Algal 14C and total carbon metabolisms. 1. Models to account for the physiological processes of respiration and recycling

A consistent set of equations has been written to describe thenet rate of algal ¹⁴CO₂ uptake (and where appropriate respirationand photosynthesis) which take into account separately complicationsdue to respiration of the labelled photosynthetic products andthe recycling of respiratory CO₂. Written specifically intothe equations is the concept of ‘new’ and ‘old’carbon, the coefficient q is used in the respiration model toallow for the differential respiration of organic material fromthe ‘new’ and ‘old’ carbon pools. Analyticalintegrals have been found for respiration and recycling models,and the behaviour of the models studied over periods of 12 h(i.e. up to 70% of the intrinsic generation time). The rateconstant for respiration has a greater effect on the behaviourof the recycling than the respiration model. Over short timecourses (up to 30% of the intrinsic generation time), the effectsof respiration and recycling on net ¹⁴CO₂ uptake are quite distinct,especially at high P/R ratios, and not complicated by assumptionsover the value of q. Although the value of q will have a time-dependentsecondary effect on the modelled total carbon-specific respirationrate, this was found not to give rise to major problems of interpretation.Beyond 50% of the intrinsic generation time, the separate treatmentof respiration and recycling in the models becomes less satisfactory.It was concluded that the present equations, which are not constrainedby mass balance considerations, would not be appropriate fora model that combines the two processes. The pattern of recyclingat low P/R values is identified as one of the major uncertaintiesin producing models of ¹⁴C uptake. The effect of the releaseof dissolved organic material can be anticipated in a generalway. The models have been used to define an experimental strategyto establish the separate effects of respiration and recyclingon the time course of net ¹⁴C uptake. The initial rates givethe clearest resolution of the two processes and it would appearthat with photosynthetic rates in the region of 1 day^–1,incubation periods up to 3–6 h would be suitable to determinethe importance of recycling in controlling net ¹⁴C uptake. Withthe present models, only in the absence of recycling could theeffect of respiration be studied and the value of q established.