9.5.pdf

Osmotic Pressure in Microorganisms: Most natural environments have lower solute concentrations than the inside of microbial cells. Rigid cell walls protect cells from bursting in dilute environments. High osmotic pressure can cause water to leave the cell, leading to cell s...

Osmotic Pressure in Microorganisms: Most natural environments have lower solute concentrations than the inside of microbial cells. Rigid cell walls protect cells from bursting in dilute environments. High osmotic pressure can cause water to leave the cell, leading to cell shrinkage (plasmolysis) and death. This is why salt is used to preserve food (e.g., brining meat and fish). Some microorganisms, called halophiles (salt-loving), need high salt concentrations to grow. Halophiles are found in marine environments (around 3.5% salt concentration). Extreme halophiles, like Dunaliella salina and Halobacterium, thrive in very salty lakes (3.5–10 times saltier than the ocean). Dunaliella salina uses glycerol and pumps out salt to cope with high salt. Halobacterium accumulates potassium and other ions, with proteins that only function well at high salt concentrations. Halotolerant organisms can survive high salt but do not require it for growth. Some halotolerant bacteria (e.g., S. aureus, Bacillus cereus, V. cholerae) can cause food-borne illnesses due to their salt tolerance. Water Activity (aw): Microorganisms need water to grow, measured as water activity (aw). Water's aw is 1.0; bacteria need high aw (0.97–0.99), while fungi can grow in drier conditions (e.g., Aspergillus spp. grows at 0.8–0.75). Reducing water content in food (drying, freeze-drying, or using brine) helps prevent spoilage. Barophiles: Microorganisms that thrive under high atmospheric pressure are called barophiles. Bacteria at the ocean's bottom endure extreme pressures, but their characteristics are mostly unknown due to challenges in studying them in labs. Light Photoautotrophs (like cyanobacteria, green sulfur bacteria) and photoheterotrophs (like purple nonsulfur bacteria) need light to grow. They capture light with pigments, converting it into chemical energy for processes like carbon fixation. The light they absorb is called photosynthetically active radiation (PAR), which ranges from 400 to 700 nanometers (nm) in the visible spectrum and can extend into the near-infrared for some bacteria. Accessory pigments (like fucoxanthin in brown algae and phycobilins in cyanobacteria) help these organisms use more light, especially in deep water.  Some microorganisms, like certain archaea (Halobacteria), use light to power their proton and sodium pumpsthrough a pigment called bacteriorhodopsin. Photosynthetic bacteria can be found in aquatic environments, soil, and in symbiosis with fungi in lichens. Watermelon snow is caused by a microalga called Chlamydomonas nivalis, which has a red pigment (astaxanthin) that gives the snow a pink color.