Diet of copepods (Scopalatum vorax) associated with mesopelagic detritus (giant larvacean houses) in Monterey Bay,California

The feeding structures or houses of the giant larvacean Bathochordaeus sp. serve as both habitat and food for the calanoid copepod Scopalatum vorax. Gut contents of S. vorax include both microbial and metazoan associates of larvacean houses, and possibly the house-mucus matrix itself. Copepods were observed and collected from larvacean houses between 100 and 500 m in Monterey Bay, California, using a submersible ROV (remotely operated vehicle) from the Monterey Bay Aquarium Research Institute. Gut contents were compared to potential food items on the houses and in the open water (not associated with the house). Copepods were generalist feeders, with amorphous detritus, diatoms, and copepods or other crustacean parts dominating gut contents. Protozoans and algae other than diatoms were rarer in guts. Houses contained a diverse assemblage of microplankton and metazoans, both intact specimens and detrital remains of these. Numbers of diatoms and fecal pellets were enriched by 1 to 3 orders of magnitude on houses compared to numbers in surrounding water. Many of the abundant species of diatoms and copepods on houses occurred in S. vorax guts. This observation coupled with S. vorax feeding habits observed in situ and in the laboratory provide evidence for feeding on houses. S. vorax appears to possess special adaptations to living in a resource-limited environment, such as gorging as a feeding adaptation, chemosensory structures to help locate houses, and the ability to change feeding modes. Consumption of detritus at depth by S. vorax provides evidence that metazoans contribute to remineralization of particulate organic carbon in the mesopelagic zone.     

Field behavior and adaptive strategies of appendicularians (Chordata: Tunicata)

The behavior of 7 species of appendicularians from the family Oikopleuridae was observed using SCUBA in the Gulf of California and the Florida Current. The frequency and orientation of feeding and the pattern of swimming while within the house varied considerably among species. Appendicularians expanded new houses in 1 1/2 to 5 min. House-expansion behavior was complex and variable among species. Appendicularians rarely abandoned the house in response to predation. Predators included the sergeant major (Abudefduf sp.), medusae, chaetognaths and ctenophores. The abandonment of the house was based on a cost-benefit behavior strategy which maximized time spent inside the house while minimizing predation and energy investment in house secretion. Strategies of predator avoidance are discussed.     

Organogenesis and histogenesis in the planktotrophic veliger of Doridella steinbergae (Opisthobranchia: Nudibranchia)

Development of the planktotrophic veliger of the dorid nudibranch Doridella steinbergae (Lance) was studied by histological examination of 4, arbitrarily defined larval stages. Following an embryonic period of 7 1/2 to 8 days (12° to 15°C), the newly hatched veligers possess a functional digestive tract, a pair of nephrocysts, a secondary kidney, a pair of cerebral ganglia, a larval shell consisting of a two-thirds whorl, and the metapodial component of the foot. Development during Stage I mainly involves growth of the larval shell and the visceral organs. Stage II is marked by the retraction of the mantle fold from the shell aperture and the appearance of the eyespots, gonadal rudiment, larval heart, and the optic, pedal, and pleural ganglia. At Stage III the radular sac rudiment evaginates from the esophageal wall, the buccal ganglia differentiate, and the propodial rudiment begins to develop on the ventral surface of the metapodium. Stage IV veligers, which are competent to metamorphose, possess 6 pairs of radular teeth, lipid deposits in the left digestive gland, rudiments of the adult kidney and the oral lip glands, an hypertrophied mantle fold, a propodium, and densely packed cilia over the entire ventral surface of the foot. The length of the obligatory larval period, from hatching of the veliger until the attainment of metamorphic competence, is 25 to 26 days under laboratory culture conditions and the larval shell grows from 142 to 168 m in length. The sequence of morphogenetic events and the structure of the competent veliger of D. steinbergae is compared to that of other opisthobranch veligers. It is suggested that the relatively small maximal shell size attained by D. steinbergae results from precocious retraction of the mantle fold. It is further suggested that interspecific differences in the kinds of structures that develop during the veliger phase of opisthobranchs may relate to variations in the requirements of the juvenile phase. The functional adaptations of the gut of planktotrophic veligers are discussed and compared to those of lecithotrophic veligers.     

Dinoflagellate luminescence increases susceptibility of zooplankton to teleost predation

The burglar alarm theory of bioluminescence was investigated by determining predation rates of a nocturnal teleost predator,Porichthys notatus, on nonluminescent kelp mysids illuminated by dinoflagellate flashes, between the fall and spring of 1989/1990. Mysids (Holmesimysis costata) were placed in aquaria containing varying concentrations (0 to 40 cells/ml) of the dinoflagellatePyrocystis fusiformis and a single midshipman fish. Controls usedP. fusiformis during their luminescence-inhibited day phase. Mysid swimming movements readily stimulated dinoflagellate luminescence. Flashes and prey strikes were observed simultaneously by image-intensifying and infrared video cameras on a splitscreen monitor. Predation rates increased at dinoflagellate concentrations of 3 to 15 cells/ml and decreased below controls at levels>20 cells/ml. Videotape analysis showed that at low concentrations (2 to 5 cells/ml), strike success rates exceeded 75% if prey were previously illuminated by a flash, but dropped below 50% at higher cell densities. Increased predation was attributed to luminescence revealing prey position. The decrease at higher concentrations was considered to be due to greater flash frequency providing a more diffuse and confusing target. The study demonstrates the effects of secondary luminescence on zooplankton predation at normally encountered dinoflagellate concentrations.     

Acquisition of breeding coloration depends on badge size in male house sparrows Passer domesticus

Summary The black throat badge of the male house sparrow Passer domesticus, which functions as a status signal of dominance rank, changes in size during late winter because light feather tips gradually wear off. Males change the size of their visible badge earlier if their final badge size is large. The differential timing of the change from winter to breeding coloration in relation to badge size is partially controlled by the behaviour of the individual male. Feather abrasion of the badge is mainly due to preening and dust bathing. During late winter, male house sparrows preen the badge area more than females preen the homologous area, and males with large badges preen more than males with small badges. The seasonal change in size of the visible badge of male house sparrows may reflect the balance of different selection pressures. A large badge size signals dominance status in autumn and winter while predation by visually searching avian predators may constitute the main opposing selection pressure. Sexual selection causes a stronger selection pressure for a large badge in spring and summer. Correspondence to: A.P. Moller     

A comparative study of photoadaptation in four diatoms isolated as endosymbionts from larger foraminifera

Four endosymbiotic diatoms were isolated from 2 species of larger foraminifera collected in the Red Sea and Hawaii. The photoadaptive responses of the cultured diatoms were measured at 312, 19 and 7 W cm^-2. Two of the diatoms (Fragilaria shiloi and Nitzschia laevis), both isolated from Amphistegina lessonii, grew fastest at 312 W cm^-2. The other two diatoms (N. valdestriata and N. panduriformis) which were isolated from Heterostegina depressa, grew best at 19 W cm^-2. Of the four diatoms, F. shiloi grew best at high light levels. Also in F. shiloi, chlorophyll c content per cell was directly proportional to light intensity; in contrast chlorophyll a and carotenoids increased to maxima at 19 W cm^-2. The chlorophyll a and c and carotenoid content of N. valdestriata were also maximal at 19 W cm^-2. Photosynthetic rates, measured by respirometry, suggested that the diatoms were photoinhibited at higher light intensities and did well at moderately low light intensities (175W cm^-2). The photocompensation points of all 4 diatoms were about 2% of the light available in the spring at 1-m depth at Elat on the Red Sea. At Elat the photocompensation point would lie between 40 and 50 m if the algae were free in nature. The amount of attenuation of light by the shells of the host has not yet been measured. Presumably photocompensation of the algae within hosts is reached at depths less than 40 m.     

Current uses of the EPA lead model to assess health risk and action levels for soil

The EPA lead model predicts mean blood lead levels and risk of elevated blood lead levels in children based on lead uptake from multiple sources. In the latest model versions, environmental data from individual homes within a community can be used to predict the overall blood lead distribution and percent risk of exceeding a specific blood lead level (i.e. 10 g dl^–1). Recent criteria used by the EPA to evaluate this information include no more than 5% of houses with a greater than 5% lead risk, and a community weighted-average risk below 5%. Environmental (primarily soil) and blood lead data from a residential community near a smelter were used to illustrate recent uses of the model. Scheduled remediation in the community will remove soil for approximately 60% of the houses (i.e. those with lead levels > 1000 mg kg^–1). After remediation, the model results indicate a relatively low community risk (0.5–1.9%), although the percentage of houses with lead risks above 5% ranged from 3 to as high as 13%, depending on the variation in blood lead and assuming the model's 7 g dl^–1 increase in blood lead with each 1000 mg kg^–1 increase in soil lead level. A comparison of the limited blood lead data with soil lead levels below 1000 mg kg^–1, however, indicated no apparent relationship. Given these uncertainties, less invasive actions than additional soil removal (e.g. exposure intervention, monitoring conditions, and follow-up as necessary) may be appropriate under the new EPA guidance for lead in soil.     

A simple technique for preservation and staining of the delicate houses of oikopleurid tunicates

The houses of oikopleurid larvaceans are of such a delicate structure that their preservation and storage for reference collections and post-collection study has been essentially impossible. However, houses ofOikopleura dioica, O. labradoriensis andO. vanhoeffeni, ranging in size from 0.7 to 7 cm, may be dried onto glass slides or onto paper or membrane filters so that most structural details are maintained. The handling of houses during the drying process is greatly facilitated if they are prestained by particulate or colloidal dyes while still inhabited. Such staining also reveals a wealth of structural detail that is difficult to obtain by other means. Staining the house after mounting and drying is also possible, but more difficult. The described technique may be used for making permanent records of oikopleurid houses of all sizes, from the first post-metamorphic house of <1 mm diameter to the largest known oikopleurid house of perhaps 1 m diameter. Such records may serve both as taxonomic tools and as specimens for detailed structural and chemical analysis by high-power microscopes and microbeam instruments.     

Gill morphogenesis in the oyster Ostrea chilensis

Despite the importance of the gills in the acquisition of food by suspension-feeding bivalve mollusks, there is almost no information on gill organogenesis. By means of a series of stereoscan electron micrographs, this paper describes gill development in the Chilean oyster, Ostrea chilensis, from the brooded larval stages to 1-month-old spat. A single gill rudiment was observed on each side of the mantle at a shell length of 320 μm, and the rudiments increased in number and size until the end of the brooding period. During metamorphosis the gill filaments increased in number from 5 or 6 to between 7 and 9. The loss of the velum and the absence of functional gill filaments during metamorphosis are consistent with previous observations of weight loss during this critical period of the life history, because the newly settled juvenile lacks the ability to remove particles from suspension. The end of metamorphosis (100% of spat with dissoconch edge) was reached 36 h after larval settlement, when the gill filaments began to grow cilia, which increased in density and differentiated as the spat developed and acquired the capability of suspension-feeding, accounting for the increase in body weight previously recorded during this stage. The larval rudiments gave rise to the inner demibranchs. The outer demibranchs were observed 10 days after settlement, located between the inner demibranch and the mantle. In 1-month-old spat, the gill did not show differentiation between primary and secondary filaments, indicating that the heterorhabdic condition characteristic of adult oysters had yet to be attained. Received: 11 December 1998 / Accepted: 21 August 2000     

Effects of zinc on epidermal ultrastructure in the larva of Clupea harengus

In March, 1983 changes in epidermal ultrastructure were examined in Clupea harengus L. larvae hatched from eggs incubated in four zinc concentrations (0.5 2.0, 6.0 and 12.0 ppm). In addition to the outer and inner epidermal cell types described previously, a third type of cell is present. Ultrastructurally this resembles the epidermal chloride cells of Sardinops caerulea Girard, characterised by numerous mitochondria, extensive smooth endoplasmic reticulum and a free surface exposed to the environment. These cells occur on the yolk sac, head and in the regions of the trunk just above the yolk sac. In larvae treated in 12.0 ppm zinc, the cells contain fewer mitochondria with fewer cristae than those in the controls. Smooth endoplasmic reticulum is much reduced forming nodular masses and contains granular osmiophilic inclusions. In larvae hatching from eggs previously incubated in 6.0 and 12.0 ppm zinc, the epidermal cells contain more vesicles, intracellular spaces, and swollen mitochondria and show signs of necrosis. The viability of these larvae is discussed.     

Stable lead isotope profiles in smelter and general urban communities: a comparison of environmental and blood measures

High-precision lead isotope ratios and lead concentrations have been compared statistically and graphically in women of child-bearing age (n = 77) from two smelter communities and one general urban community to evaluate the relative contributions to blood lead of tissue lead stores and lead from the contemporaneous environment (soil, floor dust, indoor airborne dust, water, food). Blood lead (PbB) contents were generally low (e.g. <10 g dL^–1). Statistically significant isotopic differences in blood and environmental samples were observed between the three cities although isotopic differences in blood for individual subjects living in close proximity (200 m radius) was as large as the differences within a city. No single environmental measure dominated the biological isotope profile and in many cases the low levels of blood lead meant that their isotopic profiles could be easily perturbed by relatively small changes of environmental exposure. Apportioning of sources using lead isotopes is possibly not feasible, nor cost effective, when blood lead levels are <5 g dL^–1. Interpretations based on statistical analyses of city-wide data do not give the same conclusions as when the houses are considered individually. Aggregating data from multiple subjects in a study such as this obscures potentially useful information. Most of the measures employed in this study, and many other similar studies, are markers of only short-to-medium integration of lead exposure. Serial sampling of blood and longer sampling times, especially for household variables, should provide more meaningful information.     

Effects of light of altered spectral composition on coral zooxanthellae associations and on zooxanthellae in vitro

Pocillopora damicornis (Linnaeus) and Montipora verrucosa (Lamarck) were collected from Hawaiian reefs. In two experiments (September 1979-January 1980: ca. 4 mo; August-October 1980; ca. 2 mo), these reef corals were grown under sunlight passed through filters producing light fields of similar quantum flux but different spectral composition. In vitro cultures of symbiotic zooxanthellae (Symbiodinium microadriaticum Freudenthal) from M. verrucosa were cultured under similar conditions for 15 d. Blue or white light promoted more coral skeletal growth than green or red light. In both coral species, blue light increased the total amount of chlorophyll a of the coral-zooxanthellae association. In the perforate species, M. verrucosa, the pigment concentration was elevated by an increase in the density of zooxanthellae, but the pigment concentrations per algal cell remained unchanged; in the non-perforate species, P. damicornis, it appears that pigment concentration was elevated by an increase in pigment per algal cell, and not by an increase in density of zooxanthellae. The sunloving reef-flat coral P. damicornis did not grow as rapidly as the shade-species M. verrucosa at the low quantum flux (about 10% sunlight) provided by the experimental treatments. The in vitro cultures of zooxanthellae from M. verrucosa exhibited growth rates in light of altered spectral quality that correlated with the responses of the host coral species: blue and white light supported significantly greater growth than green light, and red light resulted in the lowest growth rate.Contribution No. 678 of the Hawaii Institute of Marine Biology     

The oral cavity and bioluminescent organs of the cardinal fish species<Emphasis Type="Italic"> Siphamia permutata</Emphasis> and<Emphasis Type="Italic"> S. cephalotes</Emphasis> (Perciformes,Apogonidae)

The mechanism of bacterial-derived bioluminescence in the apogonid species Siphamia permutata and S. cephalotes presents some special structures, particularly the existence of two sites harboring bioluminescent bacteria, and not only one as previously described. One site, is the familiar disc-like, bacteria-harboring gland and paired ventral bioluminescent reflectors, typical for this genus, that merge and end at the level of the hypobranchial region. The second and more anteriorly situated site of luminescence features two sacs that originate in the gular region and protrude into the oral cavity, via the free space left by the laterally notched tongue. The apical parts of these luminous sacs harbor the bacteria whose light diffuses within the oral cavity. The tongue surface in the studied species is unique in character among the cardinal fishes, being criss-crossed anteriorly by protruding dermal ridges, rich in taste buds and mucus-producing cells. It would appear that at night when feeding, the luminous mouth cavity of these fish acts as a lure to attract the small prey, while the ridged structure of the tongues surface facilitates their collection and aggregation before they are swallowed.Communicated by O. Kinne, Oldendorf/Luhe     

Tidal variations in the photosynthesis of estuarine phytoplankton isolated in a tank

In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH₄ were observed that could be related to cyclical NH₄ uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (^B) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

Photosynthetic characteristics of marine phytoplankton from contrasting physical environments

Two suites of phytoplankton samples have been collected in consecutive years at various times over a day from selected depths within vertically mixed and stratified water columns in the western Irish Sea, in order to provide a range of possible light histories within the populations collected. Values for the maximum rate of ¹⁴C retention (P _max) and the initial slope of the ¹⁴C retention: light intensity curve () were obtained. Supra-thermocline samples from the stratified water exhibited higher P _max values than corresponding subthermocline samples. Higher values of were also generally associated with samples from the supra-thermocline zone of the stratified region. Differences in the depth distribution of P _max and in the mixed water were small, except in the presence of a shallow thermocline. In one suite of samples from the stratified water, a diurnal increase in the P _max values of the supra-thermocline samples was observed. P _max values obtained from the samples from the mixed water were interpreted in relation to the distribution obtained from the samples from the stratified zone. Data from both the contrasting sites visited for one sample suite demonstrated a two-phase relationship between the chlorophyll a concentration and both P _max and . The rates of ¹⁴C retention of the first suite of samples were estimated by two techniques. The average differences in the retention were greater in samples from the sub-as opposed to suprathermocline zone. No trends were apparent in the smaples from the mixed waters.     

Filter feeding by Oikopleura vanhoeffeni: grazing impact on suspended particles in cold ocean waters

Because of the abundance and size of Oikopleura vanhoeffeni its quantitative role as a suspension feeder in cold ocean waters needs to be defined. To minimize the effect of manipulation and containment, and to assess the effect of naturally occurring factors on clearance rate, I used an in situ latex microbead technique in Logy Bay, Newfoundland, from February 1985 to June 1986. Individual clearance rates ranged from 8–944 ml h^-1, increasing exponentially with increasing trunk length. Partial correlation and principal components analysis indicated that trunk length and the concentration of ingestible chlorophyll a accounted for a majority of the variation in clearance rate. At densities of 4–110 m^-3, O. vanhoeffeni populations removed from >1 to 13% of the standing stock of ingestible food particles each day. Grazing by near-surface populations was lowest during the spring diatom bloom (>1.4% of daily particle production removed per day), and was highest in June during the post-bloom crash (4 to 10% of daily production removed). Some populations in mid-depth waters had much higher population clearance rates (ca. 50% of daily production removed) because of a greater proportion of large animals. The median percentage daily ration (g Cxg C^-1xd^-1x100%) of 64% accounted for observed house production rates (1 to 2 d^-1, with each house=23% of body carbon).     

Symbiont photosynthesis increases both respiration and photosynthesis in the symbiotic sea anemone Anemonia viridis

Dark respiration of the symbiotic sea anemone Anemonia viridis (Forskäl) was observed to increase by 34% when anemones were exposed to hyperoxic sea water (150% oxygen saturation) overnight, and by 39% after exposure to 6 h in the light at a saturating irradiance of 300 E m^-2 s^-1 at normoxia (100% oxygen saturation). No increase due to light stimulation was observed in aposymbiotic control anemones. In darkness, the oxygen concentration of the coelenteric fluid was hypoxic. However, within 10 min of anemones being illuminated, coelenteric fluid was hyperoxic, and it remained elevated throughout a 12 h light period. When measured over a 24 h period (12 h light: 12 h dark), the dark respiration rate increased gradually over the first 6 h of the light period until it was 35% above the dark night-time resting rate. It remained elevated throughout the remaining light period and for 2 h into the following dark period, after which it fell back to the resting rate. Gross photosynthesis (P ^gross) increased significantly when anemones were exposed to either hyperoxia (150% oxygen saturation) or 300 E m^-2 s^-1 at normoxia. This increase was not observed when symbiotic anemones were illuminated at a low-light intensity of 100 E m^-2 s^-1. The results of this study suggest that respiration in the dark is limited by oxygen diffusion and that normal respiration is restored in the daytime by utilisation of the oxygen released by photosynthesis. Furthermore, it appears that the increased respiration following exposure to high-light intensities provides a CO₂-rich intracellular environment which further enhances the photosynthetic rate of the zooxanthellae.     

Time-course of photoadaptation in the photosynthesis-irradiance relationship of a dinoflagellate exhibiting photosynthetic periodicity

Cultures of the marine dinoflagellate Glenodinium sp. were light-shifted and rates of photoadaptation determined by monitoring changes in cell volume, growth rate, pigmentation, parameters of the photosynthesisirradiance (P-I) curves and respiration. To approximate physiological conditions of field populations, cells were cultured on an alternating light-dark cycle of 12hL:12hD, which introduced a daily periodicity of photosynthesis. One result of the present study was to demonstrate how specific parameters of the P-I relationship influenced by periodicity of the light: dark cycle are distinguished from photosynthetic parameters influenced by changes in light level. Under steady-state conditions, rates of both light-saturated (P_max) and light-limited photosynthesis changed in unison over the day; these changes were not related to pigmentation, and displayed their maxima midday. This close relationship between P_max and the slope (a) of the cellular P-I curves in steadystate conditions was quickly adjusted when growth illumination was altered. Rates of light-limited photosynthesis were increased under low light conditions and the periodicity of cellular photosynthesis was maintained. The short-term responses of the P-I relationship to changing light level was different, depending on (1) whether the light shift was from high to low light or vice versa, and (2) whether the high light levels were sufficient to promote maximal photosynthesis rates. Major increases in the photosynthetic carotenoid peridinin, associated with a single type of light-harvesting chromo protein in the chloroplast, was observed immediately upon shifting high light cultures to low light conditions. Following pigment synthesis, significant increases in rates of light-limited photosynthesis were observed in about one-tenth the generation time, while cellular photosynthetic potential was unaffected. it is suggested that general results were consistent with suggested that general results were consistent with earlier reports that the major photoadaptive strategy of Glenodinium sp. is to alter photosynthetic unit (PSU) size. Photoadaptive response times to high light were light-dependent, but appeared to be shower than photoadaptive responses to low light. If light intensities were bright enough to maximize growth rates, photosynthetic response times were on the order of a generation period and pigmentation fell quickly as cells divided at a faster rate. If light-intensities were not sufficient to maximize growth rates, then pigment content did not decline, while rates of light-limited photosynthesis declined quickly. In all cases, photoadaptation was followed best by monitoring fast changes in half saturation constants for photosynthesis, rather than fluctuating changes in pigmentation. Results compared well with time-course phenomena reported for other groups of phytoplankton. Overall, results suggest phytoplankton can bring about photo-induced changes in photosynthesis very quickly and thus accommodate widely fluctuating light regimes over short periods of time.     

Growth,photosynthesis and carbon metabolism in the temperate marine diatom Skeletonema costatum adapted to low temperature and low photon-flux density

The temperate diatom Skeletonema costatum (Grev.) Cleve was grown in low temperature and/or low light conditions. The cultures were acclimatized for at least three months before experiments were begun. Our data indicate that the initial slope of the photosynthesis vs irradiance curve () is controlled predominantly by light history and the light-saturated photosynthesis (P _max) by temperature. The number of divisions per day decreased with decreasing light intensity, but was identical for cultures grown at 3° or 18°C. The metabolic pathways of inorganic carbon fixation were not fundamentally affected by low temperature or low light intensity, but both these factors increased labelling of C₃ compounds, synthesized by the Calvin-Benson cycle, and decreased that of phosphoenolpyruvate (PEP) and other metabolites. This indicates an enhancement of ribulose-1,5-bisphosphate (RuBP) carboxylase activity, which is the first step in the C₃ pathway (3-phosphoglycerate and sugar phosphate synthesis); this may optimize cell functions. At low temperatures, a seven-fold increase in RuBP carboxylase activity per cell was observed. S. costatum is able to adapt to low irradiance by increasing and decreasing I _k (the ratio of P _max:, light intensity at onset of light saturation), and to low temperature by increasing its cellular chlorophyll a and RuBP carboxylase content. However, in the latter case, adaptation is not optimal. This study revealed two main features: (1) there is evidence that RuBP carboxylase has a key function in adjustment to high rates of photosynthesis at suboptimal temperatures or irradiances; (2) adaptive mechanisms are dynamic processes and the role of the time scale in physiological adaptation should be considered.