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PNS Bacteria and the American Shrimp Farmer

America Loves Shrimp. This remains a persistent truth as over 500,000 metric tons of shrimp are consumed in the United States each year. This ravenous hunger has contributed to the nation’s rising seafood deficit. Because of this, over 90% of the American shrimp supply is provided by the traditional shrimp farming nations of Southeast Asia (China, Vietnam, Philippines, Indonesia, Thailand), Central Asia (India, Bangladesh) as well as the burgeoning South American producers of Ecuador and Argentina. Recent historical events have revealed the great danger behind America's seafood dependency; interruption in global distribution networks can paralyze the distribution of seafood, inducing acute supply shortages and price spikes. Prolonged disruption of the Southeast Asian trade network alone would deprive the U.S. of 80+% of their shrimp supply, rendering the commodity virtually unaffordable to the average American.

Indeed, this shrimp dependency has come with an additional cost. Due to logistical realities of shipping perishable seafood across the globe en masse, the shrimp Americans receive are a shallow husk of the premium product enjoyed in production nations. They cannot be transported with their heads-on and thus are devoid of their rich, umami flavor. In order to avoid contamination and retain moisture, many frozen shrimp receive chemical processing (sodium triphosphate, tripolyphosphate, sodium bisulfide), reaping them of their natural color/texture and leaving them a watery shame.

So why does America not already have numerous domestic shrimp farms? The general answer is that much of the Continental U.S. is unsuited for classical methods of shrimp aquaculture which require prolonged warm seasons and vast areas of extensive production ponds. In addition, classical shrimp farming requires continuous water exchange with wild water bodies--a practice which can be detrimental to natural ecosystems and is largely prohibited by U.S. environmental regulations. Thus, with the exception of the very southern states, classical shrimp farming through pond production is nearly impossible elsewhere.

If America is to farm shrimp, it must embrace a non-classical method of production.

Nature in a machine

Biofloc Technology is an aquaculture technique which relies on dynamic ecological processes to produce market species such as shrimp with minimal feed input, water input and waste discharge. This technology allows for indoor shrimp farming that is far more profitable than clearwater production systems. More importantly, this technology allows for year-round production even in colder regions. This would allow for the localized production and distribution of fresh Pacific White-legged Shrimp (Litopenaeus vannamei), Tiger Shrimp (Penaeus monodon), Banana Shrimp (Fenneropenaeus merguiensis) and Giant Blue-Clawed River Shrimp (Macrobrachium rosenbergii) to be enjoyed by the American public.

“Superintensive shrimp culture in zero-exchange, biofloc-dominated production systems is more biosecure and sustainable than traditional shrimp farming practices. However, successful application of this technology depends upon optimizing dietary formulations, controlling Vibrio outbreaks, and managing accumulative changes in water quality and composition.” (Pragnell et al 2016).

Utilizing efficacious biofiltration agents, such as the purple non-sulfur bacterium (PNSB) Rhodopseudomonas palustris, is paramount to recycling wastes such as ammonia, nitrite and sulfides which can devastate shrimp health. R. palustris possesses a ‘swiss-army knife’ metabolism in which it is capable of processing these toxins through a variety of metabolic faculties (nitrification, denitrification, anammox, etc.) even under a wide variety of environmental conditions. Shrimp are very messy eaters, tearing their feed apart and scattering organic particles throughout the water. In a clearwater production system, these particles would rot, adding waste to the water thereby fostering pathogenic Vibrio bacteria. In a Biofloc production system, these wastes are aggressively feasted upon by R. palustris, recycling them into nutritious proteins, carbohydrates, enzymes, carotenoids and precious golden fats.

R. palustris has proven to be a potent alternative protein source in many animal feeds. Alloul et al 2021 demonstrated a diet with 5g R. palustris protein/100g feed was about to outcompete a L. vannamei commercial diet both in terms of average shrimp weight (26% higher in PNS diet) and in overall food conversion ratio (1.3g feed/g shrimp vs 1.7g feed/g shrimp). Chumpol et al 2018 demonstrated lower ammonia concentrations in L. vannamei production tanks fed feed incorporated with PNS protein than a control commercial diet. Alloul et al 2020 demonstrated that a feeding trial of L. vannamei had a survival of 8% when fed a commercial diet during an ammonia stress trial (3 ppm); the shrimp which were fed a diet enriched with active R. palustris experienced a survival rate of 63-75%!

R. palustris (as well as other PNSB such a Rhodospirillum rubrum, Rhodobacter capsulata, etc.) is able to produce highly unsaturated fatty acids (HUFAs). Some selectively-bred aquaculture strains of PNSB produce polyunsaturated fatty acids (PUFAs). PUFAs are referred to as ‘Golden Fats’ because they are able to produce incredible amounts of energy and can be metabolized with little to no inflammation. This metabolic superfuel is absolutely essential to the health of a rapidly-growing shrimp crop. Palacios et al 2001 demonstrated how dietary concentrations of PUFAs such as ARA (arachidonic acid), EPA (ecosapentanoic acid) and DHA (docosahexaenoic acid) were directly related to L. vannamei larval survival. González‐Félix et al 2003 demonstrated how HUFAs become more directly essential as L. vannamei juveniles enter the nursery stage. In conventional shrimp feeds, essential HUFAs/PUFAs are provided through ever-more expensive fish/krill oil. PNSB offer a sustainable avenue for recovering wasted/excreted feed and reconverting some of that mass back into Golden Fat capital.

Setting them far apart from other probiotic microbes, PNSB synthesize orange/red photosynthetic pigments known as carotenoids. Carotenoids, especially carotenes such as beta-carotene and astaxanthin, act as super antioxidants in that they aggressively absorb free electrons which would otherwise compromise the shrimp’s growth and health. Once consumed, carotenoids can be assimilated into shrimp tissue. This not only gives the shrimp their attractive red pigment but also exponentially increases the efficiency with which oxygen can translocate throughout its cells. Babin et al 2010 demonstrated how dietary carotenoids result in faster growth and increased immunocompetency in farmed crustaceans. The best part is that the carotenoid-rich shrimp benefit us when we eat them! Consumption of dietary carotenoids has been associated with improved cardiovascular and neurological health in humans.

There is also mounting evidence that PNSB such as R. palustris are part of the natural gut flora of many estuarine shrimp species and that live PNSB cells fed to farmed shrimp can occupy the gastrointestinal system of that crop, enhancing its overall resilience and productivity. All of these living PNSB cells release enzymes within the shrimp’s gut to enhance digestion and increase immune system competence. One of the most routine threats to shrimp aquaculture is vibrosis. This occurs when normally benign Vibrio bacteria strains exhibit pathogenic behavior; this is typically cued by excess levels of organic carbon/nitrogenous wastes or signs of shrimp stress/immuno-compromisation. PNSB such as R. palustris fight back against potentially pathogenic Vibrio strains by competing with them for space, carbon, nitrogen and other resources. Additionally, and perhaps more significantly, they produce antibiotic compounds such as streptomycin to which Vibrio is sensitive. In these ways, PNSB acts as a micro armies--feeding and fortifying their host whilst attacking its enemies.

Better yield, cleaner practice

One of the greatest woes of conventional shrimp ponds was how destructive they are to native ecosystems. Effluent released by a shrimp production pond during a routine tidal water change can ignite a harmful algal bloom and destabilize the surrounding wild waters. In a world where water is an ever more precious resource, PNSB are an incredible tool for stripping wastewater of its nutrients before it is expelled or further reclaimed for another production cycle. Biofloc production systems powered by PNSB biofilters offer a sustainable new direction for shrimp farming wherein water is utilized to its fullest extent and excess waste is processed by the farm rather than dumped into the natural environment.

The United States has a pressing mandate to produce more shrimp--both to fortify its domestic seafood supply as well as satisfy the pallets of true shrimp-loving patriots. However, it is obvious that 21st Century shrimp farming cannot depend on classical pond-based methods. Shrimp must be cultivated year-round in systems that require minimal water exchange. These systems must not only stabley house the shrimp but also recycle waste and prohibit pathogens. In short, biofloc production requires a biofilter that can do it all.

PNSB are able to aggressively consume excess nutrients and waste carbons, starving out Vibrio and other opportunistic bacteria in the process. Further, PNSB are able to utilize these wastes as they synthesize nutritious proteins, functional enzymes, antioxidizing carotenoids and golden fats. Their nutritious cells are feasted upon by ciliates, rotifers, protozoa and various other microconsortiom, eventually bringing their nutrition back into the gut of the shrimp. Numerous studies have demonstrated that R. palustris is a particularly effective probiotic that can resolve many of the confounding challenges awaiting the modern American Shrimp Farmer.

Literature cited

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Alloul, A., Wille, M., Lucenti, P., Bossier, P., Van Stappen, G., & Vlaeminck, S. E. (2021). Purple bacteria as added-value protein ingredient in shrimp feed: Penaeus vannamei growth performance, and tolerance against Vibrio and ammonia stress. Aquaculture, 530, 735788.

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