Acetabularia is a genus of large, unicellular green algae found in subtropical waters, notable for its complex morphology and single nucleus that allows for complete regeneration after cap removal. It has been used as a model organism in genetics, demonstrating that genetic information is contained in the nucleus, and has unique characteristics in growth and reproductive processes. Acetabularia is also studied for its circadian rhythms and occasionally appears in the aquarium trade, though it is considered delicate and difficult to maintain.

1. Acetabularia
Acetabularia is a genus of green algae in the family Polyphysaceae.[4]
Typically found in subtropical
waters, Acetabularia is a single-celled organism, but gigantic in size and complex in form, making it an
excellent model organism for studying cell biology.[5]
In form, the mature Acetabularia resembles the
round leaves of a nasturtium, is 4 to 10 centimetres (1.6 to 3.9 in) tall and has three anatomical parts: a
bottom rhizoid that resembles a set of short roots; a long stalk in the middle; and a top umbrella of
branches that may fuse into a cap. Unlike other giant unicellular organisms, which are multinucleate,
members of this genus possess a single nucleus located in the rhizoid, which allows the cell to regenerate
completely if its cap is removed. The caps of two Acetabularia may also be exchanged, even from two
different species. In addition, if a piece of the stem is removed, with no access to the nucleus in the
rhizoid, this isolated stem piece will also grow a new cap.[6]
A. mediterranea
In the 1930s–1950s Joachim Hämmerling conducted experiments in which he
demonstrated Acetabularia's genetic information is contained in the nucleus.[7]
This was the first
demonstration that genes are encoded by DNA in eukaryotes; earlier studies by Oswald Avery and others
had shown that this was true for prokaryotes.
Etymology
The name, Acetabularia, derives from the Latin word acetabulum, a broad, shallow cup used for dipping
bread; the upturned cap of Acetabularia resembles such a cup. For this reason, it is also sometimes
called mermaid's wineglass.[8]
In the 19th century, the same designation Acetabularia was proposed by George Edward Massee for a
genus of fungi (now Cyphellopus), but this usage is obsolete and considered invalid as the algal name
takes precedence.[9]
Anatomy and life cycle

2. Acetabularia, as well as being unicellular, is also a uninucleate organism. It has three basic parts:
its rhizoid, a short set of root-like appendages that contain the nucleus and anchor the cell to fissures in a
substrate; its median stalk, which accounts for most of its length; and its apex, where its cap forms. There
are usually several whorls of hair-like appendages close to the apex.[citation needed]
Acetabularia are among the largest single-celled organisms, having also a remarkably large nucleus.
During sexual reproduction, the nucleus undergoes multiple rounds of mitosis, forming many
daughter nuclei all within one nuclear membrane. These nuclei undergo meiosis and are transported to
the tips of the branches, the sporangia, where they are released as gametes.[10]
Hämmerling's experiment
Each Acetabularia cell is composed of three segments: the "foot" or basal segment which contains the
nucleus, the "stalk", and the "cap". Hämmerling exchanged caps between individuals from two species, A.
mediterranea and A. crenulata. A. mediterranea has a smooth, disc-shaped cap, while A. crenulata has a
branched, flower-like cap.
After the exchange, each transplanted cap gradually changed from its original form to the form typical for
the species of the base it was now attached to. This showed that the nucleus controlled the form of the
cap.
In another experiment, Hämmerling inserted a nucleus from one species of Acetabularia into an
intact Acetabularia of a different species. The Acetabularia then produced a hybrid cap with
characteristics of both species. This showed that both nuclei influenced the form of the
cap.[7]
Hammerling's results showed that the nucleus of a cell contains the genetic information that directs
cellular development.
Morphogenesis
Although a single cell, Acetabularia exhibits a remarkably complex shape and has therefore long been a
model organism for studying gene expression and morphogenesis. It seems to transport messenger RNA
molecules (in an inactive riboprotein form) from the nucleus to its apical tips, where they are translated

3. into proteins. These molecules may be activated by proteolysis of their protein carrier molecules, but this
has not been verified as yet.[citation needed]
Internal chemical gradients
In addition to its gradient in specific mRNA molecules, Acetabularia exhibits concentration gradients in
several types of molecules, such as ascorbic acid.
Circadian rhythms
Acetabularia has been used to study circadian rhythms.[11]
Studies have shown Acetabularia has
a diurnal circadian rhythm.[12]
These rhythmic changes in respiratory and photosynthetic activity are
maintained under constant conditions, even with the removal of the nucleus, showing the regulation of
the rhythm is independent of the nucleus.[13]
However, the nucleus isn't completely uninvolved, as it is
responsible for the shifting of the cycles due to external changes. In one experiment a nucleus from a
specimen trained on one circadian rhythm was transplanted into a de-nucleated plant on a rhythm that
differed by 12 hours, over a period of days the donated nucleus changed the circadian rhythm of the
receiving organism to that of the donor organism.[13]
Aquarium trade
Acetabularia species occasionally make their way into the aquarium trade. They are generally considered
to be more difficult or unappealing macroalgae to care for in the reef aquarium, a fish-only, or a FOWLR
(Fish Only With Live Rock) system, as they are delicate, readily eaten by herbivorous fish, grow slowly, and
do not have the high nutrient uptake that reef aquarium refugium species (such
as Chaetomorpha and Caulerpa) do. However, they are suitable for a macroalgae display tank, and thus
macroalgae suppliers often carry species of Acetabularia.