In search of alternative and safer sources of collagen for biomedical applications, the marine demosponges Axinella cannabina and Suberites carnosus, collected from the Aegean and the Ionian Seas, respectively, were comparatively studied for their insoluble collagen, intercellular collagen, and spongin-like collagen content. The isolated collagenous materials were morphologically, physicochemically, and biophysically characterized. Using scanning electron microscopy and transmission electron microscopy the fibrous morphology of the isolated collagens was confirmed, whereas the amino acid analysis, in conjunction with infrared spectroscopy studies, verified the characteristic for the collagen amino acid profile and its secondary structure. Furthermore, the isoelectric point and thermal behavior were determined by titration and differential scanning calorimetry, in combination with circular dichroism spectroscopic studies, respectively.

Aquaponics provides an alternative opportunity for the combined production of fish and plants. Most of the essential nutrients required for optimal plant growth can be supplied from the fish feed, except for K and Fe. These nutrients are usually inadequate in fish feed. In this study, red tilapia and rocket plants were co-cultivated in an aquaponics system along with the extra addition of K and Fe. Fish growth, morpho-anatomical characteristics, and K and Fe loading on fish gills were studied. Plant growth parameters were also determined. The addition of Fe and K slightly improved the produced fresh biomass of rocket per unit area and had no impact on tilapia growth and survival. No severe histological alterations in fish gills, liver, and midgut were detected. EDX analysis showed that the addition of K led to the enrichment of this specific ion in fish gills, but no effects of this accumulation were found on other aspects of fish growth and survival.

The effects on liver and intestinal histomorphology and on intestinal microbiota in gilthead seabream (Sparus aurata) fed diets that contained poultry by-product meal (PBM) and hydrolyzed feather meal (HFM) as fishmeal replacements were studied. Fish fed on a series of isonitrogenous and isoenergetic diets, where fishmeal protein of the control diet (FM diet) was replaced by either PBM or by HFM at 25%, 50% and 100% without amino acid supplementation (PBM25, PBM50, PBM100, HFM25, HFM50 and HFM100 diets) or supplemented with lysine and methionine (PBM25+, PBM50+, HFM25+ and HFM50+ diets). The use of PBM and HFM at 25% fishmeal replacement generated a similar hepatic histomorphology to FM-fed fish, indicating that both land animal proteins are highly digestible at low FM replacement levels. However, 50% and 100% FM replacement levels by either PBM or HFM resulted in pronounced hepatic alterations in fish with the latter causing more severe degradation of the liver. Dietary amino acid supplementation delivered an improved tissue histology signifying their importance at high FM replacement levels. Intestinal microbiota was dominated by Proteobacteria (58.8%) and Actinobacteria (32.4%) in all dietary groups, but no specific pattern was observed among them at any taxonomic level. This finding was probably driven by the high inter-individual variability observed.

The occurrence of heavy cyanobacterial blooms has become a worldwide problem, as a consequence of eutrophication of the aquatic ecosystems; furthermore, 60% to 75% of these blooms have been found to be toxic. Microcystins (MCYSTs), the predominant toxins of cyanobacterial blooms, are associated with mortality and illness in both animals and humans. Laboratory-controlled experiments studying the effects of different microcystins on the common carp (Cyprinus carpio) have revealed various histopathological alterations. The aim of the present study is to investigate the effect of chronic or subchronic exposure of fish to microcystins under natural environmental conditions by examining the possible histopathological changes associated with a dense cyanobacterial bloom and determining the microcystin contents of fish tissues. Common carps (C. carpio) were caught from Lake Karla (Greece), during a dense cyanobacterial bloom. The concentration of MCYSTs in the fish liver, kidney and muscle tissues was measured by enzyme-linked immunosorbent assay. The pseudogaster contents were analysed, and a histopathological examination was performed using light and electron microscopy. Severe alterations were detected in the liver and the kidney, suggesting that the toxic effects were caused by various pollutants that were particularly associated with microcystins. The histopathological findings are also discussed, taking into consideration the health conditions of the common carp as a commercial fish species. The mechanisms of expansion of the microcystins and the poisoning of aquatic organisms (e.g. fish) are not yet known in the Lake Karla ecosystem. Future research may focus on identifying the changes caused by microcystins and other factors that exert similar effects on fish tissues, as well as on establishing the overall combined effect of all these factors on fish health.