Tansley Review No. 117 Carbon economy in lichens

Abstract

Summary 11

I. INTRODUCTION 12

II. LICHEN BIONT CHARACTERISTICS 12

1. The mycobiont 12

2. The photobiont 13

III. LICHEN GROWTH 13

1. Allocation of resources 13

2. Growth rates and environmental limitations 14

3. Maintaining an optimal energy use efficiency 15

IV. CARBON ACQUISITION 16

1. Water relations 16

(a) Desiccation tolerance 16

(b) Activation upon re-hydration 17

(c) Diffusion of water and CO₂ 18

2. Photobiont CO₂ fixation 20

(a) CO₂ acquisition modes 20

(b) Significance of the CCM 22

3. Light and nitrogen relations 22

(a) The light-response curve 23

(b) Photosynthetic capacity 25

(c) Coping with high light 27

V. CARBON SINKS AND EXPENDITURES 28

1. Carbon translocation 28

2. Carbon sinks 29

3. Respiration 30

VI. CONCLUDING REMARKS 30

Acknowledgements 31

References 31

Lichens are nutritionally specialized fungi (the mycobiont component) that derive carbon and in some cases nitrogen from algal or cyanobacterial photobionts. The mycobiont and photobiont live together in an integrated thallus, but they lack specific tissue for the transport of metabolites and resources between them. Carbon is acquired through photosynthesis in the photobiont, which is active when the lichen is wet and exposed to light. Lichen photosynthesis is limited primarily by water, light and nitrogen, but can also be constrained by slow diffusion of CO₂ within the wet thallus. The assimilated carbon is exported from photobiont to mycobiont, which also predominates in terms of biomass, and apparently regulates the size of the photobiont population. It has therefore generally been assumed that most of the carbon is used for growth and maintenance of the fungal hyphae. However, the extent of photobiont respiration in relation to mycobiont respiration has seldom been quantified; neither do we know the pool sizes of various carbon sinks within lichens. Owing to this lack of fundamental data we do not know whether, or how, carbohydrate resources are regulated to maintain an optimal balance between energy input and expenditures in these symbiotic organisms. This review summarizes data on growth, carbon gain and carbon expenditures in lichens, with particular emphasis on factors determining the photosynthetic capacity of their photobionts. An attempt is made to introduce an economic analysis of lichen growth processes, a view that has often been adopted in studies of higher plants. Areas in which more data are needed for the construction of a model on ‘lichen resource allocation patterns’ are discussed.