Seagrasses like all aquatic plants are shade-adapted (Reiskind et al., 1989; Bowes et al., 2002), i.e. they show light saturation at fairly low irradiances and have a high a (initial slope of the P vs E curve). Like all angiosperms they have the ability to vary their photosynthetic apparatus to optimise use of the available light but have only a small ability to do this in the short-term (that is in minutes to hours) by state transitions (whereby light capture by the two photosystems is manipulated—see section VIII.I). As shade plants they optimise to rather high levels of light harvesting proteins per photosystem (high absorption cross-section) and can also vary the number of photosystems per unit of thylakoid membrane and the number of chloroplasts per cell (Major and Dunton, 2000, 2002: Cummings and Zimmerman, 2003). However, unlike many land plants with complex photosynthetic anatomies (eg palisade and spongy mesophyll) which allows them to harvest light more efficiently (Lee et al. 1990), seagrasses rely almost entirely on a photosynthetic epidermis, limiting their ability to efficiently harvest available light (Cummings and Zimmerman, 2003).
Light harvesting in seagrasses has been studied by Major and Dunton (2000,2002), in Thalassia tes-tudinum and by Cummings and Zimmerman (2003) in T. testudinum and Z. marina. All these studies show that photoacclimation is largely brought about by changes in the chlorophyll content per unit surface area (or unit weight). Chlorophyll content was shown to vary up to five fold (Cummings and Zimmerman,
2003). Major and Dunton (2002) showed that the unit size (absorption cross-section) of photosystem
I increased under low light but found that neither photosystem density (per unit chlorophyll) or the unit size (absorption cross-section) of photosystem
II changed, again consistent with a shade strategy. One way for increased photosynthesis is to increase the absorptance of a leaf by increasing the number of chloroplasts or by rearranging chloroplasts in or out of the light path (Schwarz et al., 2002). As the number of chloroplasts in the light path increases the absorption of light approaches a black body absorber (Larkum and Barrett, 1991). The result is that chloroplasts deep in the tissue (or on the under side of leaves which undergo little displacement) receive a very-much modified spectral radiation, rich in green light. This is a consequence of the package effect, the tendency for densely packed chlorophyll to absorb greater amounts of violet and red light, compared to green light. A photosynthetic system without suitable pigments such as phycobiliproteins to harvest green light can nevertheless harvest most of the available light, but only at the expense of an inefficient use of the available photosynthetic apparatus, in this case deeper chloroplasts which work at low efficiency. The package effect has been directly demonstrated in two seagrasses, T. testudinum and Z. marina (Cummings and Zimmerman (2003; see also Zimmerman, Chapter 13).
Parts of the leaves, usually the younger parts, deeper down in the canopy are also subject to this effect since the package effect means that red and blue light are differentially absorbed, dependent on shoot density and current velocity, in the upper canopy region (Zimmerman, 2003; Zimmerman, Chapter 13). Since seagrasses show reasonable photosynthetic efficiencies, on the basis of incident photons, compared with other plants (Major and Dunton, 2002, Cummings and Zimmerman, 2003) they clearly provide the necessary photosynthetic machinery for optimum light absorption despite the metabolic costs of providing that machinery.
The total primary productivity of seagrasses varies from quite high to moderate, in comparison with the most highly productive land plants and algae (Larkum, 1981; Duarte and Chiscano, 1999). This has generally been seen as caused by deployment of a large underground rhizome and root system, rather than leaves inefficient in photosynthesis. (Raven, 1984). However, as shown by Duarte and Chicano (1999) the biomass ratios and production ratios of above-ground to below-ground parts varies widely in seagrasses, so that future work should focus on just how much the proportion of below-ground parts influences overall primary production and changes with depth (see eg. Olesen et al., 2002). Some seagrasses such as Posidonia oceanica and P australis tend to have a very high proportion of underground parts while other seagrasses such as H. ovalis tend to have a low proportion, which may be analogous to the division between woody and herbaceous terrestrial plants. This overriding factor must strongly influences primary production rates, but other factors such as nutrient supply and latitude (Duarte and Chicano, 1999) have so far hindered a clear assessment of what ultimately controls primary production in seagrasses compared to, for example, algae, such as Ulva lactuca, which have a photosyn-thetic lamina in which the majority of cells are photo-synthetic, and with almost no other parts (Longstaff et al., 2002). Current technology now provides the means to answer many of these outstanding questions. However, it should be kept clear that in such studies there are two scales involved, that of photosynthesis and primary production over hours to a single day, usually under optimal conditions, and that over a year, subject to herbivory, storm damage and a variety of other environmental conditions.
Was this article helpful?