Mangrove forest protection is an essential solution for mitigating the impacts of natural disasters in coastal zones and climate change. Integrated mangrove-shrimp fanning (IMSF) system has been promoted as a sustainable livelihood that can provide income for farmers and protect mangrove forests. However, the productivity of shrimp is limited. Therefore, to enhance the revenue for farmers, improving the value of shrimp products is a good option. Organic shrimp fanning practices following the EU aquaculture organic standards have been previously applied in some areas of the Mekong delta. This study was conducted to compare technical, financial characteristics and environmental parameters between the applied (i.e., organic farms) and non-applied (i.e., non-organic farms) standards of Naturland, aiming to support the development of ecological shrimp farming and contribute towards green development. The study was carried out in Nhung Mien mangrove forest, Ngoc Hien district, Ca Mau province, Vietnam. Fifty organic farms and 50 non-organic farms were directly interviewed using structured questionnaires. And then, three farms in each system were selected for monitoring water quality. Results showed that the average mangrove coverage was 54.1% in the organic IMSF system and significantly different from the non-organic IMSF system (p<0.05). Total shrimp yield, total income and total profits tended to increase in organic IMSF system and the selling shrimp price increased by 10% compared to the conventional price. The study showed that following organic fanning methods could provide higher income for farmers and a better chance to mitigate natural disasters and climate change impacts.

Halophilic bacteria have adapted to survive in high-salinity environments by accumulating amino acids and their derivatives as organic osmolytes. L-Proline (Pro) is one such osmolyte that is also being used as a feed stimulant in the aquaculture industry. Halomonas elongata OUT30018 is a moderately halophilic bacterium that accumulates ectoine (Ect), but not Pro, as an osmolyte. Due to its ability to utilize diverse biomass-derived carbon and nitrogen sources for growth, H. elongata OUT30018 is used in this work to create a strain that overproduces Pro, which could be used as a sustainable Pro-rich feed additive. To achieve this, we replaced the coding region of H. elongata OUT30018’s Ect biosynthetic operon with the artificial self-cloned proB _m1 AC gene cluster that encodes the Pro biosynthetic enzymes: feedback-inhibition insensitive mutant γ-glutamate kinase (γ-GK ^D118N/D119N ), γ-glutamyl phosphate reductase, and pyrroline-5-carboxylate reductase. Additionally, the putA gene, which encodes the key enzyme of Pro catabolism, was deleted from the genome to generate H. elongata HN6. While the Ect-deficient H. elongata KA1 could not grow in minimal media containing more than 4% NaCl, H. elongata HN6 thrived in the medium containing 8% NaCl by accumulating Pro in the cell instead of Ect, reaching a concentration of 353.1 ± 40.5 µmol/g cell fresh weight, comparable to the Ect accumulated in H. elongata OUT30018 in response to salt stress. With its genetic background, H. elongata HN6 has the potential to be developed into a Pro-rich cell factory for upcycling biomass waste into single-cell feed additives, contributing to a more sustainable aquaculture industry. IMPORTANCE: We report here the evidence for de novo biosynthesis of Pro to be used as a major osmolyte in an ectoine-deficient Halomonas elongata . Remarkably, the concentration of Pro accumulated in H. elongata HN6 ( ∆ectABC::mCherry-proB _m1 AC ∆putA ) is comparable to that of ectoine accumulated in H. elongata OUT30018 in response to high-salinity stress. We also found that among the two γ-glutamate kinase mutants (γ-GK ^D118N/D119N and γ-GK ^D154A/E155A ) designed to resemble the two known Escherichia coli feedback-inhibition insensitive γ-GK ^D107N and γ-GK ^E143A , the γ-GK ^D118N/D119N mutant is the only one that became insensitive to feedback inhibition by Pro in H. elongata . As Pro is one of the essential feed additives for the poultry and aquaculture industries, the genetic makeup of the engineered H. elongata HN6 would allow for the sustainable upcycling of high-salinity waste biomass into a Pro-rich single-cell eco-feed.

Livestock production plays a leading role in agricultural land-use change. Producing biogas from livestock waste and subsequently using the biogas effluent as fertilizer for crops is a promising option to solve environmental problems resulting from expanding livestock production. However, it is difficult to promptly and accurately measure the nitrogen (N) concentration of effluent for farmers in developing countries, making precise N management difficult. The objectives of the current study were (1) to evaluate the feasibility of variable-timing, fixed-rate application of cattle biogas effluent using a leaf color chart (LCC) for rice (Oryza sativa L.) and (2) to determine the optimum LCC threshold for grain yield. We conducted two microcosm experiments in the Mekong Delta of Vietnam in 2018 using eight treatments of N-fertilizer application. In the Zero treatment, we applied no N. In the Estd treatment, we split-applied N as effluent (E) at fixed rate and timing as the standard method. In E2.75, E3.00, E3.25, E3.50, and E3.75, we applied effluent whenever the LCC value went below 2.75, 3.00, 3.25, 3.50, and 3.75, respectively. In U3.25, we applied N as urea (U) whenever the LCC value fell below 3.25. The total effluent-N application rate ranged from 90 to 210 kg N ha⁻¹ season⁻¹. Rice growth was normal but there was a substantial yield gap between the two microcosm experiments due to the seasonal difference in solar radiation. Rice yield tended to increase with increasing LCC threshold. There was a positive linear relationship between LCC and chlorophyll content (SPAD) values (R² = 0.73–0.79). Grain yield was well explained (R² = 0.70–0.89) by the seasonal mean LCC or SPAD value. Plant total N uptake increased with increasing LCC threshold, but the three calculated indices of N use efficiency (NUE) – apparent N recovery, agronomic NUE, and internal NUE – were not always improved with a higher LCC threshold. Our results showed that the tested variable-timing, fixed-rate strategy for the application of cattle biogas effluent was feasible and the optimum LCC threshold for grain production was 3.75 under the current microcosm conditions.

This study evaluated biogas production potentials from local biomass feedstocks comprising of rice straw (RS), steamed lemongrass (SL), bagasse (BA) and desiccated coconut (DC) on super-intensive shrimp sludge (SS) anaerobic digestion. A series of batch anaerobic digestion experiments was conducted at an organic loading rate of 50 g-VS L-1 and a C/N ratio of 25 under mesophilic conditions. The results indicate that co-digested biomass substrates are more suitable than single sludge except for DC supplementation, which exhibited a severe pH inhibition for methanogenesis activities. A reactor supplemented with BA achieved the highest overall biogas production (126.78 L kg-VSadded-1), which increased biogas yields 53.70% compared to a mono-sludge reactor. Furthermore, reactors with RS and SL increased biogas yields by 26.40% and 29.21%, respectively. Irrespective of initial materials, the H2S concentration in biogas compositions was measured at very high levels (23,710 - 65,040 ppm) after 10-15 days of digestion, while a decreasing trend was recorded for the remaining digestion period (16 - 60 days), yet still maintained relatively high levels (5,873 - 9,155 ppm). The study suggests that future works should focus on pH neutralization within the reactor with DC substrates and H2S removal in biogas composition.

Water hyacinth (WH) biomass is one of the popular materials in the Vietnamese Mekong Delta, a potential substrate for biogas production. The effectiveness of utilizing WH for producing biogas under anaerobic digestion was demonstrated in the previous studies, but the research was focused on the...

Biogas effluent (BE) is a potential source of fertilizer for rice. Alternate wetting and drying (AWD) irrigation based on surface water level can increase rice grain yield (GY), but its effect under BE application is still unclear. To test whether AWD can increase GY under BE application, we conducted a mesocosm experiment under screen-house conditions in Can Tho, Vietnam. We measured rice growth and yield under three nitrogen (N) treatments—zero-N (Z), synthetic fertilizer (SF), and BE—and two irrigation regimes—continuous flooding (CF) and AWD with a re-irrigation threshold of 15 cm below the soil surface. Chlorophyll content (SPAD) and leaf color chart (LCC) values were higher in SF than in BE, but plant height was comparable. Although GY was not significantly different between CF and AWD, AWD increased it by 12% in BE relative to CF. All N use efficiency (NUE) indices (apparent N recovery, agronomic NUE, and internal NUE) were comparable between AWD and CF in BE and SF. The results indicate that AWD irrigation is feasible under the application of cattle BE and the combination can improve rice GY.