The stimulating effect of blue light on the carbon-concentrating mechanism of Ectocarpus siliculosus was investigated by measuring the intracellular pools of inorganic carbon (Ci) in red light (low photosynthetic rates) and in red light plus blue light (high photosynthetic rates). Ci accumulated in red light to about twice the levels found in the presence of blue light, but the permeability of the cell membranes for Ci, measured as the rate of influx of tracer carbon, was similar in both light conditions. This was also true after the seawater pH was adjusted to pH 9·5, when bicarbonate should be the only inorganic carbon species to enter the cells. Activation of photosynthesis after prolonged red light caused the accumulated Ci to decline to levels similar to those observed in blue light. The time course of this decline was rapid and coincided with the time course that was expected for the emptying of a carbon pool whose existence had been demonstrated earlier in measurements of photosynthetic oxygen evolution in the absence of external carbon. The accumulation of Ci in red light required active photosynthesis, but it also occurred in the presence of inhibitors of carbonic anhydrase activity. It is suggested that blue light mobilizes the Ci pool by causing changes in intracellular pH and that the size of the pool depends on the activity of metabolic CO2-sink activities that are also under the control of blue light.

The hypothesis that carbon acquisition in phaeophytes of lower organization is controlled by a blue-light-regulated C4 mechanism was tested by various approaches with the filamentous species Ectocarpus siliculosus. Here we report analyses at the enzymatic level. Enzymes potentially participating in a C4 cycle were assayed in extracts, and activities of a phosphoenolpyruvate carboxykinase (PEPCK), of NAD- and NADP-malate dehydrogenases, and of a NADP-malic enzyme were detected. No activity could be measured of phosphoenolpyruvate carboxylase or of pyruvate-Pi-dikinase, which is essential to complete the cycle by regenerating phosphoenolpyruvate from pyruvate. The kinetic properties of all enzymes were determined. Malic enzyme and PEPCK were purified to homogeneity, as tested by gel filtration. The molecular mass of native malic enzyme was determined to be 440 kDa. In SDS gel electrophoreses it produced a single band with a relative molecular mass of 53 kDa, which suggested that the native molecule might be a homo-octamer. The molecular mass of PEPCK was 90 kDa and separation on SDS gels produced two major bands, one of 56 kDa and one of 18 kDa, which appeared not to be a degradation product of the larger polypeptide. The composition of the native enzyme is unclear. In extracts, the activities of malic enzyme and PEPCK followed a circadian rhythm, but none of the enzymes tested was found to be blue-light-dependent. The above results neither prove nor dismiss the possibility that a C4 mechanism operates in Ectocarpus.

The fine structure of the flagellar apparatus of male gametes of Ectocarpus siliculosus has been studied in negatively stained whole-mount preparations of isolated flagellar apparatuses and by transmission electron microscopy of embedded whole cells and flagellar apparatuses. Reconstructions from serial sections revealed a single absolute configuration and new structural details of the flagellar apparatus. A set of 5 microtubular roots is associated with the 2 flagellar basal bodies: the major anterior root consisting of 9 microtubules originates at the anterior basal body and loops through the anterior part of the cell. Cytoplasmic microtubules originate from this root at the apex of the cell and splay out in a posterior direction through the cell, thus determining cell shape. A broad microtubular band composed of a 9-membered bypassing root and a 3-membered major posterior root runs across the basal bodies. Two additional flagellar roots, the minor anterior and minor posterior rootlets, consist of a single microtubule each. The proximal end of the posterior basal body is laterally connected to the anterior basal body by a cross-striated connective (deltoid band). The basal bodies are connected to the nucleus by a small rhizoplast, and several other fibrous structures are associated with the basal bodies. The results are discussed in relation to earlier observations on flagellar apparatus organization in brown algae.

The ultrastructure of male gametes of Ectocarpus siliculosus is described and discussed in relation to the information available on other brown algal zoids. In general, the fine structure and the organelle associations agree with those of other brown algal cells. In particular, the male gametes typically contain a single chloroplast with a pyrenoid and a prominent concave eyespot, into which the swelling of the posterior flagellum (the photoreceptor) fits. The continuity of the chloroplast endoplasmic reticulum and nuclear membrane, as well as the presence of a perinuclear Golgi consisting of several dictyosomes, is shown. The fine structure of microbodies, lipid bodies and different types of vesicles, including physodes and coated vesicles, is also discussed. Several specializations of cellular elements and their interactions are unique and have not been reported before. These features include the connection between the nuclear envelope, the Golgi and the flagellar bases mediated by fibres resembling intermediate filaments, and the organization of the Golgi as well as the association of mitochondria and a microtubular flagellar root.