Marcus was a postgraduate in genetics at the Max-Planck Institute in Tübingen under Prof. Dr. Christiane Nüsslein-Volhard (Nobel Medicine 1995) where he took part in constructing the first radiation hybrid map of the zebrafish genome as well as a genetic screen for cell division and fertilization mutants. He demonstrated that the mutant "futile cycle" has a pronuclear congression defect, similar to a centriole attachment defect reported in infertile men. He also discovered a blood stem cell defect in the mutant "kugelig" and linked the mutation to the radiation hybrid map.

Thereafter, he collaborated with Prof. Dr. Nicholas Foulkes with whom he demonstrated that DNA replication is timed by the daily light-dark phases, which has implications for chronotherapy. At University College London, he further researched circadian rhythms and knocked down the circadian clock with a dominant-negative construct as proof of concept. He then moved to the Genomics Core unit at the European Molecular Biology Laboratory in Heidelberg where he performed in the lab of Dr. Vladimir Benes an Affymetrix GeneChip microarray screen to reveal the effect of light on the transcriptome of non-image forming cells, thereby demonstrating that visible light (non-ionizing and non-UV) induces stress response and DNA repair genes through the PAR bZip transcription factor Thyrotroph embryonic factor. Thereafter, at the University of Vienna, he demonstrated by site-directed knockout of melanopsin genes and transcriptome sequencing, that this non-visual photoreceptor exerts control over the immune system and the cell cycle in addition to promoting locomotor activity (velocity) during the diurnal wake state through repression of the melatonin pathway via homeodomain transcription factors.

At the onset of the Covid-19 pandemic he joined the lab of Prof. Dr. Johannes Zuber at the Research Institute of Molecular Pathology where he contributed to establishing a SARS-CoV-2 testing laboratory and high-throughput monitoring. Moreover, he developed a highly specific and sensitive RT-qPCR diagnostic test for broad SARS-CoV-2 variant detection. His expertise and interests are in molecular diagnostics, chronopharmaceutics and personalized medicine.