The biochemist N. G. Carr founded a research school that made seminal contributions on the metabolism, physiology, molecular biology and ecology of cyanobacteria or blue-green algae perhaps the most numerous organisms on Earth.
Cyanobacteria are involved in today's most urgent scientific issue, the rise in atmospheric CO2, causing global warming. The present composition of the atmosphere, discussed at a recent meeting of the Royal Society, results from the "Great Oxidation Event" over two billion years ago, when oxygen gas first accumulated in the air and CO2 levels plummeted.
These changes were brought about by cyanobacteria using the energy in sunlight to split water into oxygen, released as a gas, and hydrogen, which transformed CO2 into organic matter. In the current era this "oxygenic photosynthesis" also occurs in chloroplasts of plants; and chloroplasts evolved from cyanobacteria. Cyanobacteria, then, changed the conditions for life on Earth and are ultimately responsible for creating the food we eat and the air we breathe.
Noel Gordon Carr was born in South Shields in 1935. The family moved to Leeds where his father, a pharmacist, established a small chain of chemist shops. Noel attended Roundhay Grammar School. At the age of 14 he was crippled in both legs by polio. For much of his life he discounted the handicap; walking with two sticks he moved at a dizzying pace and negotiated stairs with particular daring. In 1954 he won a state scholarship to Leeds University, where he read biochemistry. On graduating he moved to Trinity College, Oxford, researching the metabolism of purple bacteria for his DPhil with June Lascelles.
Carr's first postdoctoral position was at the University of California, San Francisco. He worked on lipid metabolism but his career-changing experience was attending C.B. van Niel's course in microbial ecology at the Hopkins Marine Station, Stanford University. When Carr returned to the UK in 1962, to take up an ICI Fellowship in the Biochemistry Department at Liverpool, he turned his attention to certain blue-green algae, convinced of their pivotal position in biology.
Carr and his students demonstrated that this algae produced a substance (poly-hydroxybutyrate) known only in bacteria and they showed that the ribosomes of blue-greens were of the bacterial type. These and other features supported the reclassification of blue-green algae as cyanobacteria, formally proposed in 1970 by Roger Stanier, of the Institut Pasteur, Paris.
In the mid-1960s an old idea, that plant chloroplasts evolved from cyanobacteria engulfed by plant ancestors, was revived, separately, by the biologists Patrick Echlin and Lynn Sagan. Carr's group provided compelling evidence for this by DNA-RNA hybridisation: chloroplast genes in a green flagellate showed a closer affinity to those in free-living cyanobacteria than to the corresponding genes in the flagellate nucleus. Carr's lab produced the first extensive genetic maps of the cyanobacterial chromosome by observing the frequency of specific mutations obtained by applying mutagens at different stages of the cell replication cycle. All of these studies anticipated the findings that later depended on DNA sequencing techniques.
In the 1970s and 1980s Carr's group investigated the biochemical pathways of cyanobacteria, discovering new enzymes and explaining modifications of metabolic cycles that made these organisms dependent on photosynthesis for growth. They also determined the conditions required for differentiation of heterocysts, cells specialised for nitrogen fixation.
Appointed to a Chair of Biology at Warwick University in 1984, Carr, with his colleagues Nick Mann and Dave Scanlan, developed molecular methods of investigating nutrient uptake by cyanobacteria. They identified a gene involved in phosphate uptake, which becomes active only when phosphate is limiting; measurement of the gene's activity provided an assay for phosphate limitation in the ocean. A similar system was developed for iron. These assays were used to investigate the productivity of the minute picoplanktonic cyanobacteria, which account for much of the photosynthesis and CO2 uptake in the oceans. Recently there have been trial fertilisations of the oceans with iron to increase CO2 absorption from the atmosphere.
Noel Carr enjoyed collaborating with people. He became founding editor of Blackwell's Studies in Microbiology series. He and the botanist Brian Whitton edited two important works on their speciality: The Biology of the Blue-green Algae (1973), setting the classical studies in the context of the modern biochemistry and electron microscopy; and The Biology of the Cyanobacteria (1982).
Carr generated numerous ideas and exploited a number of them commercially with companies including Schweppes Research Ltd and the Helicon Foundation, California. He was co-inventor of a patent to use cyanobacteria for the biosynthesis of fine radiochemicals, which, as he told it, considerably enriched the Radiochemical Centre, Amersham.
At conferences Carr would remark that scientists were such nice people. He saw them at their best, uplifted by his own generous spirit. But he deflated pomposity. At a meeting in which the first DNA sequences from cyanobacteria allowed the tentative construction of phylogenetic trees, an august plenary speaker cautioned, "Only God can make trees." Noel responded by sketching a New Yorker cartoon of two lumberjacks leaning on their axes, surrounded by felled timber, saying "We may not be able to put 'em up but we sure-as-hell can knock 'em down."
Visiting Carr, especially in the Liverpool days, I saw him much involved with his research students. After a long day at the lab he would take everyone to the pub. From there we would move on to Noel's house where his children came and went while his wife Di produced bacon and eggs for everyone. Di was always supportive to Noel, which became increasingly important as he struggled with the legacy of his polio and, finally, his inability to fight off bacterial infections.
Carr was a keen theatre-goer and a connoisseur of wine and art; he became chair of the Friends of the Mead Gallery at Warwick University. With characteristic enthusiasm he raised funds for the gallery's notable collection. In retirement he played bridge most afternoons at the historic Leamington Spa Tennis Court Club.