Mechanistic interpretation of carbon isotope discrimination by marine macroalgae and seagrasses

John A. Raven, Andrew M. Johnston, Janet E. Kübler, Rebecca Korb, Shona G. McInroy, Linda L. Handley, Charlie M. Scrimgeour, Diana I. Walker, John Beardall, Mathew Vanderklift, Stein Fredriksen and Kenneth H. Dunton

Abstract

The literature, and previously unpublished data from the authors’ laboratories, shows that the δ¹³C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from –3 to –35‰. While some marine macroalgae have δ¹³C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between –30 and –35‰). The organisms with very negative δ¹³C (lower than –30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ¹³C values (higher than –10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ¹³C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ¹³C values lower than –30‰ have pyrenoids. Previous work showed a good correlation between δ¹³C values lower than –30‰ and the lack of CO₂ concentrating mechanisms for several species of marine red algae. The extent to which the low δ¹³C values are confined to organisms with diffusive CO₂ entry is discussed. Diffusive CO₂ entry could also occur in organisms with higher δ¹³C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ¹³C values more positive than –10‰ (i.e. more positive than the δ¹³C of CO₂ in seawater) must involve HCO₃^- use.

Keywords: carbon isotope discrimination, Phaeophyceae, Rhodophyta, seagrasses, Ulvophyceae.

Functional Plant Biology 29(3) 355 - 378 (2002) doi:10.1071/PP01201