NOVELTY - Magnetotactic alga cell comprises an alga cell expressing a nucleic acid encoding a bacterial MagA ferrous transporter, and/or a nucleic acid encoding a bacterial Mms6 magnetite binding protein.    USE - The magnetotactic alga cell is useful: for producing magnetic nanoparticles (claimed), where the magnetic nanoparticles are used as a selectable marker for transformed algal cells, in biomedical applications (including in imaging for magnetic resonance imaging contrast agents), in tumor therapy by magnetic hyperthermia, in electronics for high-density data storage, in pigments, and in environmental catalysts for oxidation of carbon monoxide or oxidative pyrolysis of biomass; in waste water treatment; for producing biofuel; and for selection of genetically engineered algae. No biological data given.    ADVANTAGE - The magnetotactic alga cell: can be harvested by magnetic separation with low-maintenance and highly scalable separation method, thus reducing the harvesting costs by up to 90%; and provides magnetic nanoparticles which does not require special growth conditions or oxygen concentrations for culture of anaerobic magnetotactic bacteria.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) producing (M1) magnetotactic algae, comprising transforming an alga cell with the nucleic acid encoding the bacterial MagA ferrous transporter and/or the nucleic acid encoding the bacterial Mms6 magnetite binding protein, and cultivating the transformed alga cell or its progeny that express the bacterial MagA ferrous transporter, and/or the bacterial Mms6 magnetite binding protein, under conditions to produce algae comprising magnetic nanoparticles, thus producing magnetotactic algae;    (2) producing (M2) a product from algal cells, comprising cultivating an alga cell expressing a nucleic acid encoding a bacterial MagA ferrous transporter, and/or a nucleic acid encoding a bacterial Mms6 magnetite binding protein, or its progeny that express the bacterial MagA ferrous transporter, and/or the bacterial Mms6 magnetite binding protein, under conditions to produce algae comprising magnetic nanoparticles, magnetically collecting the algae cells comprising magnetic nanoparticles, and isolating the product from the collected algae cells; and    (3) selecting a transformed algal cell, comprising cultivating a population of algae cells transformed with a first nucleic acid molecule encoding a bacterial MagA ferrous transporter, and/or a bacterial Mms6 magnetite binding protein, and a second nucleic acid encoding a protein of interest, or its progeny that express the bacterial MagA ferrous transporter, and/or the bacterial Mms6 magnetite binding protein, under conditions to produce algae comprising magnetic nanoparticles, and magnetically collecting algae cells comprising magnetic nanoparticles, thus selecting the transformed alga cell.