Photosynthetic and Atmospheric Evolution

Questions re Raven's paper

1. H. Griffiths

Q. Have you done one of your back-of-the-envelope calculations to determine the rate of diffusion of CO2 through concrete or substances equivalent to a stromatolite structure which would calculate the concentration of CO2 concentration likely in a cyano-like cell deposited within a stromatolite at the ambient CO2 of 2.6 Ga - without a CCM?

A.A very good point. No, I have not done such calculations; they would be very useful.

2. John Ellis

Q. Could you comment on the possibility of expressing the genes for cyanobacterial CCMs in the chloroplasts of higher plants where such a transformation is possible?

A. The possibility of transforming C3 crop plants so that they express a CCM has been suggested repeatedly; I recall Professor Ellis, with typical presecience, raising this possibility with me in the context of cyanobacterial or algal CCMs about 20 years ago. Experiments so far have mainly involved expression of one of more enzymes from C4 plants in C3 plants, with varying degrees of success in establishing C4 characteristics. The International Rice Research Institute is exploring the possibility of making ‘C4 rice’. The possibility of engineering CCMs into plants is discussed in the paper of Price et al. (in press) that is cited in the paper resulting from my talk.

3. Paul Falkowski

Q (i). During the last few months three papers have appeared from three independent groups showing that photosynthesis and growth in the marine, nitrogen fixing cyanobacterium Trichodesmium, is not saturated with CO2 at the present time. Do you think that's a rare or general phenomenon?

A (i). The three papers that appeared in 2007 on Trichodesmium show this organisms is an exception to the received doctrine (that I had a hand in establishing) that the growth of (almost) all marine phytoplankton organisms is inorganic carbon saturated in the present ocean. Recently published data on other marine phytoplankton organisms show that organic carbon per cell increases at growth CO2 concentrations higher than the present even when growth rate measured as increase in cell numbers is saturated at present levels of CO2.

Q (ii). Over the past 15 years a number of people have tried to reconstruct the historical record of pCO2 based on the isotopic composition of carbon in specific organic compounds preserved in marine sediments. It is my understanding that in so doing, an assumption is made that CO2 diffuses into the cells. However, if there is a CCM, the assumption seems to be invalid. Is my analysis fundamentally flawed, or do we have a palaeobarometer for pCO2 through the last 100 million years of Earth’s history?

A (ii). The possibility of a palaeobarometer for pCO2 based on the natural abundance of carbon stable isotopes in taxon-specific compounds from marine is indeed a seductive one. However, Dr Falkowskis analysis is correct: the occurrence of a CCM means that such a palaeobarometer only works over at best a very limited pCO2 range even for unialgal cultures, and could not be used in the context of molecules extracted from sedments. Regretably, that we do not have this sort of palaeobarometer.

4. Tony Walsby

Q. A comment about cyanobacteria in stromatolites. They live in little tubes and migrate to and fro so that can come to the surface to get their CO2. If there's a lot of UV light the ones that are on the shaded side of the stromatolite could come up and then go back into the rock later. You suggested that aquaporin might provide a way in for CO2. Which membrane would contain the aquaporin?

A. This very good point about movement of caynaobacteria is now addressed in the paper resulting from the talk. As for the location of CO2-selective aquaporins, these were originally found in the plasmalemma of C3 terrestrial plants, and have recently been reported from the inner chloroplast envelope membrane of these plants.

5. Dianne Edwards

Q. You mentioned the somewhat atypical 'physiological' attributes (low CO2 sensitivity, differing Rubisco) of dinoflagellates. They also have a rather strange fossil record in that they came in in abundance in the mesozoic and the recent hypothesis has been that the endosymbiosis occurred then rather than in deep time. Is it more realistic that they evolved much earlier than their fossil record suggests?

A.Yes, they could have evolved much earlier than their fossil record suggests. The Mesozoic and recent fossils are essentially all based on cysts of those dinoflagellates that live in coastal waters and form resting cysts that can sink to the sediment and subsequently stand a biologically significant possibility of being resuspended. Dinoflagellates that do not form recognised fossilised cysts could have occurred before the Mesozoic.

6. Roger E. Summons

Q. The evidence based on chemical fossils for at least an ancestor of the dinoflagellates is quite strong. They made a very distinctive steroid and that steroid is ubiquitous at least in the Neoproterozoic and probably older. Although you can't say that's the same dinoflagellate as appeared in the Mesozoic, at least a very distinctive aspect of dinoflagellate biochemistry is evident in the Neoproterozoic and probably the Mesoproterozoic.

A. A very good point. Dinosterane, derived diagenetically from dinosterol, has been found in sediments from the Neoproterozoic, and perhaps occurred even earlier. This is a good indicator of a least biochemistry like that of extant dinoflagellates.