ASCORBIC ACID IN AQUATIC ORGANISMS: STATUS AND PERSPECTIVES

ASCORBIC ACID IN AQUATIC ORGANISMS: STATUS AND PERSPECTIVES. Konrad Dabrowski (ed.). 2001. CRC Press LLC, Boca Raton, Florida. ISBN 0-8493-9881-9. 288 p. $149.95 (hardbound).—This book is composed of chapters written by experts in the field and edited by one of the leading researchers in the area of vitamin C or ascorbic acid (AA) in aquatic organisms. The chapters vary in detail and breadth of coverage. However taken together, the volume comprehensively covers the subject of ascorbic acid in aquatic organisms, with emphasis on fishes. It is an important and useful compendium of information for fish (and shellfish) nutritionists and culturists. However, because of the numerous effects AA has on fish physiology, disease resistance, reproduction, and behavior, it should also be an important volume for anyone involved in basic or applied fish research and hence holding and feeding fishes. For instance, chapter 16 addresses an aspect of nutrition not generally examined—the influence of micronutrients such as AA on fish behavior. Behavioral patterns such as the fright response or predator avoidance, aggressive behavior, and schooling behavior can have significant effects on survival. The chapter specifically addresses these in terms of “seedlings” or fishes raised for stock enhancement of wild populations. However, researchers involved in fish-behavior research should also be aware of these effects.

Subjects range from the difficulty in assaying for AA in tissues (chapters 3 and 17) and in vitro methods of ascorbic acid absorption (chapter 15) to the many roles AA plays in the general health of an organism. Organisms, like fishes, that do not synthesize AA, require a membrane transport process that allows dietary AA to be efficiently absorbed as chyme moves through the intestine. Because AA is water-soluble and large, movement between compartments requires facilitated diffusion and active transport. Unfortunately, relatively little characterization of these transport mechanisms has been done with fishes. In vitro methods such as isolated intestinal segments, intestinal rings, Schultz chambers and membrane vesicles are discussed. Expression cloning, using oocytes of Xenopus laevis to gain molecular information on the membrane transport proteins, and development of transgenic fishes with the ability to synthesize AA by the introduction of an active gene for l-gulono-γ-lactone oxidase activity are also discussed.

Vitamin C functions in reproduction, disease resistance, growth, and bone formation are discussed in a number of chapters. Vertebrates differ in their ability to synthesize ascorbic acid from D-glucose or D-galactose—a process that requires gulonolactone oxidase. Methods for the detection of the enzyme, as well as its distribution among vertebrates, are reviewed in Chapter 4. Most vertebrates, with the exception of some birds, primates, guinea pigs, bats, and teleost fishes, can synthesize some AA. All nonteleost fishes examined to date can synthesize AA, suggesting that modern teleosts lost this ability as a result of a single event in their evolution.

Because vitamin C is not synthesized by most cultured fishes and is destroyed in most food processing procedures, it must be added to the diet. Hence, for cultured species, knowledge of requirements, functions, and deficiency signs are very important. AA is required in many enzymatic reactions as a reducing compound. Conversion of iron and folic acid to reduced forms for metabolism and proper blood cell formation and function, hydroxylation-mediated detoxification of various xenobiotics, hydroxylation of lysine and proline to convert procollagen to collagen for proper bone, cartilage, and connective tissue development are all associated with AA's role as a reducing agent. In addition, AA is extremely important in protecting membrane phospholipids, particularly in reproduction and disease resistance, from oxidative damage. Requirements and deficiency signs are reviewed for cool and coldwater species (chapter 5), warmwater fish species (chapter 9), freshwater versus marine fishes (chapter 6), an individual fish species (chapter 7), and crustaceans (chapter 8), which also appear to require a dietary source of AA. In fishes, deficiency signs include reduced growth, lethargy, deformities (of fins, vertebrae operculum, and supporting cartilage), anemia, reduced resistance to infectious diseases, and reduced reproductive success. Studies on the tissue distribution and storage of AA are also discussed in these chapters.

The difficulty of establishing requirements is also well developed in the above chapters. First, the establishment of requirements depends a great deal on the criteria used—deficiency signs, growth, survival rate, or AA concentrations in the organs and cells. Requirements are also affected by age, size, metabolic rate, reproductive status, other dietary components, health status (including infectious disease, immune response, and wound healing), and environmental factors. The authors also note the need to take into consideration interindividual variability, which is often overlooked. Environmental factors such as salinity and temperature changes, handling/trauma, and environmental contaminants all induce changes in AA stores, but variations may be tissue specific. It was concluded in chapter 5 that the vitamin C requirement should be assessed for specific purposes and culture conditions, that further information on the effects of environmental, health, and husbandry conditions is necessary, and that the role of AA at the cellular levels and AA status based on tissue concentrations should be defined. The authors of chapter 6 note the high variability among studies is often caused by the form of AA used and the instability of many forms of dietary vitamin C. Chapter 8 addresses this problem in crustacean feeds as well. Requirements of shrimp using various derivatives such as l-ascorbyl-2-sulfate, l-ascorbyl-2-monophosphate-Mg, L-ascorbyl-2-monophosphate-Na or L-ascorbyl-2-polyphosphate are presented. Cross-comparison of these compounds shows great variation (100% to 25%) in their ability to meet the vitamin C requirement.This is true for fishes as well as crustaceans. The relative ability to utilize the various protected forms results, at least in part, from differences in absorption. Hence, more information is required for individual species on the biopotency of these derivatives. A close analysis of information suggested that quantitative requirements for AA are very similar between freshwater and marine species as measured by optimal weight gain and normal development. Differences among species for the most part are attributed to methodological artifact.

Chapter 10 addresses the impact of other micronutrients on the requirement of AA in crustaceans and fishes. An important concept not set forth in any other chapter is the three categories of AA status: absolute deficiency—with accompanying deficiency signs; suboptimal AA status—indicated by anemia, low AA tissue concentrations, reduced immunity and increased secondary infections; and high or pharmacological doses—indicated by AA saturation of tissues and positive effects on detoxification, disease resistance, stress responses, specific immunity, and wound healing. A number of micronutrients such as vitamin E, retinoids, and carotenoids may act synergistically with AA against lipid peroxidation. AA acts as a radical scavenger in the aqueous phase whereas vitamin E is considered the most important antioxidant in the lipid portion of animal tissue. Interactions of AA with other vitamins, such as folate, and numerous minerals within the diet as well as in the tissues are discussed.

Chapter 11 evaluates the role of ascorbic acid and its derivates in resistance to environmental and dietary toxicity. Stress such as handling, confinement, and capture have been shown to reduce AA stores, because of its role in cortisol production. AA has a role in the metabolism of a variety of organic xenobiotics transformed through the cytochrome P450 enzyme system. This chapter also examines the relationship between AA and reproductive failures in fish. AA deficiencies have been shown to affect sperm concentration and motility, as well as egg hatchability and mortality. Both the female AA status and dietary AA influence normal development and survival of fry. The issue of early mortality syndrome (EMS) in the Great Lakes and Baltic Sea is reviewed. Thiamine deficiency is believed to play a major role in this syndrome. However, other factors such as oxidative stress from organic contaminants or deficiencies of other vitamins, including AA, have been suggested.

Chapter 12 examines the effects of AA on the immune response and disease resistance of fishes. It provides a brief overview of the fish immune system, both innate (nonspecific) and acquired (specific). The potentiating role of AA in disease resistance has generated much interest, in human as well as fish nutrition. However, as pointed out in this and other chapters, the results are often contradictory because of differences in species or strain responses, nutritional status, form of AA used, diet formulations, and experimental conditions. Despite the contradictions, there appears to be general agreement that AA deficient or marginally deficient organisms are more susceptible to infectious diseases.

Chapter 13 provides a critical review of AA concentrations in algae and other live feeds and its transfer between trophic levels. Microalgae are fed to cultured bivalves, as well as larval stages of some crustaceans and fishes. They are also fed to zooplankton that are then fed to juveniles of some fishes and crustaceans. The factors affecting the biochemical composition and hence nutritional value of these feed sources are reviewed, as are the effects of processing to produce microalgal pastes and powders. AA appears to be a critical nutrient during larval development—probably required at higher levels than those required by juveniles and adults. Enrichment of these live food sources with various forms of AA has been shown to improve physiological condition of larvae. Chapter 14 follows up on the transfer of vitamin C between trophic levels with a study on live food-mediated transfer to sea bass (Dicentrarchus labrax) during first feeding.

The final chapter, by the editor, is an excellent conclusion that covers history, present and future of ascorbic acid research in aquatic organisms, and concentrates on the discovery of ascorbate essentiality to fishes, new and/or controversial areas in AA research, and areas where progress needs to be made in our understanding of the functions of vitamin C. These include the interaction of plant-derived flavonoids affecting bioavailability, gastric and enterointestinal ascorbate circulation, and mechanisms of ascorbate action beyond antioxidant and enzyme coactivator roles.