In their natural environment, seaweeds grow in exceptionally diverse and dynamic light climates. Water transparency and the continual ebb and flood of tides have profound effects on the quantity and quality of the light that reaches seaweeds at their growth sites, adding greatly to the variation already present in the irradiance at the Earth’s surface. The primary importance of light to seaweeds is in providing the energy for photosynthesis, energy that ultimately is passed on to other organisms. In addition, light perceived as a signal also has many photoperiodic and photomorphogenetic effects (see secs. 2.3.1, 2.3.3, 2.6.2). Thus, light is the most important abiotic factor affecting seaweeds, and also one of the most complex.

The principles of photosynthesis are similar in algae and higher plants, and indeed some principles (e.g. the Calvin cycle) were worked out using (mostly unicellular) algae. However, there are several important features of seaweeds and their habitats that stand in sharp contrast to those in higher, and mostly terrestrial plants, and it is on these that we shall focus. Such features include the diversity of pigmentation among marine algae and the diversity of the light climate in the oceans, the nature of carbon supply in the sea, and the diversity of photosynthetic products in different algal classes. This chapter focuses on the processes in eukaryotic algae. Reference is also made to the prokaryotic cyanobacteria, only to highlight evolutionary or functionally important differences or commonalities. It is assumed that the common details of photosynthetic mechanisms and pathways have been covered in introductory courses; they will be reviewed only briefly in the following section. Textbooks on plant physiology and biochemistry offer extensive treatments of all aspects of angiosperm photosynthesis (e.g. Buchanan et al. 2000; Raven et al. 2005). The accounts of radiation climate, light harvesting, and carbon metabolism presented here with respect to aquatic ecosystems owe much to the detailed books by Falkowski and Raven (2007) and Kirk (2010), which readers should consult for more information and references.