TECHNOLOGY FOCUS - BIOLOGY - Preparation: No preparation method is given. Preferred Method: The nucleic acid is operably linked to a transcription control element. Preferred Components: The engineered microalgae are a diatom, where the diatom is grown in a biofilm. The engineered microalgae is Phaeodactylum triconutum or Thalassiosira pseudonana. The engineered microalgae contain a nucleic acid, which encodes a protein that facilitates carbon assimilation operably linked to an exogenous transcription control element. The protein that facilitates carbon assimilation is: pyr-decarboxylase, rubsico, beta carbonic anhydrase and chloroplast hydrogen carbonate(HCO3) transporter; and nitrate reductase, nitrate transporter, nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) nitrate reductase, ferredoxin nitrate reductase and glutamine synthetase II. The nucleic acid and the transcription control element are both exogenous. The transcription control element is the fucoxanthin binding protein promoter. The engineered microalgae contain a nucleic acid encoding an RNA interference (RNAi) molecule that is an inhibitor of the expression of the protein that facilitates nitrogen assimilation. In the engineered microalgae, a gene encoding a nitrogen assimilation protein contains a mutation that reduces the expression or activity of the protein. EXAMPLE - Phaeodactylum triconutum and Thalassiosira pseudonana encode a single copy of nitrate reductase (NR), a enzyme that controls the rate-limiting step of nitrogen assimilation in all known eukaryotic marine algae. Transgenic diatoms that expressed a NR-yellow fluorescence protein (YFP) N'-terminus fusion revealed that NR is localized in both the cytoplasm and the peroxisome. This localization was further verified using immuno-gold labeling transmission electron microscopy (TEM). To generate an engineered diatom having reduced nitrogen assimilation, vectors encoding an RNA interference (RNAi) molecule that inhibited NR were developed and woked up to obtain engineered microalgae.