Misconceptions in attributing CO 2 -concentrating mechanisms in marine algae

John Raven (2021, verbal communication), to whose memory we dedicate this paper, agreed that “C 4 metabolism in marine plants [is rare and] should not be inferred based solely on plant responses known for terrestrial plants”. However, ascribing CO 2 concentrating mechanisms (CCM) activity to algae is frequently based on insufficient information or misunderstood features. In this contribution we critically assess the usefulness of the various experimental approaches used to provide evidence for the operation of CCMs in marine algae and argue specifically that techniques such as genomics and measurements of the levels of carboxylating enzymes do not in themselves allow definitive attribution of biochemical CCMs based on C 3 versus C 4 metabolism.