Estimation of Arctic copepod grazing rates in vivo and comparison with in-vitro methods

Gut evacuation rates were measured in Calanus hyperboreus and C. glacialis from two stations in Jones Sound, Northwest Territory (NWT) and one station in an Ellesmere Island Fjord during late summer of 1984. Gut content decreased exponentially with a rate constant, that, for Stage V C. glacialis at least, was independent of food type and time of day. Gut filling rates were measured in Stage V C. glacialis in the light and in the dark, at noon and midnight. Nighttime gut filling rates were very similar for both light intensities, and also similar to the daytime rate in the dark, whereas the daytime rate in the light was much lower. Ingestion rates were calculated for these latter experiments, including a rate term for defecation, and these results were compared to the values obtained from the observations of gut filling rates in vivo as reported in Head et al. (1985) and from long-term (2–3 d) bottle incubations as reported in Head et al. (in press). The following points were made: (1) in-vivo and in-vitro ingestion rates were very close if appropriate in-vitro experimental conditions were used with respect to light intensity and time of day; (2) copepods could fill their guts at a rate apparently higher than their normal nocturnal ingestion rate; and (3) the calculated rations were dependent on the shape of the observed diurnal feeding patterns.     

Effects of non-circadian light/dark cycles on the growth and moulting of Palaemon elegans reared in the laboratory

The growth and moulting of Palaemon elegans Rathke has been compared under a circadian (12 h light: 12 h dark) and two non-circadian (8 h light:8 h dark and random light:dark) light/dark cycles. In prawns reared individually from hatching to the late juvenile phase, growth, measured as increase in total length, was significantly retarded in the non-circadian regimes, during zoeal, post-larval and early juvenile development. This effect was primarily the result of reduced increments at moulting in the non-circadian regimes. Growth of prawns reared from hatching to the post-larval phase, measured as wet and dry weights, was similarly reduced in a non-circadian regime. These effects support the hypothesis that the integrity of an animal's physiology is partially dependent on maintaining, through the action of daily environmental cycles, correct timing relationships between its oscillating sub-systems.     

Heterotrophic nutrition of the marine pennate diatom Nitzschia angularis var. affinis

The nutritional pattern for heterotrophic growth of Nitzschia angularis var. affinis (Grun.) Perag. is more complex than for other diatom species studied previously. This species grew slowly in the dark in the presence of single amino acids, either glutamate or alanine; other amino acids when supplied singly were not used as substrates. Carbon from glutamate was converted to cell carbon with an efficiency of 43%. Glutamine was inhibitory both in the light and in the dark, and aspartate inhibited heterotrophic growth on glutamate. Glucose and tryptone supplied singly did not support heterotrophic growth, but when combined, together they allowed for rapid growth of N. angularis (generation time of 16 h). Glucose in combination with glutamate, alanine, aspartate, or asparagine (but not with any other amino acids) also supported growth in the dark, at a rate considerably more rapid than with glutamate alone. In the presence of excess glucose and limiting concentrations of glutamate, approximately 50% of the cell carbon for heterotrophic growth came from glucose, while in combination with tryptone about 25% of the cell carbon came from glucose. Amino acids were taken up by cells grown either photoautrophically or in the dark in the presence or absence of organic substrates; uptake rates were some-what higher for dark-grown than for light-grown cells. Glucose was taken up only by dark-grown cells; induction of a glucose uptake system in the dark required the presence of glutamate but not of glucose. The rates of uptake of glutamate and glucose by cells incubated in the dark with glutamate were sufficiently high to account for the observed rates of growth on these substrates in the dark. The uptake systems of N. angularis have relatively high affinities for glucose (K _s =0.03 mM) and glutamate (K _s =0.02 mM).Contribution No. 890 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Temporary cyst enables long-term dark survival of <Emphasis Type="Italic">Scrippsiella hangoei</Emphasis> (Dinophyceae)

We examined the dark survival strategy of the cold-water dinoflagellate Scrippsiella hangoei from the Baltic Sea. Cultures of S. hangoei were placed in dark and light and we followed the morphological developmentand, determined respiration rate and activity of extracellular leucine aminopeptidase (LAP). S. hangoei had LAP activity in the light, but not in the dark, suggesting that the degradation and use of organic substrates is not part of the dark survival strategy. After prolonged time in darkness (>5 days), S. hangoei started to shed flagella and theca, and produced a previously undescribed temporary cyst. The transformation from vegetative cell into the temporary cyst initially increased respiration rate tenfold, but after the transformation the respiration rate decreased to almost undetectable levels. The presented temporary cyst enables survival through long periods in dark by reducing the respiration rate.     

Fungi in corals: black bands and density-banding of Porites lutea and P. lobata skeleton

 Dark coloration of coral skeleton forming black bands is commonly observed in fractured, massive-coral colonies (Porites lutea and P. lobata) collected from May- otte Island in the Mozambique Channel and Moorea Island in French Polynesia. Black-banding was similar in corals from the two areas and was associated with an assemblage of microbial endoliths: Ostreobium queketti, a common siphonal chlorophyte, and a type of Aspergillus-like fungus. Fungi of coral skeletons are capable of euendolithic growth entirely within the skeleton, and of cryptoendolithic growth whereby they spread from the skeleton into the skeletal pores. The morphology and size of fungal hyphae differs significantly between euendolithic, cryptoendolithic and reproductive phases. Reproductive phases involve formation of conidiophores. Insoluble residues in black bands involve a dark pigment and a dark membranous veil. When attacked by fungi, the algae are usually destroyed. They darken and are threaded by dense, dark-brown, fibrous excrescences. The fungi excrete a dark pigment that stains the surrounding skeletal carbonate black. The pigment is organic, and its presence correlates with higher concentrations of polysaccharides. Black bands match high-density bands of the coral skeleton. Both black bands and high-density bands form at the end of the rainy season in Mayotte. Thus, black-banding in the corals studied is caused by a series of events, beginning with an increase in the abundance of endolithic algae followed by an increase in skeletal density. The algae are then attacked by fungi, which produce more cryptoendolithic hyphae and conidia that are associated with production of the dark pigment. Received: 29 January 1999 / Accepted: 29 September 1999     

Metabolic integration between symbiotic cyanobacteria and sponges: a possible mechanism

Metabolic relationships between symbiotic cyanobacteria and host sponge have been investigated in the marine species Chondrilla nucula and Petrosia ficiformis (collected in the Ligurian Sea in 1992). DNA, RNA, total protein, cytosolic protein, total sugar, cytosolic sugar, total lipid, nonprotein sulfhydryl groups, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were assayed in cortex-free sponge tissue, where cyanobacteria are all but absent. For both species, biochemical parameters were determined in specimens living in illuminated habitats and in dark caves, where sponges are virtually aposymbiotic for cyanobacteria. As C. nucula is unable to colonize dark sites, specimens of this species were artificially transferred to a cave and maintained in dark conditions for 6 mo. Results showed that in the absence of light (i.e., in the absence of cyanobacteria) C. nucula undergo metabolic collapse and thiol depletion. In contrast, P. ficiformis activates heterotrophic metabolism and mechanisms which balance the loss of cell reducing power. This suggests that cyanobacteria effectively participate in controlling the redox potential of the host cells by the transfer of reducing equivalents. Only P. ficiformis is capable of counteracting, by means of heterotrophic metabolism, the loss of the contribution from symbionts which is caused by dark conditions. This explains the differences in the ecological requirements of the two species. Because cyanobacterial symbionts release fixed carbon in the form of glycerol and other small organic phosphate (Wilkinson 1979), a model based on the glycerol 3-phosphate shuttle (typically occurring in chloroplasts and mitochondria) is suggested. The mechanism proposed appears to be an ancient biochemical adaptation which arose among ancestral symbiotic systems, and further developed in the relationships between endosymbiotic organelles and cytoplasm.     

Utilization of glutamate and glucose for heterotrophic growth by the marine pennate diatom Nitzschia laevis

Nitzschia laevis Hustedt grew in the dark in the presence of either glutamate or glucose as substrate. Complex mixtures of yeast extract or tryptone plus lactate also supported good heterotrophic growth, while tryptone alone only supported very slow growth in the dark. The observed growth rates of N. laevis in the dark at different concentrations of glutamate or glucose could be accounted for by the measured uptake rates of these compounds. The affinity of the uptake systems for glutamate and glucose (K _s =0.03 mM for each) was quite high, and similar for dark- and light-grown cells. The lack of a lag-phase when cells were transferred from photoautotrophic to heterotrophic growth conditions can be explained by the presence of uptake systems for glutamate and glucose in ligh--grown cells, as well as in dark-grown cells. However, the uptake capacity was generally higher in the latter than the former. N. laevis also took up alanine and lactate according to Michaelis-Menten kinetics, with a K _s for alanine of 0.02 mM and for lactate of 0.4 mM. Malate and glycerol were not taken up to a significant extent by the cells. Cells grown in continous light had a doubling time of 18 h. The shortest doubling time observed in the dark on glutamate was 48 h and on glucose 24 h. Glutamate was used for heterotrophic growth with an efficiency of 43% and glucose with an efficiency of 48%.Contribution No., 945 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

The relative contribution of dinoflagellate photosynthesis and stored lipids to the survivorship of symbiotic larvae of the reef-building corals

The long-distance dispersal of larvae provides important linkages between populations of reef-building corals and is a critical part of coral biology. Some coral planulae have symbiotic dinoflagellates (Symbiodinium spp.) that probably provide energy in addition to the lipids provisioned within the egg. However, our understanding of the influence of symbionts on the energy metabolism and survivorship of planulae remains limited. This study examines the relative roles of symbiotic dinoflagellate photosynthesis and stored lipid content in the survivorship of the developing stages of the corals Pocillopora damicornis and Montipora digitata. We found that survivorship decreased under dark conditions (i.e. no photosynthetic activity) for P. damicornis and M. digitata at 31 and 22 days after release/spawning, respectively. The lipid content of P. damicornis and M. digitata planulae showed a significant decrease, at a higher rate, under dark conditions, when compared with light conditions. When converted to energy equivalents, the available energy provided by the depletion of lipids could account for 41.9 and 84.7% of larval metabolism for P. damicornis (by day 31) and 38.4 and 90.1% for M. digitata (by day 21) under light and dark conditions, respectively. This finding indicates that not all energy requirements of the larvae are met by lipids: energy is also sourced from the photosynthetic activities of the symbiotic dinoflagellates within these larvae, especially under light conditions. In addition, the amounts of three main lipid classes (wax esters, triglycerides, and phospholipids) decreased throughout the experiment in the planulae of both species, with the wax ester content decreasing more rapidly under dark conditions than under light conditions. The observations that the planulae of both species derive considerable amounts of energy from wax esters, and that symbiotic dinoflagellates enable larvae to use their stores at lower rates, suggested that symbiotic dinoflagellates have the potential to extend larval life under light conditions.     

Photosynthesis,photorespiration, and dark respiration in eight species of algae

The rates of photosynthesis and dark respiration for 7 marine algae and 1 fresh-water alga were measured and compared. The dinoflagellates Glenodinium sp. and zooxanthellae have high dark respiration rates relative to photosynthetic rates, which may decrease their net growth rates. Photorespiration in the 8 algal species was studied by examining the effects of the concentration of oxygen on the rates of photosynthesis, on the incorporation of ¹⁴CO₂ into the photorespiratory pathway intermediates glycine and serine, and on the postillumination burst of carbon dioxide production and oxygen consumption. A combination of these results indicates that all the algae tested can photorespire, but that Glenodinium sp., Thalassiosira pseudonana, and zooxanthellae either have a photorespiratory pathway different from that proposed for freshwater algae (Tolbert, 1974), or an additional pathway for glycolate metabolism.     

Effects of light intensity on photosynthesis and dark respiration in six species of marine phytoplankton

Using an oxygen polarographic electrode, the shapes of photosynthetic curves and the effects of light on dark respiration in 6 species of marine phytoplankton wer examined. The species used were Skeletonema costatum, Ditylum brightwellii, Cyclotella nana (Thalassiosira pseudonana) (all Bacillariophyceae), Dunaliella tertiolecta (Chlorophyceae), Isochrysis galbana (Haptophyceae), and Gonyaulax tamarensis (Dinophyceae). A hysteresis was observed in all species examined with respect to increasing and decreasing light. Compensation light intensities varied by over 4 orders of magnitude, suggesting that the 1% light depth is an ambiguous measure of the euphotic zone. The data suggest that dark respiration accounts for ca. 25% of gross photosynthesis, but is species-dependent. In addition, respiration versus cell size does not describe an inverse exponential function over the size scales examined.This research was performed under the auspices of the United States Energy Research and Development Administration under Contract No. EY-76-C-02-0016.     

Diurnal and circadian rhythms in the turbidity of growing Skeletonema costatum cultures

Skeletonema costatum (Grev.) Cleve grown in batch culture at low light intensity under a 14 h light: 10 h dark photocycle showed exponential cell proliferation (1.1 doublings d^-1) without significant phasing of the cell division by the light: dark cycle. The growth in carbon concentration was, however, restricted to the light period. The turbidity of the culture closely followed the carbon oattern, and was not affected by the increase in the cell number during the dark period. It was found that a trustule suspension had only approximately 1% of the turbidity of the corresponding intact algae. Culture turbidity was therefore regarded as a biomass parameter similar to the carbon concentration, without direct correlation to the timing of the cell division. The short-time variations in the turbidity of growing algal cultures were further studied in a cage culture turbidostat. The growth rate (based on turbidity) increased rapidly during the first half of the light period, decreased slightly towards the evening and was zero throughout the dark period. When transformed to continuous light, the growth of the culture continued to show damped oscillations for up to 1 wk, but with a period of 26.7 h instead of 24 h. The same circadian rhythm was observed in chlorophyll content, and is thus possibly a reflection of a freely oscillating internal biological clock. The cage culture turbidostat was found to be a suitable device for studies of the photocycle related regulation of biosynthesis in S. costatum.     

Sources and biochemical composition of suspended particulate material in a submarine cave with sulphur water springs

Sources, biochemical composition and nutritional value of suspended particulate material were investigated from February 1994 to February 1995 in a submarine cave (Grotta Azzurra, Capo Palinuro, southwestern Italy) with hot sulphur springs and associated mats of chemolithoautotrophic bacteria in its innermost dark part (Snow Hall). Concentrations of total suspended material (TSM), particulate inorganic material (PIM), organic carbon (POC), organic nitrogen (PON), chlorophyll a (Chl a), phaeopigments (Phaeo), carbohydrates (TCH), proteins (TPR) and lipids (TLI) were measured at four stations along an outside–inside transect, in order to address whether the quantity and quality of suspended particles varied over time with increasing distance from the entrance of the cave and estimate the relative contribution of chemosynthesis versus photosynthesis in supplying POC to cave heterotrophs. The abundance and biochemical composition of suspended material available to filter-feeders varied over time, but no significant quantitative or qualitative differences were detected along the outside–inside transect. Concentrations of TSM and of its different compounds (PIM, POC, PON, Phaeo, TCH, TPR, TLI) were homogeneous among the four stations or fluctuated without any consistent trend, with no apparent ageing and degradation of organic material in the innermost dark station. Conversely, concentrations of Chl a significantly decreased from outside to inside the cave at all sampling periods. It is suggested that suspended POC in Grotta Azzurra consists of mixed assemblages of particles coming from advection of photosynthetic material from the open sea and local inputs of carbon by sulphur-oxidizing bacteria. Based on POC/Chl a ratios, the relative contributions of chemosynthesis versus photosynthesis in supplying POC to benthic heterotrophs was estimated to be 31 and 69%, respectively. Received: 9 December 1996 / Accepted: 14 January 1997     

Dark survival strategies in marine phytoplankton assessed by cytometric measurement of metabolic activity with fluorescein diacetate

Cytometric quantification of cellular fluorescence upon cleavage of fluorescein diacetate (FDA) is presented as a sensitive and rapid technique to assess phytoplankton metabolic activity during exposure to prolonged darkness of 10 to 12 d. Two distinct types of metabolic response to darkness are distinguished: Type I cells (Brachiomonas submarina, Pavlova lutheri, Chrysochromulina hirta) adapt to prolonged darkness by reducing their metabolism to a lower level of activity (∼10% of initial in P. lutheri, C. hirta, ∼0.5% in B. submarina) within few days, whereas Type II cells (Prymnesium parvum, Bacteriastrum sp., unidentified pennate diatom) continue with unchanged activity. Type I cells were able to maintain their initial cell abundance and commenced rapid cell growth upon re-illumination after 12 d of darkness. Among Type II cells, diatoms were able to maintain cell abundance and growth capacity as well, whereas P. parvum was not. Type I cells are expected to exhibit competitive advantages in environments with frequent or long dark periods. Bacterivory further supports dark survival in C. hirta. Received: 10 May 1999 / Accepted: 20 September 1999     

Herring (Clupea harengus) filter-feeding in the dark

The behaviour of juvenile herring (Clupea harengus L.) feeding on Artemia sp. nauplii in both light and dark was recorded using an infra-red-sensitive television-recording technique. In the light, two modes of feeding were observed, particle biting and filtering, but in the dark only filtering was observed. Marked differences in swimming behaviour were seen between light and dark. In the light, the fish continued to school while feeding in both modes; in the dark, the school dispersed, the fish swam slower in tight circular paths and fed only by filtering. In the dark, filtering fish swam faster (0.11 m s^-1) than non-filtering fish (0.07 m s^-1). In the light, no difference in speed was measured between filtering and non-filtering fish (0.34 m s^-1). Owing to the lower filtering speeds in the dark, the removal rate of nauplii from the water was much lower than in light, except in the highest prey concentrations. This suggests that if night-time filter-feeding takes place in the sea, it will be of importance only when exploiting dense patches of food.     

Bacterial colonization and endocytosis on the gill of a new limpet species from a hydrothermal vent

Calcification rates in different fragments along branches of the hermatypic coral Stylophora pistillata were tested in the laboratory using a new technique, the optic glassfiber method. By this method, the tested colony remains constantly in dark conditions while a narrow beam of light, transferred through the optic fiber, illuminates a small distinct point of coral tissue (on a branch tip or base). The selected illuminated portion of the branch serves as the experimental fragment, while all the other parts of the same colony serve as the dark controls. The results indicate that significantly more calcium is incorporated in the tip fragments than in the bases, both in light and in dark conditions (4.1 to 13.2 times more). Illumination of the tips or the bases did not stimulate or enhance the calcification rates of these fragments. Thus, in all colonies tested, the calcification rates of the illuminated fragments were not significantly different from the average rates of other similar, non-illuminated fragments of the same colony. It is suggested that light does not directly enhance calcification in hermatypic corals, but rather, that light enhances O₂ production, which consequently stimulates coral metabolism. Our preliminary results indicate that calcification rates recorded in aerated dark experiments are significantly higher than calcification rates of non-aerated dark controls.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

Algal chloroplasts in the protoplasm of three species of benthic foraminifera: taxonomic affinity,viability and persistence

The ultrastructure and pigment content of algal chloroplasts (derived from Bacillariophyceae or Chrysophyceae) are described from 3 benthic species of brackish-water foraminiferans.Elphidium williamsoni Haynes contains 4×10⁶ chloroplasts mg^-1, whereas the contents ofNonion germanicum (Ehrenberg) andE. excavatum (Terquem) are about 10% of this value. The two former contain chlorophyllsa andc and fucoxanthin, but these pigments were not detectable in the latter.E. williamsoni andN. germanicum had a net uptake of¹⁴C–HCO ₃ ^- , proportional to their content of chlorophyll and number of chloroplasts, increasing linearly up to approximately 10 Klux. At light saturation the former assimilates 2.3x10^-3 mg C mg^-1 h^-1 and the latter only about 20% of this value. Dark uptake was insignificant in all cases. Uptake could not be demonstrated inE. excavatum. The photosynthesis effected by these species is trivial in terms of the total benthic carbon fixation effected by the microflora. The chloroplasts survived longer in forminiferans kept in the dark than in light/dark adapted individuals. To keep a steady state population of chloroplasts under light/dark conditions,E. williamsoni must eat at least 65 chloroplasts individual^-1 h^-1, whereas the minimum consuption rate inN. germanicum is 20.     

Cell cycle and toxin production in the benthic dinoflagellate Prorocentrum lima

Profiles of diarrhetic shellfish poisoning (DSP) toxins produced throughout the growth cycle and the cell cycle of the toxigenic marine dinoflagellate Prorocentrum lima were studied in triplicate unialgal batch cultures. Cells were pre-conditioned at 18 ± 1 °C, under a photon flux density (PFD) of 90 ± 5 μmol m⁻² s⁻¹ on a 14 h light:10 h dark photoperiod. In exponential growth phase, cultures were synchronized in darkness for 17 d. After dark synchronization, cultures were transferred back to the original photoperiod regime. Cells were harvested for DSP toxin analysis by LC-MS (liquid chromatography with mass spectrometry), and double-stranded (nuclear) DNA was quantified by flow cytometry. The cell populations became asynchronous within approximately 3 d after transition from darkness to the 14 h light:10 h dark photoperiod. This may be due to the prolonged division cycle (5 to 7 d) that is not tightly phased by the photoperiod. Unlike other planktonic Prorocentrum spp., cytokinesis in P. lima occurred early in the dark and ceased by “midnight”. Cellular levels of the four principal DSP toxins, okadaic acid (OA), OA C8-diol-ester (OA-D8), dinophysistoxin-1 (DTX1) and dinophysistoxin-4 (DTX4), ranged from 0.37 to 6.6, 0.02 to 1.5, 0.04 to 2.6, and 1.8 to 7.8 fmol cell⁻¹, respectively. No toxin production was evident during the extended period of dark synchronization nor during the initial period when NH₄ was consumed as the major nitrogen source. Soon after the cells were returned to the 14 h light:10 h dark cycle and they began to take up NO₃, cellular levels of all four toxins gradually increased. This increase in DSP toxins usually occurred in the light, marked by a rise in DTX4 levels that preceded an increase in the cellular concentration of OA and DTX1 (delayed by 3 to 6 h). Thus, DTX4 synthesis is initiated in the G1 phase of the cell cycle and persists into S phase (“morning” of the photoperiod), whereas OA and DTX1 production occurs later during S and G2 phases (“afternoon”). No toxin production was measured during cytokinesis, which happened early in the dark. The evidence indicates that toxin synthesis is restricted to the light period and is coupled to cell cycle events. Received: 3 September 1998 / Accepted: 30 March 1999     

Chlorophyll fluorescence as a proxy for microphytobenthic biomass: alternatives to the current methodology

Pulse amplitude modulated (PAM) fluorescence has been used as a proxy of microphytobenthic biomass after a dark adaptation period of 15 min to stabilise the minimum fluorescence yield (F _o ¹⁵). This methodology was investigated for in situ migratory and ex situ engineered non-migratory biofilms, comparing dark adaptation to low (5% ambient) and far-red light treatments over different emersion periods. Far-red and low light reduced potential errors resulting from light history effects, by reversal of non-photochemical quenching after 5 min of treatment, compared to over 10 min required by conventional dark adaptation. An in situ decline of minimum fluorescence yield over 15 min was observed during the dark adaptation for migratory biofilms, but was not observed in the non-migratory biofilms indicating that the major cause of decline was downward vertical migration of cells into the sediment. This pattern occurred in far-red light after 10 min, but not for the low light treatment, indicating that low light maintained the biomass at the surface of the sediment. It is therefore concluded that low light treatment is a better option than conventional dark adaptation for the measurement of minimum fluorescence as a proxy of microphytobenthic biomass.     

Respiration,photosynthesis and carbon metabolism in planktonic ciliates

Release of¹⁴C-labelled carbon dioxide from uniformly labelled cells was used to measure respiration by individual ciliates in 2-h incubations in 1989 and 1990. In a strictly heterotrophic ciliate,Strobilidium spiralis (Leegaard, 1915), release of labelled carbon dioxide was equivalent to ca. 2.8% of cell C h^–1 at 20°C, and there was no difference between rates in the dark and light. In the chloroplast-retaining ciliatesLaboea strobila Lohmann, 1908,Strombidium conicum (Lohmann, 1908) Wulff, 1919 andStrombidium capitatum (Leegaard, 1915) Kahl, 1932, release of labelled carbon dioxide was less in the light than in the dark in experiments done at 15°C. InL. strobila release of radiolabel as carbon dioxide was equivalent to ca. 2.4% of cell C h^–1 in the dark but ca. 1% at 50µE m^–2 s^–1, an irradiance limiting to photosynthesis. InS. conicum release of radiolabel as carbon dioxide was equivalent to ca. 4.4% of cell C h^–1 in the dark, but at an irradiance saturating to photosynthesis (250 to 300µE m^–2 s^–1) there was no detectable release of labelled carbon dioxide. InS. capitatum release of radiolabel as carbon dioxide was equivalent to ca. 4.3% of cell C h^–1 in the dark but at an irradiance saturating to photosynthesis was ca. 2.4% of cell C h^–1. These data, combined with data from photosynthetic uptake experiments, indicate that¹⁴C uptake underestimates the total benefit of photosynthesis by 50% or more in chloroplastretaining ciliates.Contribution no. 7510 from the Woods Hole Oceanographic Institution