Most life on Earth is dependent upon photosynthesis, the process by which plants make energy from sunlight. However, at hydrothermal vents in the deep ocean a unique ecosystem has evolved in the absense of sunlight, and its source of energy is completely different: chemosynthesis. Chemosynthesis is the process by which certain microbes create energy by mediating chemical reactions. So the animals that live around hydrothermal vents make their living from the chemicals coming out of the seafloor in the vent fluids! Because they are a local food source, hydrothermal vents typically have high biomass, in stark contrast to the very sparse distribution of animals outside of vent areas where animals are dependent on food dropping down from above.Chemosynthetic microbes provide the foundation for biological colonization of vents. Chemosynthetic microbes live on or below the seafloor, and even within the bodies of other vent animals as symbionts. Where microbial mat covers the seafloor around vents, grazers such as snails, limpets, and scaleworms eat the mat, and predators come to eat the grazers. Tubeworms flourish in small clumps, waving in the warm fluids. A typical picture of an active hydrothermal vent is therefore one with shimmering warm hydrothermal fluids, tubeworms and many other vent species, all densely clustered around the vent, with white microbial mat material covering the surrounding area.

Chemosynthesis

Photosynthetic vs. Chemosynthetic food chains. Microscopic views of chemosyntetic microbes.

The chemicals in hydrothermal vent fluid would be toxic to most forms of life familiar to us, but amazingly, a unique ecosystem has evolved to live at hydrothermal vents. These organisms are not dependent on sunlight and photosynthesis, but instead rely on chemosynthesis, a process in which certain microbes use chemicals in the vent water to produce energy. They in turn form the base for an entire food chain of animals. Chemosynthetic microbes grow on and below the seafloor and even within other animals at the vents.

Mimosa pudica or Sensitive Plant, a native of Tropical America, is a fun plant to grow. When we have tours of school children, their favorite plant is the Sensitive Plant. The leaves fold up when touched. If

Chemosynthesis

touched with a bit more force, the petioles will also bend at their pulvinis and the leaves droop. These movements are called seismonastic movements (reaction to physical shock). At night, the leaves will also fold and bend in movements known as nyctonastic movements (reaction to absence of light).Blooming Time: The plant flowers in mid to late summer. The flowers are globe shaped, about 1 inch across.Culture: Mimosa pudica need full sun to partial shade, with a rich moist soil. We use a soil mix consisting of 2 parts peat moss to 2 parts loam to 1 part sand or perlite. The soil should be kept evenly moist but not saturated. During the growing season, the plant are fertilized on a weekly basis with a balanced fertilizer diluted to ½ the strength recommended on the label. During the winter months, fertilize on a monthly basis. Winter temperatures should not fall below 65° F; if they do get chilled, then the plant suffers with yellowing of leaves and stems.Propagation: Mimosa pudica is propagated from seed. Seed will germinate in 14 to 21 days at 70° F.Mimosa pudica was featured as Plant of the Week October 12-22, 2001.

Plant movementLike a number of other plant species, it undergoes changes in leaf orientation termed "sleep" ornyctinastic movement. The foliage closes during darkness and reopens in lightThe leaves also close under various other stimuli, such as touching, warming, blowing, or shaking. These types of movements have been termed seismonastic movements. The movement occurs when specific regions of cells lose turgor pressure, which is the force that is applied onto the cell wall by water within the cell vacuoles and other cell contents. When the plant is disturbed, specific regions on the stems are stimulated to release chemicals including potassium ions which force water out of the cell vacuoles and the water diffuses out of the cells, producing a loss of cell pressure and cell collapse; this differential turgidity between different regions of cells results in the closing of the leaflets and the collapse of the leaf petiole. This characteristic is quite common within the Mimosoideae subfamily of the legume family, Fabaceae. The stimulus can also be transmitted to neighboring leaves. It is not known exactly why Mimosa pudica evolved this trait, but many scientists think that the plant uses its ability to shrink as a defense from predators. Animals may be afraid of a fast moving plant and would rather eat a less active one. Another possible explanation is that the sudden movement dislodges harmful insects.