen to keep it rather than digest, thus completing the first endosymbiosis to form chloroplast.
- Pyramimonas gelidicola is “phagotrophic”, but this character has been subsequently lost in most green algae.
Seaweed: At a Glance
The most familiar types of marine algae are those popularly known as seaweeds. “Seaweed” is a rather unfortunate name. For one thing, the word “weeds” does not do justice to these conspicuous and often elegant inhabitants of rocky shores and other marine environments. So, some biologists opt for the more formal name of ‘macroalgae’.
- All seaweeds and unicellular algae are considered protists (kingdom Protista), whereas seagrasses, salt-marsh grasses, and mangroves are true plants (kingdom Plantae).
- “red algae” and “red seaweeds” mean the same thing. But not all algae are seaweeds and all seaweeds are algae.
In informal words, seaweed refers to several species of macroscopic, multi-cellular marine algae.
- The largest and most complex marine algae are called seaweeds. It includes some types of red, brown and green algae.
- Some tuft-forming blue-green algae are sometimes considered to be seaweed.
- Seaweed can be classified by ‘Usage’.
Seaweeds in the Global Perspective
- Seaweed’s appearance somewhat resembles non-arboreal terrestrial plants.
- Algae base dynamic species counts shows that there are about 10,000 species of seaweeds, of which 6,500 are red algae 2,000 are browns and 1,500 are greens.
Massive Marine Potential
People have used seaweeds since time immemorial. Samples from the oldest human settlement in the Western Hemisphere, in southern Chile, show that the first Americans probably used seaweeds as food and medicine. People around the world still harvest seaweeds to be used in many ways. The most obvious use is as a food source. People from different cultures have discovered that many seaweeds are edible, especially some of the red and brown algae. They are consumed in a variety of ways. The farming, or aquaculture, of seaweed is big business in China, Japan, Korea, and other nations.
- More than 70% seaweed is used as food whereas alginate, carrageenan and agar use 15-20% of world seaweed production.
- Oceans cover 71% of the earth but yield only 1.5% of our food (FAO 2012).
- The EU market for sea vegetables is growing by about 7-10% per annum.
So, the marine potential of seaweeds is massive.
- Adl, S. M., Simpson, A. G. B., Farmer, M. A., Andersen, R. A., Anderson, O. R., et al. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. J. Eukaryot. Microbiol. 52: 399–451.
- Archibald, J. M. 2009. The puzzle of plastid evolution. Curr. Biol. 19: R81–R88.
- Cavalier-Smith, T. 1981. Eukaryote kingdoms: seven or nine? Biosyst. Eng. 14: 461–481.
- Cavalier-Smith, T. 1998. A revised six-kingdom system of life. Biol. Rev. Camb. Philos. Soc. 73: 203–266।
- Cavalier-Smith, T. 2006. Cell evolution and earth history: stasis and revolution. Philos. Trans. R. Soc. Lond. B Biol. Sci. 361: 969–1006.
- Colbath, G. K. 1983. Fossil prasinophycean phycomata (Chlorophyta) from the Silurian Bainbridge formation, Missouri, USA. Phycologia 22: 249–265.
- Hedges, S. B., Blair, J. E., Venturi, M. L., and Shoe, J. L. 2004. A molecular timescale of eukaryote evolution and the rise of complex multicellular life. BMC Evol. Biol. 4: art no. 2.
- Jeffrey, C. 1971. Thallophytes and kingdoms – a critique. Kew Bull. Addit. Ser. 25: 291–299.
- Jeffrey, C. 1982. Kingdoms, codes and classification. Kew Bull. 37: 403–416.
- Keeling, P. J. 2010. The endosymbiotic origin, diversification and fate of plastids. Philos. Trans. R. Soc. Lond. B Biol. Sci. 365: 729–748
- Knoll, A. H. 2003. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton University Press, Princeton, NJ.
- Leliaert, Frederik, et al. “Phylogeny and molecular evolution of the green algae.” Critical reviews in plant sciences 31.1 (2012): 1-46.
- Parke, M., Boalch, G. T., Jowett, R., and Harbour, D. S. 1978. The genus Pterosperma (Prasinophyceae): species with a single equatorial ala. J. Mar. Biol. Assoc. U. K. 58: 239–276.
- Sluiman, H. J., Roberts, K. R., Stewart, K. D., and Mattox, K. R. 1983. Comparative cytology and taxonomy of the Ulvophyceae. IV. Mitosis and cytokinesis in Ulothrix. Acta Bot. Neerl. 32: 257–269.
- Tappan, H. 1980. Palaeobiology of Plant Protists. Freeman, San Francisco, CA
- Yoon, H. S., Hackett, J. D., Ciniglia, C., Pinto, G., and Bhattacharya, D. 2004.
A molecular timeline for the origin of photosynthetic eukaryotes. Mol. Biol.
Evol. 21: 809–818.