Amino Acids, Peptides, and Proteins
Amino acids are the building blocks of all proteins. There are roughly 20 different amino acids, with half being essential and the other half non-essential.  Non-essential amino acids can be produced by animal cells when the building blocks are available, while essential amino acids are not produced by animal cells and must be acquired from the environment. The ultimate source of essential amino acids is marine algae and those who consume it up the food chain, which is why gut-loading is so important. Peptides are amino acid chains with 2-50 amino acids, and proteins are groups of amino acids and peptides that are metabolically active, and can be various sizes, from small to very large. During digestion, some proteins are ingested whole, others broken down into peptides and absorbed, and a small amount is broken down into simple amino acids for absorption. Much of the immune response is mediated by ingesting whole molecules that can be recognized as antigens.

In figure #4, the amino acid content of a common fish (salmon) is listed; the non-essential amino acids are in bold and the essential amino acids are italicized. In this example, the ratio of essential to non-essential amino acids is approximately 1:1. There is no sense in providing only non-essential amino acids in very small quantities, unless you enjoy teasing your tank inhabitants into thinking real food is available; it is the deliverance of amino acids to the olfactory organs that initiates the feed response.

Figure #4

Feeding a diet in which the amino acid profile is deficient in one or multiple amino acids will limit protein manufacturing, limit retention of other peptides and amino acids, and likely force their deamination and catabolism to produce ammonia. If you feel you must use amino acid supplements, which clearly initiate the capture phase of feeding, make them the appetizer and not the whole meal. Ever wonder why some corals just seem to fade away? It could be an amino acid deficiency; creating a feeding response, polyp extension in sessile organisms and hunting in mobile organisms, expends energy without providing real nutrients.

Sugars and Complex Carbohydrates

There are many simple sugars, which are part of the metabolic pathways in any healthy organisms. Some organisms, like arthropods, convert simple sugars into repetitive complex structures or complex carbohydrates as a protective exoskeleton call chitin. Plants use the same concept to create their protective barrier called cellulose; both provide protection, but when ingested by animals, their digestion is inefficient, energy-expensive, and time-consuming. The digestion of these complex carbohydrates requires the presence of a healthy bacterial flora in the gut and at best are only partly digested by cleaving 2 carbon chains. These molecules represent a major portion of the detritus that is eliminated undigested. Their digestion is similar to the fermentation process used to produce your favorite alcoholic beverage. Grains from terrestrial sources have no place in a healthy marine diet, and only add more indigestible complex carbohydrates, which creates more detritus and stresses the waste elimination pyramid for your ecosystem.

Lipids, Fats, HUFA, and Essential Fatty Acids

Lipids are fats, sterols, fat-soluble vitamins, and phospholipids, and have many roles in basic cellular metabolism. Fats are mono-, di-, and triglycerides, and the principal sterol is cholesterol. C18 sterols are female hormones including estrogens and progesterone, C19 sterols are male hormones or androgens, C21 sterols are glucocorticoids and mineralocorticoids, and bile salts that bind fat-soluble vitamins for digestion are also sterols. HUFA are highly unsaturated fatty acids, while phospholipids are structural molecules, and are part of cell membranes.

Essential fatty acids deserve significant attention, as all animal life depends on their presence. Two of the most important are the Omega-3 and Omega-6 fatty acids. In animals, linoleic acid and linolenic acid can be converted into Omega-3 and Omega-6 fatty acids; the process involves multiple enzyme reactions and is extremely inefficient, to the point that it generally cannot meet metabolic demand. The Omega-3 fatty acids are EPA(Eicosapentaenoic Acid) and DHA (Dosocapentaenoic Acid).

1. The sole origin of these omega -3 fatty acids is single-cell marine algae. That includes spirulina and the many commercially available marine algae in the hobby. Each different alga has varying levels of these two essential fatty acids; Nanno, for instance, has higher levels of EPA and negligible levels of DHA. A varied diet of several species of marine algae is necessary for a healthy ecosystem.
2. DHA represents between 40% and 60% of the lipid membrane profile of brain, nerves, retina, and sperm in most animals. EPA is critical to immunity, inflammation, and blood clotting among other things.
3. Arachidonic Acid is an Omega-6 fatty acid and is also essential. It is found in many higher plant species and is abundant in muscle and liver tissue. It is a precursor of prostaglandins and necessary for the repair and growth of muscle fibers.

Vitamins

Vitamins are vital nutrients, and though they are required in very limited quantities, they cannot be produced in sufficient amounts to meet metabolic demand. Some vitamins act as hormone-like compounds, some as antioxidants, and others as co-enzymes for essential metabolic processes. There are two classes of vitamins based on their solubility. Water-soluble vitamins are the B-complex and C vitamins; they are easily absorbed in the GI tract, have the shortest half-life, and commensal bacteria in the GI tract can produce many of the water-soluble vitamins from existing nutrients. Fat-soluble vitamins are hormone-like and require binding by bile salts for absorption, and excess fat-soluble vitamins are stored in  fat, unlike water-soluble vitamins. Vitamin deficiencies are rarely seen in acute starvation, as their microscopic requirements long outlast both protein and calorie availability. Complex natural sources of vitamins are available in nature and all animals consume this as part of an all-natural diet.

Conclusion

We hope to stimulate a healthy conversation about all-natural, wholesome food choices for the aquarium hobbyist. The more you know and understand, the better choices you will make. We believe that following in the footsteps of Mother Nature (biomimicry) will lead to the greatest success, and we hope that this information helps you sustain healthy and happy long-terms pets.

Citations

Nutrient Requirements of Fish and Shrimp by the National Research Council of THE National Academies

The Physiology of Fishes by Evans, Claiborne, & Currie

Plankton Culture Manual by Hoff and Snell

The Omnivores Dilemma: A Natural History of Four Meals by Michael Pollan

In Defense of Food by Michael Pollan

The Most Important Fish in the Sea by Bruce Franklin