Acetabularia acetabulum

Classification

Eukaryota; Chlorophyta; Dasycladophyceae; Dasycladales; Polyphysaceae; Acetabularia; Acetabulum. (5) Common name: Mermaid’s wine glass (4)

Description and significance

Acetabularia acetabulum has much significance in human history in molecular biology. During a set of experiments done by Joachim Hämmerling in the 1930s-1950s, he proved that the nucleus contained the genetic information. He did this by taking two species of Acetabularia (acetabulum and crenulata) and exchanging their caps. When attached, the cap gradually changed in form to the original cap structure dictated by the base of the algae. Additionally, he introduced a nucleus from another species into an acetabulum organism and saw that the organism produced a hybrid cap. This showed that the nucleus holds the genetic information that produces the cap morphology (1). Acetabulum also provided the first evidence of mRNA (7). In addition, A. acetabulum is a great model organism used in the research of nucleus to cytoplasm relationships, the cytoskeletal organization, and in the study of circadian rhythms (1).

Genome structure

A. acetabulum has 20 linear chromosomes (7, 10). There are 96 known encoded proteins including the protein rhodopsin which is a purple pigment that is involved with the perception of light (11). Since this is a green alga, it is also photosynthetic and therefore has chloroplasts that have their own DNA. About 50% of the circular chloroplast DNA is replicated by the rolling circle method (9).

Cell structure, metabolism & life cycle

Acetabularia acetabulum is a uninuclear, unicellular eukaryote that is a green color because it is a photosynthetic green alga that contains chloroplasts (12). The cell wall of this organism is made up a polysaccharide Mannan (3). This alga is surprisingly large in size at maturity for being unicellular, growing 1-10 centimeters in height, and having 3 distinct body regions or apices. At the base of the algae is the rhizoid apex. There are 8-10 rhizoid apices per algae. They grow downwards and act as the root system for the organism. The single nucleus of the organism is located here. The stalk apex is the site where the whorls of hairs grow from in the vegetative state, and the whorls of the cap grow from in the reproductive phase. The hair apices grow upwards initially and then settle out horizontally in maturity. As they grow the main structure of the hairs divide into smaller sections, or bifurcates. There can be anywhere from 10-19 whorls of hair growing from the stalk. It is believed that the apices are biochemically distinct from each other, but it is not entirely known to what extend because there is no cellular division between the sections as A. acetalum is a unicellular organism (7). The life cycle of A. acetabulum can be divided into two distinct stages, a diploid and haploid phase. During zygote development, vegetative growth, and the first portion of reproductive development, the organism is diploid. Once further into reproductive development, gametogenesis, and mating, the organism no longer has two copies of each chromosome, but has one copy and is called haploid. The organism’s life cycle runs 1-2 years in the wild and around 6 months in a laboratory (7). There are two growth phases in the life of A. acetalum, the vegetative and reproductive phases. The vegetative phase can be further broken down into the juvenile and adult phases of growth. In the juvenile phase, A. acetabulum grows up to 1 cm and produces 5-6 whorls of hair. This stage is roughly 25% of the entire life cycle. Growth is rapid, and they prefer crowded conditions. They are more resistant to the cold than adults. In the adult phase, the organism grows the next 2-3 cm and produces 10-14 more whorls of hair. The hairs produced in this phase contain chloroplasts that are used in photosynthesis. It comprises roughly 20% of the life cycle. This includes the early adult phase, where the stalk is thinner, and the late adult phase where the stalk becomes thicker and stronger with more branching of the hairs. The growth in this phase is also rapid, but only in regions that are not crowded. Once the stalk apex begins to produce the cap, the adult phase is complete and the organism enters the reproductive phase of growth (7). Once in the reproductive phase, the cap begins to grow. It does so mostly from the center and not around the perimeter of the cap. During the first three and a half weeks of this phase, the shape of the cap changes from concave to flat to saddle (13). The cap structure becomes encrusted with calcium carbonate and causes the algae to look whiter in color in their natural marine environment (8).

A. acetabulum has sexual reproduction, conducted by the release of flagellated haploid gametes from the cap structure. The organism is grounded to something such as a rock by the rhizoids and lives in the water. When the gametes are released, they can swim and fuse with anther gamete, creating a diploid zygote. This zygote then attaches to a rock and starts the vegetative growth phase (2). A. acetabulum is a photosynthetic green alga. It uses the carbon in the environment to make glucose and the byproduct oxygen. It uses the inorganic carbon, in the form of bicarbonate, in the ocean to create these products. Because the ocean is alkaline, the bicarbonate does not freely diffuse across the membrane. This means that the algae had to develop mechanisms to transport the carbon source into themselves for photosynthesis. The algae lower the pH at the base of the organism by proton efflux which increases the amount of carbon dioxide molecules per inorganic carbon. Along with respiration, this allows the algae to get carbon and perform photosynthesis. Most of the photosynthesis occurs along the stalk of the algae. The glucose produced is then used by the organism in any process needing energy (14).

Ecology (including pathogenesis)

Acetabularia acetabulum is a marine organism that lives primarily in the Mediterranean. It has been found in Morocco, Algeria, Libya, Egypt, Turkey, Greece, and most prominently in Italy, France, and Spain (5). It grows in the waters of the eastern Atlantic off of the coast of North Africa, Mediterranean Sea, Red Sea, and the Indian Ocean. Its optimal temperature for growth is 10-25°C. Specifically, its habitat for growth is the shallow region of these waters on the subtidal rocks. It grows in clusters by attaching itself to rocks or shells that are covered in sand on these rocky coastlines (8). A. acetabulum must live in a marine environment because their form of sexual reproduction is dependent on water in which the flagellated gametes can swim to fuse with another gamete (2). As photosynthetic algae, they produce oxygen as a byproduct which it released into the environment. A. acetabulum is also a food source for many herbivores including sea urchins and fish. Its peak growth time is in the summer. This effects the environment in that herbivores increased their consumption of these algae during their peak season. This then in turn decreased the overall biomass of the algae (6).

Interesting feature

An interesting feature of A. acetabulum is that it has the ability to produce the reproductive cap without a nucleus. Depending on the time at which the cell was amputated and how much of the stalk was removed, the cap can be regenerated. Enucleated algae that are just in the early adult phase of the vegetative state could be amputated at the first whorl of hairs up to 35 days after cap formation and still regenerate a new cap. Those amputated in the late adult phase were 3 times more likely to regenerate a second cap though. The early adults also require more cell mass, or more of the stalk to be left behind than the late adults. Another interesting aspect of cap formation is that the algae can stop the formation of the original cap, repeat part of the vegetative phase by growing more stalk, and then form a second cap. Eventually the original cap turns white because the cellular contents are withdrawn back into the stalk, leaving it with no chloroplasts that create the green color. This happens naturally in 5% of the A. acetabulum population. This information shows that this alga has developed many different mechanisms to ensure that there is cap formation and that it can reproduce and continue the species (12).

References

(1) “Acetabularia.” Webster’s Online Dictionary. Web. 30 October, 2011 http://www.websters-online-dictionary.org/definitions/Acetabularia (2) Angulócello, Ahaie Tengwar. Who needs intelligent design? Web. 30 October, 2011. http://glaurung-quena.dreamwidth.org/tag/wonderful+life (3) Dunn, Erin K, et al. “Spectroscopic and Biochemical Analysis of Regions of the Cell Wall of the Unicellular ‘Mannan Weed’, Acetabularia acetabulum.” Oxford Journals: Plant and Cell Physiology. 48.1 (2006): 122-133. Oxford University Press. Web. 30 October, 2011. http://pcp.oxfordjournals.org/content/48/1/122.short (4) Guiry, M.D., Dhonncha, Nic E. “Acetabularia acetabulum.” Taxonomy Browser. NCBI. (2005) Web. 30 October, 2011. http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=35845&lvl=3&lin=f&keep=1&srchmode=1&unlock (5) Guiry, M.D. & Guiry, G.M. 2011. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Web. 30 October, 2011 http://www.algaebase.org/search/species/detail/?species_id=351 (6) Hereu, Bernat, Mikel Zabala, and Enric Sala. “Multiple Controls of Community Structure and Dynamics in a Sublittoral Marine Environment.” Ecology. 89: 3423-3435. Ecological Society of America. Web. 30 October, 2011. http://www.esajournals.org/doi/full/10.1890/07-0613.1 (7) Mandoli, Dina F. “Elaboration of Body Plan and Phase Change During Development of Acetabularia: How Is the Complex Architecture of a Giant Unicell Built?” Plant Physiology. 47 (1998): 173-198. Annual Reviews. Web. 30 October, 2011. http://scitec.uwichill.edu.bb/bcs/courses/Biochemistry/BL38B/pdf1/mandoli98annurevpb.pdf (8) “Mermaid’s Wineglass Acetabularia acetabulum.” Marine Wildlife Encyclopedia. Oceana. 2010. Web. 30 October, 2011. http://na.oceana.org/en/explore/marine-wildlife/mermaid8217s-wineglass (9) Mazza, A. et al. “A minicircular component of Acetabularia acetabulum chloroplast DNA replicating by the roling circle.” Biochemical and Biophysical Research Communications. 93.3 (1980): 668-674. Elsevier Inc. Web. 30 October, 2011. http://www.sciencedirect.com/science/article/pii/0006291X80911304 (10) “Nucleotide Database.” NCBI. Web. 30 October, 2011. http://www.ncbi.nlm.nih.gov/nuccore/?term=txid35845[Organism:noexp] (11) “Protein Database.” NCBI. Web. 30 October, 2011. http://www.ncbi.nlm.nih.gov/protein/?term=txid35845[Organism:noexp] (12) Runft, Linda L., Mandoli, Dina F. “Coordination of cellular events that precede reproductive onset in Acetabularia acetabulum: evidence for a ‘loop’ in development.” The Company of Biologists. 122 (1996): 1187-1194. Web. 30 October, 2011. http://dev.biologists.org/content/122/4/1187.full.pdf (13) Serikawa, Kyle A., Mandoli, Dina F. “An analysis of morphogenesis of the reproductive whorl of Acetabularia acetabulum.” SpringerLink: Planta. 207. 1 (1998): 96-104. Web. 30 October, 2011. http://www.springerlink.com/content/qxj0a2vyew90d7ud/ (14) Serikawa, Kyle A., Marshall D. Porterfield, and Dina F. Mandoli. “Asymmetric Subcellular mRNA Distribution Correlates with Carbonic Anhydrase Activity in Acetabularia acetabulum.” Plant Physiology. 125.2 (2001): 125.2.900. Amercian Society of Plant Physiologists. Web. 30 October, 2011. http://www.plantphysiol.org/content/125/2/900.full