The secondary metabolites geosmin and 2-methylisoborneol (MIB) are known to taint fish with an undesirable, earthy-muddy taste and odor. In an earlier study on a zero-discharge recirculating aquaculture system (RAS), it was found that geosmin and MIB were removed by microbial communities residing in sludge from the digestion basin of the system. In the present study, 16S amplicon sequencing was used to identify changes in relative abundances of bacterial taxa in geosmin and MIB-enriched crude sludge. The removal of geosmin and MIB by the sludge was accompanied by increased abundances of 12 operational taxonomic units (OTUs). The most prominent increase in abundances was recorded for OTUs affiliated with bacterial genera known to harbor denitrifiers. Among these were the Betaproteobacteria genera Thauera, which utilizes terpenes to fuel denitrification, and Comamonas, which was previously isolated from the digestion basin of the same system and is capable of growth on geosmin and MIB as sole carbon and energy sources. Thus far, denitrification has been associated with bacteria capable of utilizing terpenes other than geosmin and MIB. The significant increase in the abundance of denitrifying bacterial genera in sludge in which geosmin and MIB comprised only 0.06% of the total carbon content might indicate that such bacteria play a major role in the removal of these compounds in anoxic environments. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Intensive pond cultivation of Gracilaria may achieve maximal yields by enrichment of seawater with ammonia and CO2. In the present study, we examined the use of organic waste material as a source of ammonia and inorganic carbon. A biofiltration system was constructed, consisting of several fermentation tanks and a fluidized bed reactor for the required bioconversion of organic wastes. Gracilaria conferta was cultured in a 30m2 pond from which seawater was circulated through the biofiltration system for a period of nine months. As compared to the filtered seawater used for water supply, the outlets of the fermentation tanks and the fluidized bed reactor showed significant increases in dissolved inorganic carbon and ammonia concentrations and significant decreases in oxygen concentrations and redox potentials. The most efficient organic waste compound tested had the highest C/N ratio. No significant differences were found between the Gracilaria yields of the control and biofiltration pond systems. These results might contribute a fundamental improvement in the economy of Gracilaria pond cultivation by water recycling through such a biofiltration system. Statement of relevance The system might contribute a fundamental improvement in the economy of Gracilaria pond cultivation.

The accumulation of geosmin and 2-methylisoborneol in culture water and fish is a common problem encountered in recirculating aquaculture systems (RAS). In these systems, like in other aquaculture systems, abatement of this problem is usually conducted by purging the fish with clean water prior to their marketing. In indoor RAS, mainly heterotrophic prokaryotes underlie the production of geosmin and MIB. While production of these off flavor compounds has been linked with the organic-rich parts of these systems, no further information on factors that promote the growth of the geosmin and MIB-producing microorganisms under these conditions is currently available. Thus far, geosmin and MIB removal from RAS has mainly been conducted by ozonation, albeit with limited success. Biodegradation of geosmin and MIB might serve as an additional method for removal of these compounds from RAS. Geosmin and MIB degradation has been detected in the latter systems, yet factors which promote the activity of geosmin and MIB degraders in these systems remain largely unknown. In the present review, the current knowledge on geosmin and MIB production and removal in RAS is presented. Emphasis is placed on the biodegradation of these off-flavor compounds, a process which has received little attention thus far.

Recirculating aquaculture systems (RAS), offering many economic and fish husbandry benefits, are characterized by an accumulation of dissolved organic matter (DOM) and, specifically, humic substances (HS). As reported in a number of studies, HS may affect biological activity in both invertebrates and vertebrates. Given the accumulation of HS in RAS, it is therefore of great interest to characterize DOM and, specifically, its HS fraction in the RAS. The present study was aimed at characterizing long-term changes in fluorescent DOM composition in the culture water of RAS systems, which were operated in a novel, zero water exchange mode. Two such zero-discharge recirculating systems (ZDS) were examined: a freshwater system, stocked with hybrid tilapia (Oreochromis aureus x Oreochromis niloticus) and a marine system, stocked with gilthead seabream (Sparus aurata). Excitation-emission matrices (EEMs) of fluorescence, coupled with parallel factor analysis (PARAFAC), were used to characterize and quantify the different DOM components in the ZDS. In the culture water, one tryptophan-like and four HS-like components were identified. The fluorescence intensities of three of the HS-like components as well as the tryptophan-like component increased at comparable rates during ZDS operation while a much slower accumulation of these compounds was observed in a parallel operated, flow-through, freshwater aquarium. The ZDS examined in this study comprised a sludge digestion stage where a considerable accumulation of all fluorescent components was detected. A HS-like components and a tryptophan-like component in blood of tilapia from the freshwater ZDS were similar to components found in the culture water. Blood levels of both components were higher in fish cultured in the DOM-rich ZDS than in fish raised in the control, flow-through freshwater aquarium. Fluorescence of the HS-like component found in the fish blood increased also with time of ZDS operation. The finding that fish blood contains a HS-like fluorescent component may have important implications for the understanding of the physiological effects of HS in fish and the possible benefits of these substances in aquaculture.

Off-flavor in fish poses a serious threat for the aquaculture industry. In the present study, removal of 2-methylisoborneol (MIB), an off-flavor causing compound, was found to be mediated by adsorption and bacterial degradation in sludge derived from an aquaculture system. A numerical model was developed which augmented Langmuir equations of kinetics of adsorption/desorption of MIB with first order degradation kinetics. When laboratory-scale reactors, containing sludge from the aquaculture system, were operated in a recirculating mode, MIB in solution was depleted to undetectable levels within 6 days in reactors with untreated sludge, while its depletion was incomplete in reactors with sterilized sludge. When operated in an open flow mode, removal of MIB was significantly faster in reactors with untreated sludge. Efficient MIB removal was evident under various conditions, including ambient MIB levels, flow velocities and sludge loads. When operated in an open flow mode, the model successfully predicted steady MIB removal rates with time. During steady state conditions, most of the MIB removal was found to be due to microbial degradation of the adsorbed MIB. Findings obtained in this study can be used in the design of reactors for removal of off-flavor compounds from recirculating aquaculture systems.

The exposure of guppy fish (Poecilia reticulata), infected with the monogenea Gyrodactylus turnbulli and Dactylogyrus sp. to humic-rich culture water and feed, reduced both the infection prevalence (% of infected fish) and the infection intensity (parasites per fish) of the two parasites. Specifically, among fish exposed to: (a) humic-rich water and sludge from a recirculating system (RAS) and (b) synthetic humic acid (HA), infection prevalences of Gyrodactylus turnbulli were 17% and 25% respectively, as compared with an infection prevalence of 52% in the control group. The lower infection prevalence was accompanied by a significant reduction in the infection intensity: from 3.8 in the control group to 0.2 and 0.3 parasites per fish in the RAS and HA treated fish respectively. The infection prevalence and intensity of Dactylogyrus sp. were significantly lower (infection prevalence: 2.5%; infection intensity: 0.3 parasites/fish) in guppies exposed to RAS water and sludge than in the control group (infection prevalence: 50%; infection intensity: 0.8 parasites/fish).

Abstract When challenged with atypical Aeromonas salmonicida subsp. salmonicida, exposure of the common carp (Cyprinus carpio L.) to different humic-rich compounds resulted in a significant reduction in infection rates. Specifically, in fish exposed to (i) humic-rich water and sludge from a recirculating system, (ii) a synthetic humic acid, and (iii) a Leonardite-derived humic-rich extract, infection rates were reduced to 14.9%, 17.0% and 18.8%, respectively, as compared to a 46.8% infection rate in the control treatment. An additional set of experiments was performed to examine the effect of humic-rich components on the growth of the bacterial pathogen. Liquid culture medium supplemented with either humic-rich water from the recirculating system, the synthetic humic acid or the Leonardite humic-rich extract resulted in a growth reduction of 41.1%, 45.2% and 61.6%, respectively, as compared to the growth of the Aeromonas strain in medium devoid of humic substances. Finally, in a third set of experiments it was found that while the innate immune system of the carps was not affected by their exposure to humic-rich substances, their acquired immune system was affected. Fish, immunized against bovine serum albumin, displayed elevated antibody titres as compared to immunized carps which were not exposed to the various sources of humic substances.

BackgroundFuran fatty acids (F-acids) are valuable minor fatty acids which are appraised for their protective role against lipid oxidation of polyunsaturated fatty acids (PUFAs). The most relevant dietary source for F-acids is fish with the predominant occurrence of up to five dimethyl- or monomethyl-substituted homologues. During the screening of fish from a zero discharge aquaculture (ZDA) system we noted the potential presence of unusual F-acids. Methods We developed a method by gas chromatography with mass spectrometry operated in the selected ion monitoring mode for elucidation of the structures of the uncommon F-acids. Results Carp from the ZDA system contained seven non-methylated F-acids with dominance of 8-(5-hexylfuran-2-yl)-octanoic acid (8F6). Non-methylated F-acids have never been detected before in fish. Subsequent analysis of other fish species and a batch of the fish feed confirmed the presence of non-methylated F-acids. Conclusions F-acids in fish are derived from the feed. Our investigation indicates that more emphasis should be put on the F-acid concentrations in fish from aquaculture, which appears to depend on the quality of the fish feed.

Reuse of grey water (GW) enables to reduce fresh water consumption, but a treatment is required to prevent potential transmission and propagation of pathogenic organisms. This study presents results on the removal of pathogenic bacteria from GW as well as reduction of turbidity, TSS COD, and BOD by a novel treatment system. Compared to previous studied methods, three new elements are presented in the current treatment of GW: (1) A granulated complex of micelles of the organic cation octadecyltrimethylammonium (ODTMA) with montmorillonite was employed in filtration of GW. This complex was efficient in purifying GW due to its large surface area, positive charge and existence of hydrophobic domains. The granulated complex enabled flow when present exclusively in the filter; (2). A moving bed reactor for decomposition of part of the organic matter in the GW. This pretreatment stage, prior to the micelle-clay filter, was also efficient in removing pathogenic bacteria; (3) A regeneration stage of the micelle-clay filter conducted by passing either dilute solutions of Na-hypochlorite or HCl through the micelle-clay complex, or by heating the complex. Incubation of GW for either two weeks or one day in the pretreatment stage yielded a 10- and 7-fold enhancement in the volume filtered, which did not contain fecal coliforms, i.e., 300 and 210L for 40g of complex, respectively. The capacity of purified volume per gram of the complex increased further several-fold (>23L/g) for filters filled exclusively with granules. Regeneration of the complex in the filter further enhanced the capacity.