Algae can be produced according to a great variety of methods, from closely-controlled laboratory methods to less predictable methods in outdoor tanks. Indoor culture allows control over illumination, temperature, nutrient level, contamination with predators, and competing algae, whereas outdoor algal systems, though cheaper, make it very difficult to grow specific algal cultures for extended periods. Open cultures such as uncovered ponds and tanks (indoors or outdoors) are more readily contaminated than closed culture vessels such as tubes, flasks, carboys, bags, etc. Axenic cultivation can be also chosen, by using algal cultures free of any foreign organisms such as bacteria, but this cultivation is expensive and difficult, because it requires a strict sterilization of all glassware, culture media, and vessels to avoid contamination. These constraints make it impractical (and very expensive) for commercial operations. On the other hand, non-axenic cultivation, though cheaper and less laborious, are more prone to crash, less predictable, and often of inconsistent quality.