A fully automated system for measurements of photosynthetic oxygen exchange under immersed conditions: an example of its use in Laminaria digitata (Heterokontophyta: Phaeophyceae)
A new, fully-automated, closed-chamber system was developed for measuring photosynthetic activity in aquatic plants, algae, or corals during immersion. The performance of this system, which monitors oxygen exchange, was evaluated both in the laboratory and in situ under natural conditions using the seaweed Laminaria dtgitata. Intact, large individuals were placed inside the chamber and kept in place by a plastic grid in a transparent Perspex dome. The grid separated the upper incubation chamber containing the alga from the detectors that were situated in the lower chamber. Oxygen was measured using a novel method based on lifetime optical fluorescence sensor technology that provides an extremely stable and precise measurement of dissolved oxygen. The circulation and homogenlzation of the medium between the samples and the detectors in this closed system were provided by two pumps. The medium could be renewed by another pump that opened to the external ambient seawater and controlled by a solenoid valve. All the mechanics were driven by an electronic card that allowed choice of filling time, time of measurement, and time of medium renewal. This system provides a new tool to study, In detail, the photosynthesis of whole aquatic organisms under natural field conditions during Immersion, combining high time-resolution of oxygen exchange with a long temporal scale of In situ measurements. By allowing automatic and very accurate measurements without any Intervention during monitoring, this system will be useful for estimating and comparing production of primary producers or for assessing the influence of environmental factors on production.
Accurately quantifying the primary production of aquatic photosynthetic organisms is fundamental in developing our understanding of energetics and tropho-dynamics in aquatic systems. Indeed, in shallow coastal waters, benthic algal communities, whose structural complexity and biodiversity is similar to that of terrestrial forests, are responsible for much of the primary production (Chapman 1974; Gattuso et al. 1998; Gazeau et al. 2004; Mann 1973; Mann 1982). They also provide food and valuable habitat for a wide range of associated fauna such as invertebrates and fishes, while being important contributors to food webs (l)uggins et al. 1989).
However, even if production in macroalgae has been extensively investigated, especially in iMminaria species (Davison et al. 1991; Drew 1983a,1983b; Dunton and Jodwalis 1988; Gerard and Du ISois 1988; Hatcher 1977; Sakanishi et al. 1991) using a variety of techniques, none of these methods is very satisfactory, with each of them presenting its own disadvantages. Estimates of aquatic plant and algal production based on biomass and changes in standing crop (Aleem 1956), which rely on periodic harvesting, do not take into account tissue loss due to grazing and sloughing. In addition to being destructive, they give such variable results that estimation tends to be done with other methods such as gas exchange.
Measuring dissolved oxygen In open water around kelps (Mcfarland and Prescott 1959) suffers from possible Interference by water movement and activities of other organisms. Carbon fixation (MC) of intact kelp blades held In polyethylene bags has also been used (Johnston and Cook 1968; Towle and Pearse 1973) but is a short-term incubation and is heavy to operate. Oxygen production by isolated blade discs was also developed (Clendenning 1964; Sargent and Lantrip 1952) but docs not closely replicate conditions found within a kelp hed. Delieu and Walker (1981) developed a polarographic method of measuring photosynthetlc oxygen evolution by leaf discs, which was then commonly used by authors to study photosynthesis in algae, such as Henley et al. (1991) and Henley (1993) who measured and interpreted photosynthetlc light-response curves In the context of photoinhlbition and diel changes. Most of the gas exchange measurements were thus made in the laboratory, on some parts of the thallus, whereas only a few were performed directly in the field. As pointed out by King and Schramm (1976a), the correlation of laboratory measurement of gas exchange with biomass estimations (with or without sampling) Is confounded by the pronounced morphological and physiological differentiation of the thallus as displayed by Lamiruiriii iligitata and many other algae. Finally, assessing primary production of bcnthlc marine algae by establishing relationships using laboratory measurement of photosynthesis is fraught with difficulty as only measurements of some aspects of the photosynthetlc process are taken into account. Nevertheless, these laboratory investigations are necessary to determine the physiological responses of algae to individual environmental factors as the ecological factors in the field situation are complex.