Common Anabaena, Calothrix, and Leptolyngbya cyanobacteria are shown to form Au, Ag, Pd, and Pt nanoparticles of well-controlled size. These nanoparticles are synthesized intra-cellularly, and naturally released in the culture medium, where they are stabilized by algal polysaccharides, allowing their easy recovery. The size of the recovered particles as well as the reaction yield is shown to depend on the cyanobacteria genus. Investigations of nanoparticle formation indicate that the intracellular nitrogenase enzyme is responsible for the metal reduction but that the cellular environment is involved in the colloid growth process.

While magnetic nanoparticles offer exciting possibilities for stem cell imaging or tissue bioengineering, their long-term intracellular fate remains to be fully documented. Besides, it appears that magnetic nanoparticles can occur naturally in human cells, but their origin and potentially endogenous synthesis still need further understanding. In an effort to explore the life cycle of magnetic nanoparticles, we investigated their transformations upon internalization in mesenchymal stem cells and as a function of the cells’ differentiation status (undifferentiated, or undergoing adipogenesis, osteogenesis, and chondrogenesis). Using magnetism as a fingerprint of the transformation process, we evidenced an important degradation of the nanoparticles during chondrogenesis. For the other pathways, stem cells were remarkably “remagnetized” after degradation of nanoparticles. This remagnetization phenomenon is the direct demonstration of a possible neosynthesis of magnetic nanoparticles in cellulo and could lay some foundation to understand the presence of magnetic crystals in human cells. The neosynthesis was shown to take place within the endosomes and to involve the H-subunit of ferritin. Moreover, it appeared to be the key process to avoid long-term cytotoxicity (impact on differentiation) related to high doses of magnetic nanoparticles within stem cells. Significance Naturally occurring magnetic nanoparticles have been detected in human cells; however, their origin and role remain unknown. In the present paper, the observation of long-term transformations of man-made magnetic nanoparticles internalized in human stem cells displays a mechanism regarding their assimilation. Indeed, the nanoparticles are first degraded by the stem cells, and then new magnetic nanoparticles are synthesized in situ from the released iron. This “remagnetization” phenomenon is observed under specific pathways only, involves the ferritin protein, and seems linked to a detoxification mechanism in case of iron excess. It also brings a direct experimental proof that magnetic nanoparticles can be synthesized by human cells.

Klebsormidium flaccidum algal cells exhibiting the ability to form gold nanoparticles intra-cellularly in suspension were encapsulated within silica gels. Optical and electronic microscopy indicate that entrapped cells maintain their ability to reduce gold salts. A difference in the kinetics of gold colloid formation within silica in the absence or presence of cells could be followed by UV-visible absorption spectroscopy, confirming the bio-mediated nature of the reduction process. Study of the photosynthetic activity of the algae showed that the encapsulation process protects the cells from lethal effects arising from gold toxicity. Moreover, the first in situ imaging of entrapped cells using Raman spectroscopy allowed the investigation of the influence of the gold colloids on the photosynthetic system of the algae, in particular through modification of chlorophyll fluorescence and carotenoid signals. Such a coupling of sol–gel encapsulation and Raman imaging should allow the future development of novel photosynthesis-based cellular biosensors.