The search for novel compounds of marine origin has increased in the last decades for their application in various areas such as pharmaceutical, human or animal nutrition, cosmetics or bioenergy. In this context of blue technology development, microalgae are of particular interest due to their immense biodiversity and their relatively simple growth needs. In this review, we discuss about the promising use of microalgae and microalgal compounds as sources of natural antibiotics against human pathogens but also about their potential to limit microbial infections in aquaculture. An alternative to conventional antibiotics is needed as the microbial resistance to these drugs is increasing in humans and animals. Furthermore, using natural antibiotics for livestock could meet the consumer demand to avoid chemicals in food, would support a sustainable aquaculture and present the advantage of being environmentally friendly. Using natural and renewable microalgal compounds is still in its early days, but considering the important research development and rapid improvement in culture, extraction and purification processes, the valorization of microalgae will surely extend in the future.

Sexual reproduction is an obligatory phase in the life cycle of most diatoms, as cell size decreases with successive vegetative divisions and the max

A new species of raphid pennate diatom producing a blue pigment, Haslea karadagensis Davidovich, Gastineau & Mouget, sp. nov., was recently isolated from the Crimean coast of the Black Sea (Ukraine). This organism is very similar to the well-known Haslea ostrearia, the first described ‘blue’ diatom, which produces marennine, the pigment involved in the greening of oysters. The Ukrainian diatom, H. karadagensis, differs slightly from H. ostrearia in the structure of its frustule, and the two organisms are unable to interbreed. Two molecular markers, rbcL and the ITS1–5.8S–ITS2 sequences, showed 2% and >50% differences, respectively, between the two species. UV-visible spectrophotometry and in vivo confocal micro-Raman spectroscopy were used to compare the pigment of H. karadagensis with marennine. Both pigments showed absorption bands in the UV and red regions, but the positions of the maxima differ between the pigments. Significant differences were observed by micro-Raman spectroscopy in the 1000–1700 cm⁻¹ wavenumber range, revealing that the pigments are different molecules. Haslea karadagensis is the first example of a new ‘blue’ diatom and produces a novel blue pigment.