The maintenance of metabolically active algae is essential because of the conserva­tion of stock cultures, attainment of explicit morphological and physiological sta­tus, or mass production. As described earlier, conservation of stock cultures is by routine, serial sub-culture and storage, preferably under suboptimal temperature and light regimes that may be similar for most algae. In addition, the nature of the media also plays a role in the frequency of transfer interval. But, for the achievement of desirable physiological cultures or for mass cultures, optimal growth conditions are vital, and this varies greatly with strains. In fact, algae poorly adapted to a specific medium may alter morphological features, as in the case of Chlamydomonas, where loss of functional flagella and some cyanobacteria may lose cell surface features. Another concern needing emphasis, specifically in continuous culture systems, is that culture conditions such as pH, nutrient content, oxygen level, etc. tend to change over time, despite the fact that the external environment remains unchanged and the limiting substrate concentration is at the required concentration. Some microalgae having an absolute requirement for vitamin B12 at very low concentrations can be grown without supplementing vitamin B12 in the culture medium for a number of generations. Complementing medium with vitamins B1 or B12 helps in stimulating the growth of certain algae. Another intrinsic phenomenon of some diatoms is that the cell size eventually becomes too small during continuous vegetative propagation to remain viable. A better alternative is to allow sexual reproduction of the culture to regenerate large, new vigorous cells. To propagate indefinitely, some Dasycladales are subjected to undergo periodic sexual reproduction.

One should appraise whether a particular alga strain would be best maintained for long periods in liquid medium or in agar slants. This is influenced by many envi­ronmental factors, including the habitat of the strain. A soil-water biphasic medium favors the growth of filamentous green algae and euglenoids. In fact, the addition of a soil phase directs the coccoid green algae to retain morphology, and a medium without soil extract promotes the accumulation of starch granules or lipid droplets. Hence, a choice of suitable culture medium specific to the strain is crucial. Second, light intensity must also be considered. For long-term culturing and maintenance of most microalgae, coupling subdued temperature with light intensities between 10 and 30 mmol photons m-2s-1 is vital. Excessive light can cause photo-oxidative stress in some algae. That is one of the reasons that some marine algae of tropical open-ocean are killed by continuous light (Graham and Wilcox, 2000). Furthermore, low light intensities are usually preferred by algae with phycobilisomes, while most dinoflagellates often need higher light intensities (60 to 100 mmol photons m-2s-1). This directs most culture collections to vary the light:dark regimens between 12:12 and 16:8 hours light:dark. However, preserving algae from extreme environments needs specific insight, as suggested by Elster et al. (2001). Third, the temperature of storage is vital. Variations in temperature can more easily influence marine strains than freshwater strains. In general, microalgal cultures are successfully conserved at temperatures between 15°C and 20°C. Indeed, some larger service repositories such as the Culture Collection of Algae at the University of Texas (UTEX) preserve algal strains at 20°C. Prolonged maintenance at 20°C leads to cellular damage resulting from photo-inhibition. And, alternatively, increased light intensities coupled with incubation temperatures higher than 20°C can be employed. However, temperatures above 20°C are mostly incorrect for conserving stocks at comprehensive transfer cycles. One should note that the evaporation of the medium effectively regulates the interval of their transfer cycles. Fourth, the frequency of transfer cycles is considered a key factor. For routine maintenance, sub-culturing is done toward the end of the exponential growth phase of the culture. The shortest transfer cycle is about 1 to 2 weeks for sensitive strains, while some green algae and cyanobacteria, on agar slants at low light and 10°C, is sub-cultured only once every 6 months. However, a safe transfer interval for a specific strain can be predictable to one quarter the time a strain can survive maximally. Usually, a post-transfer period at higher light and temperature regime is valuable in regular quality control assessment. Moreover, perpetual maintenance over longer periods may alter the morphological features and physiological characteristics of some strains, and hence a short interval of mainte­nance at optimal growth conditions is recommended to refresh the culture. In case of an unknown or a newly isolated strain, the cultural characteristics should be fully understood prior to maintenance, or the long-term maintenance procedure should be framed by optimization of survival at varied light and temperature conditions (Lokhorst, 2003). It is often sensible to screen and configure a suitable medium for the new isolate. Finally, the culture maintenance chamber or room should be con­trolled for humidity—for not only preventing evaporation of cultures, but also to avoid the contaminating fungi and molds.

In spite of these, the selective and synthetic nature of the media as well the incu­bation conditions, as opposed to the native ecological conditions, limit the success of perpetual transfer. Furthermore, continuous transfers lead to the loss of morphologi­cal and/or genetic characteristics (Warren et al., 2002). Not to mention that serial sub-culturing is a labor-intensive and time-consuming process, which restricts main­tenance and handling of a large number of cultures. Above all, to overcome the risk of loss of strains, each strain may be maintained in secondary culture collections with sub-cultures of different ages or transfer dates. The World Federation of Culture Collections has suggested to stock backup cultures at various locations to expedite all possible chances of revival (Anon, 1999).