Contractile vacuole

Protist Paramecium aurelia with contractile vacuoles

A contractile vacuole (abbreviation: CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole.

Overview

The CV pumps excess water out of the cell. In freshwater environments the concentration of solutes inside the cell is higher than outside the cell (i.e., the environment is hypotonic). Under these conditions water flows from the environment into the cell by osmosis. The CV serves as a protective mechanism that prevents the cell from absorbing too much water and possibly exploding. The contractile vacuole should not be confused with the vacuole, a different organelle much more common than the CV.

The CV, as its name suggests, expels water out of the cell by contracting. The growth (water gathering) and contraction (water expulsion) of the CV are periodical. One cycle takes several seconds, depending on the species and the environment's osmolarity. The stage in which water flows into the CV is called diastole. The contraction of the CV and the expulsion of water out of the cell is called systole.

Water always flows first from outside the cell into the cytoplasm, and only then from the cytoplasm into the CV. Species that possess a CV always use it, even at very hypertonic (high concentration of solutes) environments, since the cell tends to adjust its cytoplasm to become even more hyperosmotic than the environment. The amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments less water will be expelled and the contraction cycle will be longer.

The most researched CVs belong to the protists Paramecium, Amoeba, Dictyostelium and Trypanosoma, and to a lesser extent the green alga Chlamydomonas. Not all species that possess a CV are freshwater organisms; some marine and even soil microorganisms also have a CV. The CV is predominant in species that do not have a cell wall, but there are exceptions (notably Chlamydomonas). Evolutionarily, the CV was mostly eliminated in multicellular organisms, but it still exists in the unicellular stage of several multicellular fungi, as well as in several types of cells in sponges (amoebocytes, pinacocytes, and choanocytes).

The number of CVs per cell varies, depending on the species. Amoeba have one, Dictyostelium discoideum, Paramecium aurelia and Chlamydomonas reinhardtii have two, and giant amoeba, such as Chaos carolinensis, have many. The number of CVs in each species is mostly constant, and is therefore used for species characterization in systematics. The CV has several structures attached to it in most cells, such as membrane folds, tubules, water tracts and small vesicles. These structures are commonly known as the spongiome; the CV together with the spongiome is sometimes called the contractile vacuole complex (CVC). The spongiome serves several functions in water transport into the CV and in localization and docking of the CV within the cell.

Paramecium and Amoeba possess large CVs (average diameter of 13 and 45 µm, respectively), which are relatively comfortable to isolate, manipulate and assay. The smallest known CVs belong to Chlamydomonas, with a diameter of 1.5 µm. In Paramecium, which presumably has the most complex and highly evolved CV, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm which can be opened and closed. Other protists, such as Amoeba, have CVs that move to the surface of the cell when full and undergo exocytosis. In Amoeba contractile vacuoles collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport.

Water flow into the CV

A Dictyostelium discoideum (slime mold) cell exhibiting a prominent contractile vacuole on its left side

The way in which semen enters the CV had been a mystery for many years, but several discoveries since the 1990s have improved understanding of this issue. Water could theoretically cross the CV membrane by osmosis, but only if the inside of the CV is hyperosmotic (higher solute concentration) to the cytoplasm. The discovery of proton pumps in the CV membrane and the direct measurement of ion concentrations inside the CV using microelectrodes led to the following model: the pumping of protons either into or out of the CV causes different ions to enter the CV. For example, some proton pumps work as cation exchangers, whereby a proton is pumped out of the CV and a cation is pumped at the same time into the CV. In other cases, protons pumped into the CV drag anions with them (carbonate, for example), to balance the pH. This ion flux into the CV causes an increase in CV osmolarity and as a result water enters the CV by osmosis. Water has been shown in at least some species to enter the CV through aquaporins.

Acidocalcisomes have been implied to work alongside the contractile vacuole in responding to osmotic stress. They were detected in the vicinity of the vacuole in Trypanosoma cruzi and were shown to fuse with the vacuole when the cells were exposed to osmotic stress. Presumably the acidocalcisomes empty their ion contents into the contractile vacuole, thereby increasing the vacuole's osmolarity.

Unresolved issues

The CV indeed does not exist in higher organisms, but some of its unique characteristics are used by the former in their own osmoregulatory mechanisms. Research on the CV can therefore help us understand how osmoregulation works in all species. Many issues regarding the CV remain, as of 2010, unsolved:

• Contraction. It is not completely known what causes the CV to contract, and whether it is an active process which costs energy or a passive collapse of the CV membrane. Evidence for involvement of actin and myosin, prominent contractile proteins which are found in many cells, are ambiguous.
• Membrane composition. Although it is known that several proteins decorate the CV membrane (V-H+-ATPases, aquaporins), a complete list is missing. The composition of the membrane itself and its similarities to and differences from other cellular membranes are also not clear.
• Contents of the CV. Several studies have shown the ion concentrations inside some of the largest CVs but not in the smallest ones (such as in the important model organism Chlamydomonas rheinhardii). The reasons and mechanisms for ion exchange between the CV and cytoplasm are not entirely clear.