Similar to all living organisms, plants require appropriate supplies of metal micronutrients for their normal metabolism, growth, and development. Plants being sessile in nature respond to external variations (both types and concentrations) of these metal and metalloid elements by employing a complex network of membrane transport system for efficient uptake, translocation, and compartmentalization of metal(loid)s in order to maintain the ion homeostasis. Among the different gene families involved in metal(loid) transport in plants, the ubiquitous cation diffusion facilitator (CDF) is a family of transmembrane transporters that efflux divalent cations from the cytoplasm to either subcellular locations or outside the cell. In plants, CDFs are called metal tolerance proteins (MTPs) and have shown specificity in transporting Zn2+, Mn2+, and Fe2+ but can also transport Cd2+, Co2+, and Ni2+, and some of these are also toxic to plants. Thus, the MTPs are presumed to carry out important and essential roles in mineral nutrition maintenance, stress tolerance, and homeostasis of metal(loid)s in plants. This chapter summarizes the recent developments through both in silico genome-wide analyses and functional characterization studies of MTP transporters in both dicot and monocot model plants, such as Arabidopsis and rice, respectively, together with other plant species of known genome sequence. A comprehensive understanding of the MTP family of proteins will help us to grasp clearly their roles in plant metal(loid) tolerance and cellular homeostasis at the physiological and biochemical levels. Further, it is anticipated that an in-depth study of the transcriptional and posttranscriptional regulation of the MTP genes in different plant species during metal(loid) stress will help to identify the candidate genes, which could be employed for crop biofortification and environmental bioremediation in future.
Bangladesh has a high potential for aquaculture development because the country has favorable conditions in terms of natural habitats, such as ponds, ditches, rice fields, rivers, lakes, estuaries, flood plain and coastal areas. Since the beginning of the 1980s, the aquaculture sector in Bangladesh has realized a momentum in terms of culture area, production, target species and degree of management intensity. Production based on aquaculture has been increasing rapidly in comparison with that from capture fisheries, particularly in recent years. Currently, although the freshwater shrimp culture industry is not equal to the scale of the saltwater shrimp culture industry, Macrobrachium rosenbergii, which is indigenous, is becoming an increasingly important target species. The culture of this species, especially in rice fields, has been a traditional activity, which was based mainly on wild postlarvae collected from rivers and other freshwater bodies. However, lack of a stable supply has become a major obstacle to the further expansion and development of rice-shrimp farming systems. Thus, adaptation of environment-friendly postlarvae production technology is essential to utilize the potential vast water bodies for freshwater shrimp farming and to enhance rural development. Water recirculation systems are designed to reuse a volume of water through biofilter treatment and delivery to larval rearing tanks. Water is typically recirculated when there is a specific need to minimize water replacement and to maintain stable water quality that differs from the supply water or to compensate for an insufficient water supply. Biofiltration is an effective and efficient means of removing low concentrations of biodegradable compounds from water. Typical nitrification rates range from 0.15-1.0 gTAN/m2/day and depend on the type of medium used and methods of operation of biological filters (Losordo and Hobbs 2000). It is estimated that 250-300 g of solid waste, in the form of uneaten feed and feces, is generated for every kilogram of feed added (Losordo and Timmons 1994). That estimate can be used to calculate the expected concentration of suspended solids. In small amounts, ammonia causes stress and gill damage (Francis-Floyd and Watson 2005) and can exert sublethal stress to fish at concentrations of less than 0.05 mg/L ammonia nitrogen (NO3-N), resulting in poor growth and reduced disease resistance (McGee and Cichra 2000). Freshwater shrimp larvae exposed to low levels of ammonia over time are more susceptible to bacterial infections, poor growth and behavioral disorders. Within biological filters, populations of nitrifying bacteria are responsible for the conversion of toxic ammonia to relatively harmless nitrate. This is a two-stage process involving Nitrosomonas bacteria that convert ammonia to nitrite (NO2-N) and Nitrobacter bacteria to further convert nitrite to nitrate. Both species are aerobic, therefore requiring dissolved oxygen levels near saturation to perform the conversions at an optimal rate. Biological filters are designed to provide high surface area of media for colonization by populations of both groups of nitrifying bacteria. The bacteria occur widely in soil and water environments and can be easily inoculated into biofilters from natural sources or with material from established filters. To ensure sufficient bacterial populations to remove ammonia and nitrite at rates required during operation, a biofilter is typically conditioned for several days by adding ammonia and monitoring its breakdown prior to stocking larvae. The hatchery industry in Bangladesh needs to move toward recirculation systems to minimize the discharge of hatchery water in the nearby environment, reduce the cost of bringing brine water from the distant coastal saltpan and maintain stable water quality. This shift requires an increased awareness of the filtration equipment needed to attain the intended water quality in the most economic and environment-friendly way. The objective of the study was to develop a cost effective and technically efficient biofiltration system and to introduce a recirculation system into hatchery operation, which would help to reuse water through continual treatment and reduce water consumption as well as discharge of saline water into the surrounding environment.
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