Photosynthesis and Inorganic Carbon Usage by the Marine Cyanobacterium, Synechococcus sp

Abstract

The marine cyanobacterium, Synechococcus sp. Nageli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO2 provided during growth. The high affinity for CO2 + HCO-of air-grown cells (K1/2 < 80 nanomoles [pH 8.21) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus C02) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for 02 evolution appear to be some 2-fold greater for low CO0-grown cells, compared with high C02-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells.A number of experimental approaches to the question of whether CO2 or HCO3-is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. C02, however, is more readily taken up when provided at an equivalent concentration to HCO3-.This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HC03-pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HC03-uptake into the cell is the primary source of internal inorganic carbon.The utilization of inorganic carbon by cyanobacteria has recently been the subject of several investigations (6, 9). It is clear that these photosynthetic organisms are able to change their relative affinities for external inorganic carbon, depending on the level in the external medium, and that there is some active accumulation mechanism which allows CO2 to be concentrated inside the cell. The concentrating mechanism has been proposed to be an HCO3 pump (3), rather than a CO2 utilizing system, which would function most efficiently at alkaline pH where a large portion of the inorganic carbon is present as HCO3 . An environment where a HCO3 transport mechanism could be expected to play a major role in the fixation of carbon from the aqueous medium is seawater, where a more or less constant alkaline pH of around 8.2 exists, providing a constant high level of HCO3 . Experiments described here were aimed at characterizing the photosynthetic properties of a marine cyanobacterium, Synechococcus sp., and assessing to what extent HCO3 rather than CO2 is used for photosynthesis.We have used a combination of techniques, including silicone oil centrifugation and mass spectrometric monitoring of gas exchange, to show that this cyanobacterium does possess an inducible active CO2 concentrating system. In exploring the question of active species used by this concentrating mechanism, we are forced to conclude that both CO2 and HC03-can both act as substrates; however, durin...

Selective and Reversible Inhibition of Active CO2 Transport by Hydrogen Sulfide in a Cyanobacterium

“…4C) (Fig. 4C) was very similar to that observed for a marine species of Synechococcus in the absence of any added inhibitor (2,3,27).…”

“…The difference in peak height between experimental curves (+ Na2S) and the curve obtained in the dark was was made to calculate rates of CO2 transport (17). Nevertheless, the method provides an alternate means of assessing CO2 transport, as distinct from HCO3-transport, over a range of [CO2] and is the procedure most directly comparable to 4GCO2 pulsing techniques used in conjunction with silicone fluid filtering centrifugation (2,17,28) and isotopic disequilibrium experiments (6).…”

“…During steady state photosynthesis in Synechococcus leopoliensis UTEX 625 both species of DIC are transported simultaneously and continuously in light dependent processes (7). Although cyanobacteria rapidly remove CO2 from the medium (2,7,17,28), it is HCO3-transport which provides the bulk of the DIC to the intracellular pool when the extracellular pH is alkaline and the [DIC] low (2,6,7,10,16 …”

“…Cyanobacteria possess mechanisms for the active transport of CO2 and HC03-that lead to the accumulation of a large intracellular pool of DIC3 (2,6,10,11,13,16,17,28) which serves as the immediate source of CO2 for photosynthesis (10,16). During steady state photosynthesis in Synechococcus leopoliensis UTEX 625 both species of DIC are transported simultaneously and continuously in light dependent processes (7).…”

Two Systems for Concentrating CO2 and Bicarbonate during Photosynthesis by Scenedesmus

“…15,20,28), whereas air-adapted blue-green algae (2) and Scenedesmus at alkaline pH have been used to study HC03-as well as C02 uptake. The principle investigator of the DIC pump with Scenedesmus was Findenegg (6)(7)(8)(9), who showed that C02-grown, synchronized cultures ofScenedesmus, when changed to air levels of C02, formed a partial HC03-pump within 2 h in the light that functioned between pH 8 to 1.…”