Photosynthetic Carbon Metabolism in Seagrasses ¹⁴C-Labeling Evidence for the C₃ Pathway

Abstract

The 813C values of several seagrasses were considerably less negative than those of terrestrial C3 plants and tended The marine monocotyledonous angiosperms, the seagrasses, have many interesting adaptations to their submerged environment (9,18). One of these is the adaptation of their photosynthetic physiology to enable the direct assimilation of inorganic carbon from seawater. The leaf anatomy of seagrasses is very different from that of terrestrial plants. The leaves have no stomata, and almost all of the chloroplasts are located in a single layer of thick walled epidermal cells. The mesophyll is comprised of highly vacuolated cells, containing very few chloroplasts, interspersed with large gas lacunae (2, 10). Evidence exists that the lacunal gas may become enriched in 02, relative to atmospheric levels, during periods of active photosynthesis (11). It is possible that the photosynthetic apparatus of seagrasses may be exposed to higher than atmospheric 02 concentrations. This situation is known to promote photorespiration (6) and is rarely, if ever, encountered by terrestrial plants in the natural environment.The photosynthetic pathway used by seagrasses to fix carbon is of interest. This interest has been stimulated by reports that the carbon isotope ratios of seagrasses resembled those of C4 plants rather than C3 plants and that C4 acids were labeled early during 14C fixation by Thalassia testudinum (1)(2)(3) 10 MATERIALS AND METHODSPlant Material. Both species were collected near Magnetic Island, North Queensland. T. hemprichii (Ehrenb.) Aschers was collected from an intertidal reefflat in Cockle Bay and H. spinulosa (R. Br.) Aschers from a subtidal location at 5-to 10-m depth off Nobby Head. Leafy shoots with attached rhizomes and roots were transported to the laboratory, stored in aerated seawater in the dark overnight, and used the following day. Only young leaves, free from visible epiphytes, were used for experiments. Segments of the youngest mature T hemprichii leaves (1.5-2.0 cm long) or groups of 8 to 10 leaves at the tips of H. spinulosa branches were selected.Medium for "4C-labeling. Millipore-filtered seawater was buffered with N,N-bis(2-hydroxyethyl)glycine (final concentration, 20 mM), acidified with HCI to below pH 4 and purged overnight with humidified C02-free air at 25 C. The pH was adjusted to 8.22 with carbonate-free NaOH and then NaH14CO3 was added to a final concentration of 2.2 mm with a specific radioactivity of 26 ,uCi ,Imofl' for T. hemprichii experiments and 9 liCi tmol' for H. spinulosa experiments.14C-labeling Procedure. Experiments were conducted at 25 C in a shaking water bath (about 50 oscillations/min). Light (about 100 w m-2) was provided from a mercury vapor lamp, filtered through 10 cm of water. Leaf material was preilluminated for at least 30 min in Millipore-filtered seawater or in nonradioactive medium equivalent in all other respects to the labeling medium. It was then quickly blotted to remove adhering unlabeled medium which would otherwise decrease the s...

Destructive hydrogen peroxide production inEucheuma denticulatum(Rhodophyta) during stress caused by elevated pH, high light intensities and competition with other species

“…The high growth rate at pH > 8-5 shows the ability of U. reticulata and Thalassia sp. to utilise both available CO2 and HCO3 in the seawater, as already has been reported for other Ulva species (Beer & Israel, 1990;Drechsler & Beer, I991;Maberly, I992;Bj6rk et aI., 1993) and Thalassia species (Beer et al, 1977;Andrews & Abel, 1979;Abel, 1984).…”

“…Thalassia testudinum has been shown to have a C 4-type photosynthetic carbon metabolism (Benedict & Scott, 1976). However, Andrews & Abel (1979) suggest, on the basis of results for Thalassia hemprichii, that compartmentalisation of C i (before fixation) such that…”

Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers

“…However, results reported by Andrews and Abel (2), Abel (1), Benedict et al (7), and Beer et al (4,5) showed that most seagrasses produce PGA as the first product of ' (15). Low cytoplasmic pH would also release free CO2 from previously absorbed bicarbonate (22).…”

“…Experiments were conducted in Bicinebuffered seawater of known pH and total carbon concentration prepared as previously described (1,2 (Fig. 3).…”

δ13C is a signature of light availability and photosynthesis in seagrass

“…The remarkably heavy isotope signature of seagrasses was originally thought to be indicative of C 4 or CAM physiology, but 14 C labeling patterns were shown to be dominated by C 3 intermediates, not malate or aspartate (Andrews and Abel 1979). Following demonstrations that seagrass photosynthesis was reliant on HCO { 3 transport (Larkum et al 1989;Larkum and James 1996), the heavy isotopic signatures were interpreted as providing evidence for the role of HCO { 3 as a source of C i (Raven et al 2002).…”