Microalgae are increasingly recognised as suitable microorganisms for heavy metal (HM) removal, since they are able to adsorb them onto their cell wall and, in some cases, compartmentalise them inside organelles. However, at relatively high HM concentrations, they could also show signs of stress, such as organelle impairments and increased activities of antioxidant enzymes. The main aim of this review is to report on the mechanisms adopted by microalgae to counteract detrimental effects of high copper (Cu) concentrations, and on the microalgal potential for Cu bioremediation of aquatic environments. Studying the delicate balance between beneficial and detrimental effects of Cu on microalgae is of particular relevance as this metal is widely present in aquatic environments facing industrial discharges. This metal often induces chloroplast functioning impairment, generation of reactive oxygen species (ROS) and growth rate reduction in a dose-dependent manner. However, microalgae also possess proteins and small molecules with protective role against Cu and, in general, metal stress, which increase their resistance towards these pollutants. Our critical literature analysis reveals that microalgae can be suitable indicators of Cu pollution in aquatic environments, and could also be considered as components of eco-sustainable devices for HM bioremediation in association with other organisms.

Diatoms are unicellular eukaryotic microalgae widely distributed in aquatic environments, possessing a porous silica cell wall known as frustule. Diatom frustules are considered as a sustainable source for several industrial applications because of their high biocompatibility and the easiness of surface functionalisation, which make frustules suitable for regenerative medicine and as drug carriers. Frustules are made of hydrated silica, and can be extracted and purified both from living and fossil diatoms using acid treatments or high temperatures. Biosilica frustules have proved to be suitable for biomedical applications, but, unfortunately, they are not officially recognised as safe by governmental food and medical agencies yet. In the present review, we highlight the frustule formation process, the most common purification techniques, as well as advantages and bottlenecks related to the employment of diatom-derived silica for medical purposes, suggesting possible solutions for a large-scale biosilica production.

Heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) can have negative impacts on the marine and freshwater biota. Utilizing microbes, particularly microalgae, which are able to extract metals and hydrocarbons from environmental matrices, the impact of these contaminants in freshwaters, seas, and wastewaters can be reduced. In fact, the contaminants can be passively absorbed and actively accumulated within the organelles of microalgae, reducing their negative impact. River streams, untreated municipal, agricultural, and industrial wastewaters, represent one of the most important issues for the anthropogenic pollution. Microalgae and bacteria can grow in polluted waters containing different metals as cadmium (Cd), lead (Pb), zinc (Zn), chromium (Cr), copper (Cu), and mercury (Hg). They are able to accumulate metal elements within biomass in a dose-dependent manner and are employed in bioremediation thanks to their growth ability in polluted sites. In the following chapter, we analyze the potential of employing microalgae in phycoremediation, their mechanisms of tolerance and resistance to heavy metals, the possibility to use consortia of microorganisms instead of single specie to cope with complex polluted matrices, the possibility to metabolic engineer microalgae to gain their function, and the employment of microalgae in both liquid and solid devices.

Heavy metals (HMs) can induce both chronic and acute harmful effects on marine and freshwater biota. The environmental impact of HMs in freshwater, seawater, soil, and wastewater can be limited using microbes, including microalgae, that are able to remove metals from environmental matrices. Indeed, they can passively adsorb and actively accumulate these persistent pollutants within their organelles, limiting their detrimental effects on cellular metabolism. The Sarno River is a 30 km long freshwater stream located in Southern Italy, polluted by partially untreated municipal, agricultural, and industrial wastewaters. In spite of this, microalgal cultures from Sarno River or Sarno River Mouth have never been established. In the present study, we isolated a green algal strain from the Sarno River Mouth and determined its ability to grow in polluted seawater containing different concentrations of cadmium, lead, or zinc. This strain was found to be able to accumulate these elements within its biomass in a dose-dependent manner. Growth inhibition experiments confirm the relatively low toxicity of Cd and Pb below 50 µM, while algal growth was seriously affected in Zn-amended media. To the best of our knowledge, this is the first study focused on the ability of microalgae from Sarno River Mouth to tolerate and uptake HMs.

The planktonic diatom genus Pseudo-nitzschia contains several genetically closely related species that can produce domoic acid, a potent neurotoxin known to cause amnesic shellfish poisoning (ASP). An early identification and an adequate monitoring of the potential toxic Pseudo-nitzschia spp. are necessary. However, effective monitoring programs are time consuming due, in some cases, to the cell morphology similarities among species, determined with light microscopy, that can result in insufficient data to give a definitive species and toxins attribution. In this paper, Whole-Cell Fluorescent In Situ Hybridization (WC-FISH) has been evaluated as a powerful tool to detect and enumerate harmful cryptic and/or pseudo-cryptic Pseudo-nitzschia spp. collected in the Gulf of Naples. Fluorescently labelled probes directed against the ribosomal RNA (rRNA) of the 28S large subunit (LSU) were used. In particular, five probes detecting four cryptic species of Pseudo-nitzschia delicatissima complex and one specific for Pseudo-nitzschia multistriata gave good results for the molecular identification of potentially toxic target species in natural samples. Finally, we can state that the WC-FISH method, to identify Pseudo-nitzschia species, is faster and more cost-effective if compared with other rDNA-based methods.

Pennate diatoms belonging to the genus Pseudo-nitzschia are important components of phytoplankton assemblages in aquatic environments. Among them, the bloom-forming species Pseudo-nitzschia calliantha and Pseudo-nitzschia multistriata are known as domoic acid producers, and are thus considered harmful for aquatic organisms and for human health. For these reasons, monitoring the abundance of such species, as well as identifying the growth conditions enhancing or inhibiting their growth, could help to predict eventual risks for aquatic communities and for humans by direct or indirect exposure to these toxic algae. In this work, we assessed the effects of different parameters (irradiance, temperature, salinity, and nutrients) on six Pseudo-nitzschia species by evaluating their specific growth rates and total RNA content. Our results—corroborated by statistical analyses of regression and correlation plots of control samples and samples exposed to stressful conditions, showed, as expected, a general decrease in growth rates under suboptimal levels of temperature, irradiance, salinity, and nutrient supply (especially under silicon depletion), that was usually accomplished by a general increase in RNA content inside cells. We hypothesized that increments in RNA levels in cells exposed to unfavorable conditions could be due to a relatively fast activation of the mechanisms of stress response.