[5] After completing a BSc (Special) degree from Imperial College in 1961, he conducted biochemical research with Henrik Lundegårdh at the University of Upsala, Sweden for 1 year.

Then he continued his postgraduate research at Oxford University where he received his D.Phil in 1966 for a thesis concerned with the processes of ion uptake in plant roots.

[6][7] During his post-doctoral fellowship at the University of Cambridge, he was in the laboratory of Enid MacRobbie and studied the ionic status of chloroplasts.

At this time, he was also involved in developing early applications of underwater (SCUBA) techniques to plant physiology, particularly the ways in which algae harvest energy in situ.

[8] Under the aegis of Australian Academy of Science, he organized the first Fenner Environment Conference on Ultraviolent B Radiation Impacts in Canberra.

He has served on the editorial boards of Aquatic Botany, Marine Biology and Oceanography, Phycologia and Trends in Plant Science.

In collaboration with colleagues from the Research School of Biological Sciences at the Australian National University, he initiated a study, which for the first time detailed the problem and outlined the various types of photoinhibition and protective measures available to the more common algae of coral reefs.

[14] In addition to the major discoveries involving Prochloron, Larkum discovered four other cyanobacterial symbionts in certain deep-water sponges.

[15] Larkum's early work in this area was concerned with pioneering the use of SCUBA techniques to the field of algal ecology and physiology, in particularly to the theory of chromatic adaptation in algae.

He discovered a new species and named it Halophila capricorni; type specimens came from One Tree Island on the Great Barrier Reef, but it is also found in New Guinea and New Caledonia waters.

He conducted considerable research in the evolution of Prochloron and cyanobacteria and was also involved in phylogenetic studies of cab genes in a variety of eukaryotic algae, Pavlova lutheri, diatoms and Amphidinium.

[27] Along with his interest in Plant Sciences, Larkum was a keen student of Charles Darwin and his writings on natural selection.