Amino acids, as components of peptides, peptide hormones, structural and immune proteins, are the most important bioregulators involved in all life processes along with nucleic acids, carbohydrates and lipids.  Environmental D-amino acids are thought to be derived from organic diagenesis such as racemization and release from bacterial cell walls and even from microbial production.  50px D-amino acids were discovered in the 1950s : Auclair and Patton (1950) first reported their presence in the blood of insects and mollusks . Subsequently, D-amino acids have been detected in a variety of mammalian tissues. There are two major types of D-amino acids synthesized in mammals: D-aspartic acid (D-Asp) and D-serine (D-Ser).  D-Asp is present in vertebrates and invertebrates and is involved in the development of both the nervous and endocrine systems (Kim et al., 2010) . A large amount of D-Ser can  be detected in the brain, where it plays an important regulatory role in the development of the  central nervous system and is closely associated with learning, memory, and mammalian behavior (Snyder and Kim, 2000). In addition, a large amount of D-Ser has been detected in human urine; it plays a regulatory role in pathogenic gene expression in the urinary system (Huang et al., 1998). D-amino acids have also been detected in some tissues of marine invertebrates and might be involved in regulating osmotic pressure (Abe et al., 1999). However, it remains unclear whether these D-amino acids are synthesized by the organisms themselves or absorbed from the seawater. D-amino acids have been detected in plants as well (Brckner and Westhauser, 2003). Several D-amino acids have been found to inhibit plant growth (Forsum et al., 2008),  while some plants can absorb and metabolize or assimilate D-amino acids (Grdes et al., 2011; Hill et al., 2011) . The D-amino acids taken up by plants may be derived mainly from bacteria in the rhizosphere (Brodowski et al., 2005). In addition to higher organisms such as animals and plants, microbes (including planktonic algae, archaea, and bacteria) generally synthesize D-amino acids during development. Bacteria  have the largest capacity to utilize D-amino acids. They have been described to synthesize more than 10 kinds of D-amino acids, most commonly D-alanine and D-glutamate for crosslinking within the peptidoglycan cell wall. Extracellular D-amino acids released from bacteria also regulate remodeling of the bacterial cell wall and are  thought to function in communication among bacteria to accommodate changing environment. Besides structural function in the bacterial cell wall, D-amino acids have  been associated to growth fitness and to  processes such as biofilm development, spore germination and signaling. Some challenges associated with studies that use D-amino acids are that cell walls found in other life, such as archaea or plants/ fungi in eukaryotes, are not composed with D-amino acids. They are also naturally less digestible and utilizable as microbial growth substrates compared with L-amino acids. As a deep-sea strain, Phaeobacter sp. JL2886, which was isolated in 2012 from a 2000-m-deep sediment in the South China Sea, has been analyzed for its complete genome sequence . Another studyisolated 28 D-amino acid utilizers from 56 deep-sea sediments collected from a depth of 800–1500 m in Sagami Bay, Japan ; one strain, Nautella sp.A04V, was revealed to grow better with D-valine than with L-valine. For the enrichment and isolation of D-amino acid utilizers, liquid and solid media are often with D-amino acidsas the sole carbon source. Studies generally employ D-alanine, D-aspartate, and D-glutamate, as they represent the most common D-amino acids  in living organisms. They also serve as nitrogen sources  .
Amino acids, as components of peptides, peptide hormones, structural and immune proteins, are the most important bioregulators involved in all life processes along with nucleic acids, carbohydrates and lipids. " Environmental D-amino acids are thought to be derived from organic diagenesis such as racemization and release from bacterial cell walls and even from microbial production. " 50px Their discovery was in the 1950s . “ Auclair and Patton (1950) first reported their presence in the blood of insects and mollusks ” Furthermore, they also have been identified in various mammalian tissues. The two major types of D-amino acids synthesized in and by mammals are D-aspartic acid (D-Asp) and D-serine (D-Ser). “ D-Asp is present in vertebrates and invertebrates and is involved in the development of both the nervous and endocrine systems (Kim et al., 2010) ". An immense amount of D-Ser can also be detected in the brain, wherein it composes a significant regulatory role in the development of the “ central nervous system and is closely associated with learning, memory, and mammalian behavior (Snyder and Kim, 2000). " Additionally, a great amount of D-Ser , has also been “ detected in human urine; it plays a regulatory role in pathogenic gene expression in the urinary system (Huang et al., 1998). " Furthermore, they have also been noticed in certain tissues of some marine invertebrates, and “ might be involved in regulating osmotic pressure (Abe et al., 1999). " It is still unclear however, on as to whether they are absorbed from seawater, or are made by the organisms themselves . They have also been identified in plants (Brckner and Westhauser, 2003). A number of D-Amino acids also constrain the growth of plants (Forsum et al., 2008), “ while some plants can absorb and metabolize or assimilate D-amino acids (Grdes et al., 2011; Hill et al., 2011) ” Bacteria probably have the largest capacity to utilize amino acids. They have been known to synthesize more than 10 kinds of D-amino acids, most frequently D-alanine and D-glutamate for crosslinking within the peptidoglycan cell wall. In addition, extracellular D-amino acids released from bacteria also control remodeling of the bacterial cell wall and are moreover, thought to function amongst bacteria to accommodate frequently changing environments. Apart from structural function in the bacterial cell wall, D-amino acids have also been associated to growth fitness and to further processes such as biofilm development, spore germination and signaling. Phaeobacter sp. JL2886, a deep sea strain, that was sequestered in 2012 , from a sediment that was 2000 m deep in the South China sea, “was analyzed for its entire genome sequence (Fu et al., 2016)” (Naganuma et al., 2018). In another study, from 56 sediments collected from the deep sea (depth range 800-1500 m), “28 D-Amino Acid utilizers” were isolated, in Sagami Bay, Japan (Kubota et al., 2016). Independent attempts to sequester micro-organisms that grow healthier, due to D-Amino acids, has also been done. Most conducted studies and experiments usually also utilize mainly D-alanine, D-aspartate, and D-glutamate, as they signify the most common D-amino acids found in living organisms. Moreover, they also aid as nitrogen sources for many of the fundamental processes involved in the ocean .
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