The low diffusion rates of solutes in water call for a separation of photosynthetic carbon acquirement in aquatic plants into carbon transport and the subsequent photosynthetic reduction of CO2. This paper will focus on the transport of inorganic carbon from the external medium to the site of fixation in marine macrophytes. In accord with the much higher concentration of HCO3− than of CO2 in seawater, most marine macrophytes can utilize the ionic carbon form for their photosynthetic needs. The two known ways of HCO, utilization are (a) via extracellular, carbonic anhydrase catalyzed dehydration of HCO3− to form CO2, which then diffuses into the photosynthesizing cells, and (b) by direct uptake via a transporter. While the first way may be sufficient to support low rates of photosynthesis in temperate regions, it is viewed as futile under situations where high temperatures and irradiances would cause a high pH to form close to the uptake site of carbon and where, consequently, the CO2/HCO3− ratio would be very low. Therefore, it may well be that the direct HCO3− uptake mechanism described for Ulva from more tropical regions confers an adaptational advantage under conditions conducive to higher photosynthetic rates.