18.2.2  Ecophysiology of seagrasses

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(a)  Low and unpredictable light

Plants living in deep water receive low and unpredictable amounts of light. Light reaching seagrasses is attenuated by atmospheric (e.g. clouds, dust) and marine (e.g. suspended particles, colour, water absorption) factors. Even when light reaches seagrass leaves, it can be further attenuated by epiphytes such as bacteria, algae and sponges growing on leaf surfaces. Heavy growth of epiphytic algae can reduce light levels reaching chloroplasts so severely that plants succumb to carbohydrate shortage. For example, epiphyte growth induced by nutrient enrichment has led to loss of seagrass meadows through light deprivation in Cockburn Sound, Western Australia. Light attenuation occurs over different time periods depending on the screening agent: wind might raise turbidity transiently while the plume of a flooding river can persist for weeks. Chloroplasts of seagrasses are found predominantly in epi-dermal cells as an adaptation to low light, contrasting with terrestrial plants where chloroplasts are found in leaf mesophyll (Chapter 2). Epidermal chloroplasts capture light and exchange gases free from the barriers imposed on chloroplasts in sub-epidermal tissues. Seagrasses have diminished leaf mesophyll with little structural material, so that most of the shoot is photosynthetic. Thus seagrass leaves have photosynthetic rates and photosynthesis–irradiance relationships consistent with plants adapted to low light levels: light compensation intensities (Ic), light saturation intensities (Ik) and maximal photosynthetic rates (Imax) are all low. As a hedge against periods of low light (e.g. during turbidity), below-ground stems (rhizomes) store carbohydrate as starch and sugar which are mobilised when required to satisfy respiratory demands. Some seagrasses even grow in prolonged darkness for months using these carbo-hydrate reserves (e.g. Posidonia), while others are affected after a few days of low light (e.g. Halophila).

Waves and ocean currents are a unique influence on leaf canopies in marine environments. While seagrass meadows and terrestrial forests can both have epiphytes and dense canopies producing heavy self-shading, light environments in the two canopies differ. In forests, sunflecks moving across the forest floor are important for survival of shade plants in the understorey (Section 12.1.4). Under the sea, waves and currents buffet the canopy backwards and forwards, exposing seagrasses and associated marine plants to alternating high and low light intensities. Such canopy movement achieves a more uniform exposure to light throughout the seagrass canopy than on a forest floor and selects for the formation of isobilateral leaves in seagrasses (identical top and bottom surfaces).

(b)  Nutrients contained in sediments

Oceanic waters generally contain low concentrations of dissolved nutrients, partly due to rapid nutrient uptake into microscopic algae, or phytoplankton. Eventual deposition of plankton to the sea floor, along with inputs of organic matter from rivers, results in accumulation of organic material in nearshore soft sediments. Microbial degradation of this organic material leads to remineralisation of nutrients from sediments. Resupply of nutrients into bulk seawater is slowed by sorption of nutrients to sediment particles and by slow diffusion of nutrients through the tortuous path between sediment particles. Thus, nutrient concentrations are much higher in marine sediments than in bulk seawater.

Seagrasses draw on nutrients in both seawater and sediments via uptake into leaves and roots. Roots are believed to play a major role in nutrient acquisition because high nutrient concentrations are often found in sediments. Nevertheless, sea-water moving through a seagrass canopy provides a renewable source of dissolved nutrients for uptake via leaves. Water movement through a seagrass canopy also helps replenish nutrient de-position to underlying sediments through leaf blades acting as baffles to water motion and increasing deposition of organic material into sediments. As in terrestrial plants, nutrients taken up from external sources are effectively redistributed, helping efficiency of nutrient use.

Seagrasses also fix dissolved nitrogen gas through microbial associations. Nitrogen fixation by bacteria attached to root surfaces sometimes contributes to plant nitrogen status. Labelling studies have shown that nitrogen fixed by bacteria on seagrass roots rapidly enters plants, probably in exchange for dissolved organic material from seagrass roots sustaining the bacteria. Interestingly, the tropical seagrass species that fix nitrogen most rapidly are also preferentially consumed by dugongs, ensuring adequate protein intake for these animals.

Individual nutrients can become limiting factors for seagrass growth, as observed in terrestrial plants. For example, seagrasses growing in carbonate sediments of marine origin are primarily limited by availability of nitrogen, phosphorus and iron. In contrast, seagrasses growing in silica-based sediments of terrestrial origin are generally limited by nitrogen availability.

(c)  Chronically anoxic sediments

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Figure 18.8 O2 is transported to rhizomes and roots of seagrasses during periods of light when photosynthesis releases O2 into aerenchyma. Note the formation of an oxidised zone around roots and radial O2 loss into surrounding anoxic sediments. Both leaves and roots contain airspaces, configured, however, very differently through which O2 can diffuse. By night, almost all O2 transport ceases because seawater surrounding the leaves becomes the only source of O2 and alcoholic fermentation commences in roots. (Courtesy W.C. Dennison)

Microbial degradation of organic material in sediments results in rapid consumption of O2 and other electron acceptors. The rate of O2 diffusion is often so slow that O2 levels in soft nearshore sediments become depleted by respiration and reducing conditions are established. Therefore, while nutrients are relatively abundant in soft sediments, seagrasses must contend with the chronically anoxic nature of nutrient-laden sediments. Seagrasses have several morphological and physio-logical adaptations to anoxia. Plants form extensive networks of internal gas spaces (lacunae), similar to the aerenchyma of terrestrial plants, acting as conduits for diffusive and/or advective transport of O2 from leaves to roots (Figure 18.8). These lacunae are interrupted by a series of single-cell-thick diaphragms containing small pores able to let gases but not water pass. Thus the entire internal gas spaces cannot be flooded. Gas flowing from leaves to roots contains ap-proximately 35% O2, supporting aerobic metabolism in roots embedded in highly reducing sediments. Radial O2 loss into surrounding sediments (Section 18.1.1) oxidises sediments as well as roots, thereby improving the redox status and lowering toxicity of surrounding sediments. Transported O2 is derived from photosynthesis, hence transport virtually ceases within 15 minutes of darkness.

Seagrasses in darkness rely on anaerobic metabolism to generate ATP. As for terrestrial plants, anaerobic pathways yield ATP inefficiently but are able to sustain energy requirements for maintenance of anaerobic cells. One short-term metabolic response to anoxia by seagrasses is the reversible conversion of glutamate and glutamine into alanine and g-aminobutyric acid (GABA), producing ATP but reducing the capacity for nitrogen assimilation. Longer term survival of anaerobic con-ditions is achieved through ethanol production by alcoholic fermentation (Section 18.1.1(c) and Figure 18.8). Seagrass roots shift to alcoholic fermentation after 2–3 h of darkness, losing ethanol by diffusion into surrounding sediments. These adaptive mechanisms operate at different time scales, providing seagrasses with an integrated response to chronically anoxic sediments and ensuring their survival.

(d)  Slow diffusion rates

Universally slow diffusion of gases through water (Section 18.1.1) affects O2 and CO2 exchange in leaves of aquatic plants. O2 is only available to aerobic respiration as a dissolved gas, carried to leaf surfaces by mass flow of seawater then diffusing mainly through stomata.

On the contrary, inorganic carbon is present in water as dissolved CO2 gas and bicarbonate ions (HCO3). Freshwater plants in fast-moving water with low pH or high natural carbonate levels can derive enough CO2 to photosynthesise but the relatively high pH of seawater (about 8.2) and high salinity mean that about 90% of inorganic carbon in seawater is present as bicarbonate ions. CO2 concentration in seawater is therefore well below that required to achieve maximum rates of fixation by the dark reaction so mechanisms have evolved that exploit bicarbonate as an inorganic carbon source. Plants using both carbon sources have much lower CO2 com-pensation points and higher half-saturations (Km) for CO2 fixation than expected from simple diffusive entry of CO2. Active import of bicarbonate by leaves appears to be energised by a protonmotive force and is sometimes stimulated by cations (e.g. in Zostera). Once bicarbonate enters leaves, carbonic anhydrase in the periplasmic space converts it rapidly to CO2, providing a substrate for Rubisco. Such CO2-concentrating mechanisms allow plants to achieve photosynthetic rates much greater than might be expected in a carbon-poor environment and underpin the high growth rates observed in many submerged aquatic macrophytes.

Diffusion of bicarbonate ions through boundary layers immediately adjacent to seagrass leaves and hence to sites of assimilation can be a rate-limiting process for seagrass photo-synthesis. Diffusion rates are governed by (1) boundary layer thickness, which is largely a function of water turbulence around the leaf and (2) the bicarbonate concentration gradient from surrounding seawater to the site of photosynthesis. The process of active uptake of bicarbonate into leaves described above reduces bicarbonate concentrations within leaves and enhances diffusion from bulk water to sites of assimilation.

Seagrasses have further adaptations to acquire carbon for growth. Lacunae in seagrasses are enriched in CO2 and provide leaves with an effective mechanism for CO2 recycling. In fact, CO2 is so effectively recycled that photosynthesis in seagrasses fixes carbon with similar efficiency to terrestrial plants with C4 photosynthesis (Section 2.1; Feature essay 2.1). However, these efficiencies are achieved in seagrasses solely through morphological adaptations (lacunae) — CO2 is fixed in seagrasses by the action of Rubisco in the C3 pathway.

Highly efficient CO2 recycling can be demonstrated through estimates of natural carbon isotope discrimination based on δ13C values (Chapter 2): less negative δ13C values indicate less discrimination against the heavier 13C isotope hence more effective CO2 recycling. Seagrasses have δ13C values ranging from –3.6 to –23.8, in contrast to marine algae (–8.8 to –35), C4 terrestrial plants (–9 to –18) and C3 terrestrial plants (–23 to –34). Relatively high values in sea-grasses are evidence that they have the most efficient CO2 recycling of any plants in response to the strictures imposed by an underwater habitat. These mechanisms of extracting carbon from a scarce source against high diffusive resistance have, along with efficient nutrient acquisition, allowed seagrasses to occupy the sea floor with little competition from other macrophytes.

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