Physiological Characteristics of the Seagrass Posidonia sinuosa along a Depth-Related Gradient of Light Availability

Brief description

Rapid light curve-derived parameters, light harvesting pigments, photoprotective pigments as well as nutrient and carbohydrate concentrations were measured for Posidonia sinuosa along a depth-related gradient of light availability (1.6 - 9.0 m depth) in Cockburn and Warnbro Sound in the winter of 2002 and summer of 2003.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: - Study site- The study was conducted in Cockburn Sound (CS) and Warnbro Sound (WS) in south-western Australia, where monospecific stands of the locally dominant species P. sinuosa grow on steep sub-tidal depth gradients ranging from approximately 1 to 9 m depth. Sampling at CS was carried out in winter (June 2002) and summer (January-February 2003) at six depths; 1.6 m, 4.0 m, 5.7 m, 6.5 m, 8.3 m and 9.0 m (lowest astronomical tide) that will be referred to as sites CS1, CS2, CS3, CS4, CS5 and CS6, respectively. Sampling at WS was carried out in summer only at the same six depths, and will be referred to as sites WS1, WS2, WS3, WS4, WS5 and WS6.

Statement: - Sampling - Carbohydrates: Six replicate rhizomes and shoots were randomly collected from each depth, placed in plastic bags and immediately stored on ice prior to storage at -18 °C for analysis of non-structural carbohydrates including soluble sugars and starch. Samples were later analysed for soluble sugar concentration according to the method of Dubois et al (1956). For rhizome material, 0.1 ? 0.2 g from a segment nearer to the terminal shoot (internode 4) and an older segment (internode 10) were separately analysed (three-way ANOVA, season × depth × segment). Starch showed no difference between these segments, so data were pooled for final statistical analysis; whereas sugar data for both segments are presented separately (segment × location p < 0.01). All analyses on leaves were carried out on the youngest fully mature leaf (usually leaf one or two). Rhizome material was finely ground with acid washed sand and leaf material was ground in liquid nitrogen as grinding in sand did not effectively macerate leaves. Sugars were twice extracted in ethanol at 60°C for 20 min. A trial determined that this extraction regime was as effective as three shorter extractions. The extract was analysed for sugar content using the phenol-sulphuric acid colorimetric method. Samples were gelatinized at 100°C for 15 min and then solubilised in 70% perchloric acid. Starch content was then analysed using the phenol-sulphuric acid colorimetric method (Quarmby & Allen, 1989). Nutrient content and 13C: Leaf samples for 13C and nutrient content (%N and %C) were randomly selected from the above-ground biomass samples. Only the youngest fully mature leaf was selected as 13C may vary with leaf age (Lepoint et al. 2003). Dried samples were finely ground in a mixer mill (Retsch MM 200, Germany) and analysed for 13C, %N and %C carbon using a mass spectrometer (ANCA-NT Europa Scientific, Crewe, UK) interfaced with a 20-20 isotope ratio mass spectrometer (Europa Scientific, Crewe, UK). Isotope signatures were determined by comparison with a working laboratory reference material, which had been previously calibrated against various IAEA or NIST standard reference materials with a precision of <0.1%. All 13C, expressed using the per million (%) notation, are traceable to the internationally accepted VPDB (for 13C) or AIR (for 15N) scales. Pigments: At each depth, six replicate shoots were randomly collected and placed immediately on ice in the dark, prior to storage at -18 °C. In a darkened room, the youngest fully mature leaf was scraped free of epiphytes, finely chopped and extracted in N,N-Di-methyl formamide at 4 °C in darkness for 72 h. Spectrophotometric determination of chlorophyll concentrations of the extract were performed according to the equations of Wellburn (1994). Additional shoots were collected for analysis of accessory pigments. Whole shoots were collected, wrapped in foil whilst under water and placed in liquid nitrogen upon return to the surface. Samples were stored at -86 °C. In a darkened room, a whole mature leaf, including all material emerging from the leaf sheath to the leaf tip, were scraped free of epiphytes. Leaf material was ground in a cold, glass mortar with acid washed sand and cold (-4 °C) HPLC-grade 90% acetone. The sample was sonicated for 20 min, allowed to extract for 12 h at 4°C and sonicated for a further 20 min. The extract was then analysed for pigment concentration on a high performance liquid chromatograph (Waters) comprising of a 600 controller, 717 plus refrigerated autosampler and a 996 photodiode array detector. Concentrations of pigments were determined from a combination of standards (Sigma) and from purified pigments isolated from algal cultures. Photosynthetic characteristics: Rapid light curve parameters - Photosynthetic characteristics were measured on six replicate leaves using a Diving-PAM fluorometer (Walz, Germany). The sites were measured in a randomised order between 10:00 - 14:00 h on cloudless days to capture the midday period of maximum electron transport (Campbell et al., 2003; Ralph & Gademann, 1999). Photosynthetic data were not collected at WS5. All measurements were made on the youngest, mature leaf, 15 cm from the top of the leaf sheath (lowermid section), which is the most mature section that was consistently epiphyte free and where the highest effective quantum yield is expected (Durako & Kunzelman, 2002). The leaf was held 5 mm from the tip of the fibre-optic cable in a dark-adaptation clip. For further information see section 4.2.2.4 of thesis.

Notes

Credit
Strategic Research Fund for the Marine Environment (SRFME)

Purpose
To characterise a number of physiological characteristics of the meadow-forming seagrass P. sinuosa that have previously been reported as responsive to light availability with particular emphasis near the depth limit where light is near limiting.