Microencapsulated algal feeds as a sustainable replacement diet for broodstock in commercial bivalve aquaculture

By: David F . Willer, Samuel F urse & David C . Aldridge *

The use of live algae is driving excessive and unsustainable energy and resource use in bivalve production. Since the 1990s, the bivalve aquaculture industry has been seeking non-live alternative feeds to reduce its need for antibiotics. FAO and EU sustainable aquaculture policies identify urgent and immediate needs to reduce land, energy, and antibiotic use. Advances in algal production and microencapsulation technology offer a groundbreaking solution to reduce the environmental footprint of bivalve aquaculture. This investigation demonstrated that sustainable Schizochytrium-based microencapsulated diets could help support more effective sexual maturation in oyster broodstock.

The USD 17.2 billion global bivalve shellfish industry relies upon a supply of juvenile bivalves produced by broodstock in hatcheries. Current estimates suggest that 220 million broodstock bivalves are held in hatcheries worldwide and are usually fed with live algae, which its production drives unsustainable land, energy, and antibiotic use.

“Novel microcapsules are an ideal vehicle for delivering Schizochytriumv to broodstock. Mass production is simple and cost-effective, and the microcapsules dry and shelf-stable, thus circumventing conventional feed wastage costs.”

Artificial lighting, temperature, and air control systems are needed to support algal growth. Furthermore, algal stocks are difficult to maintain and frequently lost due to contamination and disease, so greater quantities must be produced, accounting for 50% of bivalve production costs at USD 220 kg-1 algal biomass in 2016.

Live algal feeds are the primary vector of bivalve disease, which is controlled with antibiotics. Antibiotics cause severe damage to marine ecosystems; in the world’s largest bivalve producers, no veterinary prescriptions are required for antibiotics with use essentially unregulated. Non-live diets are more sterile, and hence since the 1990s, the bivalve aquaculture industry has been seeking non-live alternative feeds to reduce antibiotic needs. FAO and EU sustainable aquaculture policies identify urgent and immediate needs to reduce land, energy, and antibiotic use.

Advances in algal production and microencapsulation technology offer a groundbreaking solution to reduce the environmental footprint of bivalve aquaculture. Schizochytrium algae can be grown heterotrophically on industrial scales at USD $1.50 kg-1, using low-cost food waste and agricultural side-streams as inputs.

For bivalve nutrition, Schizochytrium has advantages, with levels of key nutrients such as docosahexaenoic acid (DHA) exceeding 20% dry-weight: greater than twice the abundance of DHA in hatchery-grown algae. Novel microcapsules are an ideal vehicle for delivering Schizochytriumv to broodstock. Mass production is simple and cost-effective, and the microcapsules dry and shelf-stable, thus circumventing conventional feed wastage costs.

“Other advantages of replacing live algal feeds with microencapsulated feeds in bivalve aquaculture include 20-fold reductions in energy use, carbon emissions, and production costs.”

Capsule characteristics can be tailored to maximize feeding efficiency and minimize nutrient leaching to water while also being sterile and not a disease vector. The nutritional profile of microencapsulated feed compared to conventional algal feed is shown in Table 1.

Nutritional composition of broodstock diets: live algae, microcapsules, or live algae + microcapsules. All values are in g per 100 g dry weight (dw). Nutritional data is sourced directly from the manufacturer—for live algae SeaSalter Shellfish (Whistable) Ltd, for microcapsules BioBullets Ltd, and for the combined diet a mean is used.

Other advantages of replacing live algal feeds with microencapsulated feeds in bivalve aquaculture include 20-fold reductions in energy use, carbon emissions, and production costs (see Figure 1). However, for the replacement to be commercially viable, it is critical to assess whether microencapsulated feeds provide comparable sexual development in bivalve broodstock compared to conventional algal feeds. 

Results

Sexual maturation: gonad weight. We first tested the impact of replacing live algal diets with microencapsulated Schizochytrium diets on oyster gonad weight. Both microencapsulated and live algal diets resulted in greater gonad weight European oysters (O. edulis) following a 6-week broodstock conditioning period in a commercial hatchery.

“The waxy encapsulant minimizes pre-ingestive nutrient loss by allowing particles to remain stable and retain nutrients in seawater yet still be rapidly digested on entry to the bivalve gut.”

Mean wet gonad weight was significantly greater in oysters fed algae, microcapsules, or algae + microcapsules compared to oysters pre-conditioning. Gonad weight was greatest in oysters fed with microcapsules, although this value was not significantly different from oysters fed with algae.

Sexual maturation: fatty acid and lipid abundance.

Demonstration that microencapsulated Schizochytrium diets could facilitate comparable or greater increases in gonad mass than live algal diets provided macroscopic evidence that microencapsulated diets could be an effective replacement for live algae sexual maturation. We hence embarked upon a molecular investigation of gonad lipids and gametogenesis to provide further explanation.

“For a replacement to be commercially viable, it is critical to assess whether microencapsulated feeds provide comparable sexual development in bivalve broodstock compared to conventional algal feeds.”

Mass spectrometry was used to determine the abundance and profile of fatty acids. These data showed that fatty acid mass in O. edulis gonads was greater post-conditioning compared to pre-conditioning. The greatest differences in abundance (over 400 ‰) were present in 16:0, 18:0, 18:1, 20:5 (EPA), and 22:6 (DHA) fatty acids (see Supplementary Information Table S1a in original article).

The greatest difference in fatty acid abundance relative to the pre-conditioning controls was seen in oysters fed only microcapsules. For 40 of the 45 fatty acids, the difference was significantly greater for microcapsule-fed oysters than algae-fed oysters. In particular, the difference in 20:5 and 22:6 fatty acids was 12 times greater.

The post-conditioning difference in fatty acid abundance was also significantly greater in oysters fed only microcapsules compared to oysters fed algae + microcapsules in 41 of 45 cases. 

“This investigation demonstrates that Schizochytrium-based microencapsulated diets enable not only comparable but improved sexual maturation in oyster broodstock compared with conventional live algal diets.”

The abundance of other lipids in O. edulis gonads was also greater post-conditioning compared to pre-conditioning. However, there was only a significant difference between diets for 30 of the 792 lipids assessed. The significantly greater post-conditioning abundance of fatty acids and lipids in oysters fed microcapsules provide a biochemical explanation to our initial finding of strong increases in gonad mass in oysters fed microcapsules. We can again assume that the greater abundance represents an increase over time relative to the pre-conditioning controls.

Sexual maturation: histology. Histological imaging of the oyster gonads revealed that oysters fed either microcapsules or algae + microcapsules were at a more advanced stage of sexual maturation after 6-weeks of conditioning than oysters fed algae alone. Oysters fed algae alone had progressed from having largely inactive gonads to advanced spermatogenesis, with follicles filled with spermatogonia and spermatocytes. In contrast, oysters fed microcapsules in addition to or in replacement of algae had reached full maturity and had dense follicles with many spermatids.

Discussion

Microencapsulated diets enable improved sexual maturation in oysters. Our investigations demonstrate that Schizochytrium-based microencapsulated diets enable not only comparable but improved sexual maturation in oyster broodstock compared with conventional live algal diets. The gonads of oysters fed microencapsulated diets were of greater weight, contained higher levels of omega-3 fatty acids crucial for sexual maturation, and underwent accelerated spermatogenesis.

The significant increases in EPA and DHA in microcapsule relative to algae-fed oysters are especially meaningful. EPA is the primary energy source for gamete maturation, with higher levels directly increasing gamete quantity and development rate. DHA is pivotal to the structure and function of gamete cell membranes, with higher levels increasing gamete quality and egg survival rates. High levels of EPA and DHA in the Schizochytrium-based microcapsules are likely driving this increase.

“To date, there is no clear evidence that either fatty acid can be synthesized de-novo by oysters from shorter chain precursors. The more rapid advance of spermatogenesis in oysters fed a microencapsulated diet is highly likely driven by the greater levels of EPA and DHA in these animals, a causal relationship demonstrated by several previous studies. This offers strong support for the use of microcapsules as a broodstock conditioning feed.”

It is important to consider that for future application, the nutritional formulation of the microcapsules would need to be tailored further for increased protein content or fed alongside a quantity of live algae. The current protein content of the microcapsules is lower than that of live algae (6 vs. 31 g protein per 100 g dry weight, see Table 1). Protein is important in bivalve larval development and for shell formation, and if it is insufficient juvenile growth can be suppressed. There would be significant value in performing additional studies investigating the effectiveness of a higher protein formulation of microcapsules on bivalve broodstock conditioning and juvenile development.

Regarding the newly developed microcapsules, the size is tailored to maximize bivalve feeding efficiency (20–140 µm diameter) and buoyancy neutral to ensure particles remain within reach of the filter feeders. This is an improvement over a basic freeze-dried algal powder delivery system. Powders tend to float on the water surface and can clump into particles too large to be accessed by bivalves.

The waxy encapsulant minimizes pre-ingestive nutrient loss by allowing particles to remain stable and retain nutrients in seawater yet still be rapidly digested on entry to the bivalve gut. The specialized coating allows minimal leaching to the surrounding water, reducing eutrophication risks. The encapsulant also has strong antibacterial properties, and contents are sterile, which reduces disease incidence in aquaculture relative to live feeds and enables reduced antibiotic usage.

“Sustainability and commercial implications. The use of live algae is driving excessive and unsustainable energy and resource use in bivalve production. This investigation demonstrated that sustainable Schizochytrium-based microencapsulated diets could help support more effective sexual maturation in oyster broodstock.”

However, given that microencapsulated diets enabled comparable and improved sexual development in bivalve broodstock, there is potential to reap even further commercial and sustainability benefits.

Higher quality broodstock with greater lipid stores directly translates into a higher quality seed with a greater inherent survival rate. More rapid sexual maturation enables seed production earlier in the season, giving the seed a greater growing period before their first overwintering.

The corresponding greater size and cold tolerance again increase survival. Increased sexual maturation rates also mean shorter conditioning cycles and greater larvae and seed production for a given hatchery each year. As the supply of bivalve seed is one of the biggest factors limiting the growth of the bivalve industry, with demand far outstripping supply, microencapsulated feeds could play an important role in enabling the bivalve industry to expand.

Bivalve aquaculture is far more environmentally sustainable than other aquaculture and meat production forms and even some cereal crops. We spare 9 ha of land, 67 tonnes of CO2 emissions, and 40,000 L of freshwater for every new ton of protein produced from bivalve instead of fish aquaculture. Any technology, such as microencapsulated diets that might enable bivalve aquaculture to grow instead of other aquaculture, should be viewed as of great benefit and a worthwhile recipient of further research and industry attention.

Methods 

Microcapsule manufacture. Lipid-walled microcapsules containing 50% powdered Schizochytrium algae by weight were manufactured under patent by BioBullets (BioBullets Ltd, Cambridge, UK). To manufacture the particles, a premix slurry containing a waxy encapsulant with antibacterial properties and powdered algae was prepared under controlled shear conditions. The slurry was pumped into an ultrasonic atomizing nozzle at the top of a cooling chamber. The atomized particles formed near-perfect spheres as they cooled and fell to the chamber base.

Further particle cooling was achieved with an air-conveying system before discharge via cyclone to a fluid bed processor. The encapsulated particles were then coated with a proprietary non-ionic surfactant to aid dispersion in water. Further cooling in the fluid bed removed all heat of crystallization from the microparticles before packaging. All components of the formulation were food grade. The final microcapsules had a diameter between 20 and 140 µm, spherical shape, and near-neutral buoyancy.

Broodstock conditioning. Conditioning experiments on O. edulis broodstock took place under commercial production conditions in England. Experiments took place over 6-weeks and were carried out in three 25 L aerated flow-through tanks kept at ambient hatchery temperatures (18–24 °C) and salinities (26–28 ‰), offering further commercial context to our experiments. Each tank contained 15 O. edulis broodstock and received one of the following three diets: live algae, microcapsules (BioBullets), or algae + microcapsules. Each feed was fed at 3% daily, considering dw for food and broodstock. It means oysters on the algae + microcapsules diet received twice as much food as oysters on the single food diets.

“The 3% ratio is recommended as an additional algal ratio above this value has been shown to have little effect on O. edulis nutrient uptake. Feed was delivered using a continuous system. At the end of the 6-week conditioning period all broodstock were frozen and transported to the Department of Zoology, University of Cambridge, England, where they were frozen at −80°C. Before the conditioning period began, an additional sample of 15 broodstock was also frozen for use as pre-conditioning controls.”

Gonad weight analysis. The entire gonad mass was dissected from nine oysters from each diet and the control sample, keeping the samples below 0 °C on dry ice. Gonad wet weight for each oyster was measured to a precision of±1 mg. Gonad tissue was stored at −80 °C.

Fatty acid and lipid analyses. Reagents, Extraction of the lipid fraction, and Mass spectrometry are fully described in the original publication.

Sexual maturation analyses. To assess sexual maturation, all gonadal tissue was dissected from 5 oysters from each diet and the control. Tissue was fixed, sectioned, stained using hematoxylin and eosin, and imaged under light microscopy following a standard protocol.

Data processing and statistical analyses. Methods are fully described in the original publication, cited at the end of this article.

*This is a summarized version developed by Ph.D. Carlos Rangel Dávalos, researcher and professor at the University of Baja California Sur México from the article “Microencapsulated algal feeds as a sustainable replacement diet for broodstock in commercial bivalve aquaculture” written by David F . Willer, Samuel F urse, and  David C . Aldridge that was originally published on 2020 through the Journal of Scientific Reports of Nature Research (10:12577) under a creative commons 4.0 open access license. 

The original version can be accessed online at: https://doi.org/10.1038/S41598-020-69645-0

Correspondence email: dw460@cam.ac.uk