Seaweeds require inorganic carbon, water, light, and various mineral ions for photosynthesis and growth. This chapter will examine the mechanisms of uptake, the nutrient requirements, and the metabolic roles of essential nutrients (excluding C, H, and O). The importance of nutrient uptake and growth kinetics will be discussed in terms of their effects on chemical composition, growth, development, and distribution of macroalgae. Particular emphasis will be placed on nitrogen, because it is the element most frequently limiting to seaweed growth. Even though seaweeds are larger than phytoplankton and usually are attached to a substratum, their nutritional requirements are very similar, and therefore some discussion of phytoplankton nutrition is also included when little or no information exists for seaweeds.

Nutrient requirements

Essential elements

The development of defined culture media for growing algae axenically has allowed the testing of a variety of elements to determine which are essential. The criteria to define an absolute requirement for an element were established by Arnon and Stout (1939):

1. deficiency of the element makes it impossible for the alga to grow or complete its vegetative or reproductive cycle.

2. cannot be replaced by another element.

3. effect is direct and is not due to interaction with (e.g., detoxification of) other, nonessential elements, stimulation of epiflora, or the like (Levitt 1969).

C, H, O, N, P, Mg, Cu, Mn, Zn, and Mo are considered to be required by all algae (O'Kelley 1974; DeBoer 1981); S, K, and Ca are required by all algae, but can be partially replaced by other elements; Na, Co, V, Se, Si, Cl, B, and I are required only by some algae.