Properties of chloroplasts isolated from siphonous algae: effects of osmotic shock and detergent treatment on intactness

Identification of sequestered chloroplasts in photosynthetic and non-photosynthetic sacoglossan sea slugs (Mollusca, Gastropoda)

Background

Sacoglossan sea slugs are well known for their unique ability among metazoans to incorporate functional chloroplasts (kleptoplasty) in digestive glandular cells, enabling the slugs to use these as energy source when starved for weeks and months. However, members assigned to the shelled Oxynoacea and Limapontioidea (often with dorsal processes) are in general not able to keep the incorporated chloroplasts functional. Since obviously no algal genes are present within three (out of six known) species with chloroplast retention of several months, other factors enabling functional kleptoplasty have to be considered. Certainly, the origin of the chloroplasts is important, however, food source of most of the about 300 described species is not known so far. Therefore, a deduction of specific algal food source as a factor to perform functional kleptoplasty was still missing.

Results

We investigated the food sources of 26 sacoglossan species, freshly collected from the field, by applying the chloroplast marker genes tufA and rbcL and compared our results with literature data of species known for their retention capability. For the majority of the investigated species, especially for the genus Thuridilla, we were able to identify food sources for the first time. Furthermore, published data based on feeding observations were confirmed and enlarged by the molecular methods. We also found that certain chloroplasts are most likely essential for establishing functional kleptoplasty.

Conclusions

Applying DNA-Barcoding appeared to be very efficient and allowed a detailed insight into sacoglossan food sources. We favor rbcL for future analyses, but tufA might be used additionally in ambiguous cases. We narrowed down the algal species that seem to be essential for long-term-functional photosynthesis: Halimeda, Caulerpa, Penicillus, Avrainvillea, Acetabularia and Vaucheria. None of these were found in Thuridilla, the only plakobranchoidean genus without long-term retention forms. The chloroplast type, however, does not solely determine functional kleptoplasty; members of no-retention genera, such as Cylindrobulla or Volvatella, feed on the same algae as e.g., the long-term-retention forms Plakobranchus ocellatus or Elysia crispata, respectively. Evolutionary benefits of functional kleptoplasty are still questionable, since a polyphagous life style would render slugs more independent of specific food sources and their abundance.

Photosynthetic Properties of Chloroplasts from Chlamydomonas reinhardii

Chloroplasts isolated from synchronous cultures of the unicellular green alga Chlamydomonas reinhardii, SAG 11-32/b (-), fix CO(2) at rates between 25 and 50 micromoles per milligram chlorophyll per hour. The upper value is approximately half of the rate of the intact cell.During storage in the dark on ice, the chloroplast preparation loses 30 to 50% of its CO(2) fixing capability per hour. Under reducing conditions (+ 1 millimolar dithiothreitol), this loss of activity is about twice as fast. The same reducing conditions stimulate CO(2) fixation in the light.High concentrations of inorganic phosphate (>2 millimolar) inhibit CO(2) fixation. This inhibition is overcome by the addition of glycerate 3-phosphate. It is concluded that chloroplasts from C. reinhardii possess a higher plant type phosphate translocator. With respect to dependency upon light intensity, pH and Mg(2+) concentration, the results were similar to that reported for chloroplasts from higher plants. However, in contrast to higher plant chloroplasts, maximum CO(2) fixation is observed at the relatively low osmotic concentration of 0.12 molar mannitol in the reaction buffer.

Isolation of Functionally Intact Rhodoplasts from Griffithsia monilis (Ceramiaceae, Rhodophyta)

A procedure is described for isolating photosynthetically active rhodoplasts ("red algal chloroplasts") from the marine alga Griffithsia monilis. The rhodoplasts exhibited rates of CO(2) fixation and CO(2)-dependent O(2) evolution in the order of 200 micromoles per milligram chlorophyll a per hour when illuminated with red or green light and were approximately 80% intact. The response of the rate of photosynthesis to the inorganic phosphate and pyrophosphate concentrations in the medium was qualitatively similar to that previously reported for spinach chloroplasts. Osmotically shocked rhodoplasts evolved O(2) from ferricyanide in red, but not in green, light and were completely uncoupled. Rhodoplast envelope rupture appeared to be accompanied by phycobilisome loss from the thylakoids.

Purity of Chloroplasts Prepared from the Siphonous Green Alga, Caulerpa simpliciuscula, as Determined by Their Ultrastructure and Their Enzymic Content

The ultrastructure and enzyme distribution in chloroplasts and other subcellular fractions isolated from the siphonous green alga, Caulerpa simpliciuscula, are described. The isolated chloroplasts were similar in appearance to those in the tissue from which they were derived, and in typical preparations 70% or more were intact. Chloroplasts which had lost their outer envelopes could be separated from intact plastids by centrifugation at low speeds through gradients of colloidal silica. Intact chloroplasts separated in this way retained their photosynthetic capacity and were impermeable to ferricyanide ions. The chloroplast preparations separated by differential centrifugation and refractionated using either discontinuous or continuous Percoll gradients contained non-chloroplast material. It was estimated that this amounted to a maximum of 10% of the mitochondrial population and 6% of cytoplasm extracted from the plant. The contaminating material surrounded the chloroplasts in a thin layer and was surrounded by a membrane.

Preparation of intact chloroplasts by chemically induced lysis of the green alga Dunaliella marina

A method for the isolation in high yield of intact chloroplasts from the unicellular green alga Dunaliella marina (Volvocales) is described. This procedure uses chemically induced lysis of cells with the polycationic macromolecules, DEAE-dextran (M=500,000) or poly-D,L-lysine (M=30,000-70,000). Reaction conditions were optimized with respect to obtaining a high yield of intact chloroplasts, after isopycnic centrifugation in a linear sucrose density gradient, by varying the concentration of polycation and the temperature and pH of incubation. Broken chloroplasts devoid of the stromal marker enzymes fructosebisphosphate phosphatase and ribulosebisphosphate carboxylase, but containing mitochondrial (fumarase) and microbody (catalase) contamination, were banded at a bouyant density of 1.18 g cm(-3). Intact chloroplasts, as indicated by their retention of alkaline fructosebisphosphate phosphatase and ribulosebisphosphate carboxylase, were found in 30% yield (chlorophyll in intact cells, 100%) at an equilibrium density of 1.24 g cm(-3). Contamination by cytoplasmic material (pyruvate kinase), mitochondria, and microbodies was less than 8% each.

Glutamine Synthetase/Glutamine: alpha-Ketoglutarate Aminotransferase in Chloroplasts from the Marine Alga Caulerpa simpliciuscula

The enzymic capacities for ammonia assimilation into amino acids have been investigated in chloroplasts from the siphonous green alga Caulerpa simpliciuscula (Turner) C. Ag. The results show that these chloroplasts differ from those of higher plants in having present simultaneously the enzymic capacities to permit assimilation of ammonia by two pathways. Glutamine synthetase (EC 6.3.1.2) activity at levels up to 4 mumoles per mg chlorophyll per hour were found in soluble extracts of the chloroplasts. Glutamine(amide):alpha-ketoglutarate aminotransferase (oxidoreductase ferredoxin) (EC 1.4.7.1) activity at levels up to 1.4 mumoles per mg chlorophyll per hour was detected by incubation of photosynthetically active chloroplasts either in light or with reduced ferredoxin. Together these enzymes provide the capacity for the conventional pathway of ammonium assimilation in chloroplasts via glutamine. A similar level of a glutamate dehydrogenase with an unusually low K(m) for ammonia which has been described previously in these chloroplasts provides the second potential pathway.