Nutritional Strategies of Submerged Benthic Plants: The Acquisition of C, N and P by Rhizophytes and Haptophytes

Submerged benthic plants (algae and ‘higher plants’) can be classified into two groups on the basis of their attachment to the substrate. ‘Haptophytes’ are attached to the surface of rocks; ‘Rhizophytes’ have rhizoids, roots or rhizomes within the substratum (sand or mud). Haptophytes must obtain all of their nutrients from the bulk water phase, while rhizophytes can obtain some of their nutrients from the sediment. Evidence as to the sites of nutrient uptake on the surface of rhizophytes supports the view that a substantial fraction of phosphorus and nitrogen and, in some cases, inorganic carbon, is obtained from the sediment. This might be expected to make the portion of the rhizophyte which occurs in the bulk water phase more specialized for interception of photosynthetic light and C than the corresponding portion of the haptophyte which also intercepts and absorbs all other mineral nutrients. The morphology of the haptophyte and its frequent occurrence in more rapidly flowing water means that the superficial unstirred layer is less of an impedance to nutrient transport from the bulk medium to the plant surface. This in turn improves the supply of inorganic C, which is reflected in more negative delta 13 C values and higher biochemical capacities for photosynthesis per unit plant surface in haptophytes than in rhizophytes. Haptophytes frequently have colourless hairs on their surface, especially in media of low nutrient content or with poor water movement: in many cases, these hairs seem to be involved in nutrient uptake. Despite their different mechanisms for uptake of nutrients, rhizophytes and haptophytes can produce similar biomasses and chlorophyll m −2 , although rhizophytes may have rather lower productivities. The role of symbiotic associations (mycorrhizas in vascular plants; microalgae in invertebrates) in uptake of nutrients and photosynthesis is discussed in relation to haptophytic and rhizophytic strategies, as is the evolution of land plants.