Comparative persistence of marine fish larvae from pelagic versus demersal eggs off southwestern Nova Scotia,Canada

Two groupings of larval fish were repeatedly identified by principal component analyses of larval densities from four broad-scale surveys during the spring and summer of 1985–1987 off southwestern Nova Scotia, Canada. Larvae originating from pelagic eggs (four species within Gadidae and Pleuronectidae) constituted one group, which were uniformly distributed over the sampling area with densities not correlated with bathymetry, although nearly all spawning occurs on the shallow western cap of Browns Bank, 100 km offshore. Larvae from demersal eggs (five species within Pholidae, Stichaeidae, Cottidae, Agonidae) constituted the second group, which dominated the shallow-water environments both inshore and on Browns Bank. Lower patchiness indices were evident amongst larvae from pelagic eggs in small and large sampling-gear collections (average 3.4 and 3.1, respectively) compared to fish hatching from demersal eggs (average 5.1 and 4.6). Fine-scale nearshore surveys over a 5 wk period in 1987 also showed that larvae of demersal eggs had a less variable distribution along an inshoreoffshore transect. Larvae from demersal eggs appear spatially persistent through the release of well-developed larvae from non-drifting eggs. These conclusions are consistent with other studies over a range of spatial scales in temperate and tropical environments, demonstrating that single-species models of larval dispersal are inadequate to account for the distributional patterns of larval fish in general.     

Mitochondrial energetics of benthic and pelagic Antarctic teleosts

Antarctic fauna are highly adapted to the frigid waters of the Southern Ocean. This study describes the in vitro temperature sensitivity of oxygen consumption rates measured in liver mitochondria from the pelagic notothenioid Pleuragramma antarcticum between 5 and 35 °C. Oxygen fluxes were measured after the addition of millimolar levels of pyruvate, malate, succinate and glutamate (state II, LEAK) and saturating levels of ADP [state III, oxidative phosphorylation (OXPHOS)]. State III respiration significantly decreased above 18.7 °C. A comparison of the oxidative capacities among P. antarcticum and other notothenioids showed significant differences in state III respiration, where benthic species exhibited about 50 % lower rates than P. antarcticum. In addition, state III respiration rates normalized per milligram of mitochondrial protein of P. antarcticum were up to eight times higher than state III rates reported in the literature for other notothenioids. The comparatively high respiration rates measured in this study may be explained by our approach, which engaged both complexes I and II under conditions of oxidative phosphorylation. State III rates of independently activated complexes I and II were found to range from 42 to 100 % of rates obtained when both complexes were activated simultaneously in the same species. The remarkable tolerance of P. antarcticum OXPHOS toward warmer temperatures was unexpected for an Antarctic stenotherm and may indicate that thermal sensitivity of their mitochondria is not the driving force behind their stenothermy.     

Muscle structure and differentiation in pelagic and demersal stages of the Antarctic teleost Notothenia neglecta

The distribution and fine structure of muscle-fibre types has been determined for the pelagic fingerling and demersal adult stages of the antarctic teleost Notothenia neglecta Nybelin, collected from Signy Island, Antarctica, between January and February 1984. In both stages, the pectoral fin adductor muscle (m.ad.p) is largely composed of slow fibres which contain abundant mitochondria (34 to 36%). During development, the ratio of capillaries to fibres increases less than does fibre diameter, so that capillary density is significantly lower in the m.ad.p of adults (498 mm^-2) than fingerlings (1 727 mm^-2). The secondary metamorphosis from a pelagic to a demersal mode of life is associated with the disappearance of subcutaneous lipid sacs and major changes in the distribution and structure of muscle fibres in the myotomes. The trunk cross-section of adult fish is almost entirely composed of poorly vascularised fast-muscle fibres (100 capillaries mm^-2), which contain densely packed myofibrils (86.3%), and have few mitochondria (1.4%). Slow-muscle fibres in adults are restricted to a thin wedge adjacent to the lateral line canal. In contrast, slow fibres occur around the entire circumference of the trunk in fingerlings representing 24% of the total cross-sectional area at the post-anal level. Volume densities (%) of mitochondria, intracellular lipid and myofibrils in this tissue are respectively 37.0, 7.9, 38.6 for fingerlings and 13.1, 0, 70.3 for adults. Slow-muscle fibres in adult fish are of unusually large diameter (50 to 120 m) and have relatively low capillary densities (266 mm^-2). These morphological changes reflect a general decrease in activity and a shift from a sub-carangiform to a labriform mode of swimming following transition from the fingerling to adult stage of the life cycle. The results are briefly discussed in relation to the physiology and ecology of antarctic fish.Please address all correspondence and requests for reprints to Dr. I.A. Johnston     

Depth-related enzymic activities in muscle,brain and heart of deep-living pelagic marine teleosts

The activities [units^-1 wet weight tissue] of lactate dehydrogenase (LDH), pyruvate kinase (PK), malate dehydrogenase (MDH) and isocitrate dehydrogenase (IDH) in white skeletal muscle, brain and heart of 24 pelagic teleost fishes were determined. In addition, for several of the foregoing species, citrate synthase (CS) activities were examined in white skeletal muscle. In muscle, the activities of all these enzymes decrease exponentially with increasing minimum depth of occurrence of the species; this decrease closely parallels the decrease in respiratory rate found previously for these same species. The decline in enzymic activity with increasing minimum depth of occurrence is much greater than the decline in body protein content of the whole fishes, suggesting a disproportionately rapid fall in enzyme concentration in comparison to contractile and structural protein concentrations. The similar reductions in activities of both glycolytic (LDH and PK) and citric acid cycle (CS, MDH and IDH) enzymes with depth indicate that both standard and active metabolisms of deeper-dwelling species are reduced relative to shallower-dwelling forms. There is no suggestion of increased anaerobic capacity with depth or in relation to species, occurrence in the oxygen minimum layer. In brain and heart, there is no significant decrease in enzymic activity with increasing minimum depth of occurrence. These two tissues have similar capacities for metabolism in most fishes, when comparisons are based on enzymic activity per gram wet weight of tissue.     

Growth-selective survival of young jack mackerel Trachurus japonicus during transition from pelagic to demersal habitats in the East China Sea

To examine the growth-selective survival in early life stages of jack mackerel from the East China Sea, we compared growth rates of young fish collected from the surface layer in April with those of survivors collected from the near-bottom layer in May?CJune. We performed this analysis from 3?year classes with different levels of relative abundance: low in 2005, intermediate in 2008 and high in 2009. Growth rates during the larval stage of the original population were slower than the survivors in all survey years, indicating that faster-growing individuals in the surface layer had a higher probability of survival to the near-bottom habitat than slower-growing individuals. Selection intensity, defined as differences in growth rates between the two groups, was higher in year classes with a lower abundance. Hence, growth rates during the early life stages of jack mackerel are considered to be a critical determinant of successful survival to the demersal habitat.     

Phosphorus metabolism of oceanic dinoflagellates: phosphate uptake,chemical composition and growth of Pyrocystis noctiluca

The phosphorus metabolism of Pyrocystis noctiluca Murray (Schuett) 1886 has characteristics which may enhance its potential for success in orthophosphate impoverished waters. The steady-state phosphate uptake rates were equal in the light and dark, and were directly proportional to both the phosphorus cell quota and the cell division rate. In contrast, nutrient-saturated uptake rates were multiphasic, faster in the light than the dark, 2 to 4 orders of magnitude greater than steady-state rates, and were inversely proportional to both the phosphorus cell quota and the cell division rate. These uptake characteristics suggest that P. noctiluca may take up phosphate coincidently at their typically low ambient concentrations as well as to exploit episodic nutrient events in nature. Cell division rates were a hyperbolic function of the ambient orthophosphate concentration. The shortest doubling time was 8.7 d, the phosphate concentration at half the maximum division rate was 0.15 M and the threshold, concentration for cell division was ca 0.05 M PO ₄ ^3- . Division rates of P. noctiluca in the ocean are much faster than predicted from the measured ambient orthophosphate concentrations. Since this dinoflagellate has high naturally occurring alkaline phosphatase activities, and can utilize organic-P compounds, we suggest that organic-P can be as important as orthophosphate in supporting the observed division rates of P. noctiluca in the sea.     

Oxygen consumption of pelagic juveniles and demersal adults of the deep-sea fish Sebastolobus altivelis,measured at depth

Oxygen consumption rates of the deep-sea fish Sebastolobus altivelis were measured in situ on pelagic juveniles at mesopelagic depths (608 m) and on demersal adults at bathyal depths (1 300 m) in the Santa Catalina Basin in March 1982. Two pelagic juveniles were individually collected, and respiration was measured continuously for approximately 2 d with a slurp gun respirometer manipulated from the submersible “Alvin”. Oxygen consumption rates of these juveniles were highly variable and were 1.5 to 1.8 times higher during the night than during the day. Gut contents of the juveniles were mainly euphausiids (Euphausia pacifica and Nematoscelis difficilis). Four demersal adults were collected by “Alvin” and individually placed in fish-trap respirometers on the bottom where respiration was measured continuously for approximately 1 d. Weight-specific O₂ consumption rates for adults decreased with increasing body weight and were consistent in magnitude throughout the incubation period. Population O₂ consumption for demersal S. altivelis (calculated from abundance, size-frequency distribution, and O₂ consumption regression equation) was 11.01 μl O₂ m^-2 h^-1, which is two orders of magnitude less than the O₂ consumption rate for the population of the most abundant epibenthic megafaunal species in the Santa Catalina Basin, the ophiuroid Ophiophthalmus normani. O. normani is a principal prey for adults of S. altivelis based on gutcontent analysis. Given the population O₂ consumption rate as an estimate of food energy demand, the demersal population of S. altivelis would assimilate only 0.007% of the standing crop of O. normani per day.     

Energy metabolism during development of eggs and larvae of gilthead sea bream (Sparus aurata)

Developing eggs and larvae of laboratory-reared gilthead sea bream (Sparus aurata) maintained in filtered seawater (40 ppt) at 18°C, were measured for oxygen uptake, ammonia excretion, contents of free amino acids (FAA), protein, fatty acids (FA) accumulated ammonia, and volumes of yolk-sac and oil globule. Absorption of the yolk coincided with the consumption of FAA and was complete ca. 100 h post-fertilisation. Amino acids from protein were mobilised for energy in the last part of the yolk-sac stage. Absorption of the oil globule occurred primarily after hatching following yolk absorption, and correlated with catabolism of the FA neutral lipids. Overall, FAA appear to be a significant energy substrate during the egg stage (60 to 70%) while FA from neutral lipids derived from the oil globule are the main metabolic fuel after hatching (80 to 90%).     

Anaerobic and aerobic energy metabolism in ovaries of the sea urchin Strongylocentrotus droebachiensis

The gonads of sea urchins undergo large changes in mass during their gametogenic cycle. In addition, they have relatively low aerobic capacities and are poorly perfused by the circulatory system and thus are continually hypoxic or anoxic. The present study of Strongylocentrotus droebachiensis investigates seasonal changes in the relationships among mass of the ovaries, pH and P_O2 of the perivisceral coelomic fluid which bathes the ovaries, and partitioning of ovary energy metabolism into its anaerobic and aerobic components. S. droebachiensis were collected at Blue Hill Falls, Maine, USA, from August 1982 to March 1984. We found that from 76 to 92% of the heat dissipated by isolated ovaries of the sea urchin S. droebachiensis derives from anaerobic energy metabolism at partial pressures of oxygen prevailing in vivo. Ovaries from S. droebachiensis have the capacity to produce large amounts of lactate under imposed anoxia, but lactate accounts for only 37% of the total anoxic heat dissipation, which suggests that other end products of anaerobiosis are present. Seasonal changes in pH and P_O2 of the perivisceral coelomic fluid can be explained by a complex relationship among changes in temperature, reproductive condition, and anaerobic and aerobic metabolism in the ovaries, gut and body wall. Seasonal changes in the buffering capacity of the perivisceral coelomic fluid must be determined before the effects of respiratory and metabolic acid production on the acid-base status of the coelomic fluid can be fully understood.     

Respiration and nitrogen metabolism of Atlantic halibut eggs (Hippoglossus hippoglossus)

Naturally spawned and fertilized eggs of Atlantic halibut,Hippoglossus hippoglossus L., were analysed for protein, free amino acids (FAA), ammonium ions and energy content. The chemical composition was found to be size-dependent but varied little during egg development. Ammonium ions did, however, accumulate during the late embryonic stage, and the trend in FAA content was downward during the same period. Rates of O₂ uptake and NH₃ excretion followed exponential patterns. A total of 1µmol O₂ was consumed and 120 nmol NH₃ excreted between the time intervals of fertilization and 1 d post hatch. Derived O:N ratios indicated that the dominant portion of the energy metabolism was lipid- or carbohydrate-based during the mid-development period but switched to FAA as hatch was approached.Correspondence should be addressed to: Zoological Laboratory, University of Bergen, Allégt. 41, N-5007 Bergen, Norway     

Free amino acids and energy metabolism in eggs and larvae of seabass, Lates calcarifer

Contents of free amino acids (FAA), protein and ammonium ions together with rates of ammonia excretion and oxygen consumption were measured in order to study the role of FAA as an energy substrate in developing eggs and larvae of seabass (Lates calcarifer) maintained in seawater (30 ppt) at 28 °C without feeding. Initially eggs contained 25.3 nmol ind⁻¹ of FAA of which 21.5 nmol was rapidly utilised by the developing eggs and larvae during the period up to 40 h post spawning (PS) when nearly all the yolk had been resorbed. During the same period, a net increase in protein content of 1.7 μg ind⁻¹ was observed, indicating that the major part of the amino acids lost from the free pool had been polymerised into body proteins. Assuming that the balance of the FAA after protein synthesis was used entirely for energy metabolism, FAA appeared to be an important energy substrate during the embryonic stages (2 to 16 h PS); after hatching, the contribution of FAA to energy metabolism was less significant. From 50 h PS until the end of the study period at 100 h PS, amino acids derived from somatic protein were used for energy metabolism. For the overall period from just after spawning up to 100 h PS, the data indicate that ca. 14% of the total aerobic energy metabolism was derived from amino acid catabolism. Received: 26 September 1997 / Accepted: 1 April 1998     

水分管理对重金属在水稻根区及在水稻中积累的影响

选取了酸性矿山废水污灌区重金属污染水稻土,通过盆栽试验,研究了不同水分管理条件（60%最大田间持水量,80%最大田间持水量,最大田间持水量,前期淹水＋抽穗扬花期烤田,全生育期淹水）下水稻根际土壤及其不同器官（稻根、茎叶和籽粒）中As、Cd、Cu和Zn的含量变化。结果表明：土壤水分含量对水稻根际土壤中As、Cu和Zn的含量影响不大。但显著影响水稻不同器官对这3种元素的吸收积累。随着土壤水分含量的增加,水稻根、茎叶和籽粒中As的含量都显著增加;Cu的含量则逐渐减少;水稻茎叶中Zn含量也逐渐减少,但水稻根和籽粒中Zn含量则变化不明显。抽穗扬花期拷田能显著降低As在水稻中的积累,显著增加水稻茎叶中Zn的积累;但不影响水稻各器官中Cu,以及水稻根和籽粒中Zn的含量。此外,虽然这3种重金属在土壤中含量均超标,但在水稻籽粒中含量只有Zn全部超标,而Cu则都不超标,因此,农作物的超标情况并不直接与土壤的重金属超标相联系。     

Phosphorus fractionation in surface water and sediments of industrially polluted freshwater reservoir,India

Industrial pollutions are responsible for alterations in biogeochemical cycling of phosphorus (P) in freshwater systems. The objectives of the present study were to quantify the major P forms and assess the relationship between P fractions in water and surface sediments of Govind Ballabh Pant Sagar (GBPS) reservoir, India. Surface water samples (n?=?48) and surface sediment samples (n?=?48) were collected from six regions of GBPS reservoir in December 2014. The results showed that total particulate phosphorus (TPP) and total dissolved phosphorus (TDP) account for 45%–58% and 42%–55% of total phosphorus (TP) in the surface water. The authigenic-P was found to be more than 84% of the total P in surface sediments. The TPP showed a positive statistical relationship with sedimentary P fractions (authigenic-P, exch-P, and organic-P) indicated the impact of industrial pollution load on inorganic and organic P fractions in water and sediments of GBPS reservoir. The discriminant fraction analysis (DFA) revealed that the exch-P, Fe(III)-bound-P, detrital-P, organic-P, and TPP were sensitive indicators of P dynamics in the industrially polluted GBPS reservoir.     

The “pelagic larvaton” and its role in the biology of the World Ocean,with special reference to pelagic larvae of marine bottom invertebrates

Ecological subdivision of marine organisms is often based on two characteristics: presence in a defined environment, and types of locomotion (degree of free active movement) in such an environment. The use of these characteristics results in a simple scheme: (1) Inhabitants of the boundary surface “ocean-atmosphere” (a zone including not only the surface film but also the thin subsurface water layer below it and the air layer just above it, i.e., pleuston and neuston). (2) Inhabitants of the deeper water layers of the ocean i.e., excluding the zone mentioned under (1): (a) passively drifting forms with very limited locomotory capacity, moving practically in the vertical plane only (plankton); (b) actively moving forms which migrate both vertically and horizontally (nekton). (3) Inhabitants of the “bottom”-benthos (level-bottom of oceans and coastal waters, tidal zones up to the upper supralittoral, different types of drifting and floating substrata, e.g. ship bottoms, harbour structures, buoys, driftwood, sargassum, whales, etc.). This simple scheme is essentially based on characteristics of adults. If developmental stages are considered, pelagic larvae of bottom invertebrates, eggs and larvae of fishes and other forms, usually present only temporarily in the plankton, neuston, and pleuston, can be distinguished as “mero-plankton”, “mero-neuston” and “mero-pleuston”, from the permanent “holo”-components of these groups. Division into “mero”-subgroups opposes all these larvae to those of planktonic, neustonic and pleustonic forms developing within the “parental” groups and their environments. However, the last category of larvae in the light of world-wide distribution of the seasonal reproductive pattern of marine invertebrates and some other organisms — especially in temperate and high latitudes — can also be rated to some degree as “mero”-(not “holo”-) components. The present paper proposes to unite all larvae of marine invertebrates (and of other organisms) undergoing pelagic development into one biological group, the “pelagic larvaton”. The main characteristic for all forms of this group is the presence of one and the same life-cycle stage in one and the same environment. All forms of the “pelagic larvaton” are, to various degrees, biologically different from their respective adult forms. Even the pelagic larvae of the holoplanktonic species exhibit some differences. Within the “pelagic larvaton”, 3 subgroups can be distinguished on the basis of their ecological peculiarities;

1. Larvae undergoing their whole development in an environment different from that inhabited by their parents and belonging to a group different from that of their parental forms; e.g. the pelagic larvae of bottom invertebrates which develop in the plankton, neuston or pleuston.
2. Larvae undergoing development in the same general pelagic environment, but in “non-parental” ecological groups; e.g. larvae of nektonic species developing in the plankton, neuston or pleuston; larvae of planktonic species in the neuston or pleuston; larvae of neustonic and pleustonic species in the plankton.
3. Larvae undergoing development in the “parental” groups; e.g. larvae of planktonic species in the plankton, of neustonic species in the neuston, or of pleustonic species in the pleuston.

In contrast to the 5 ecological groups: benthos, plankton, nekton, neuston and pleuston, the “pelagic larvaton” represents rather a biological than an ecological group. The “pelagic larvaton” comprises the 5 ecological groups and maintains the permanent turnover of organic substances between water and bottom. This group short-circuits the interrelations between the 5 ecological groups in all possible combinations. The existence of the “pelagic larvaton” presents another illustration of the unity of the biological nature of the oceans. The present paper also discusses the specific distributional patterns of the pelagic larvae of bottom invertebrates and their biological role in the seas.     

Chemical composition and effects of water extracts of petroleum on eggs of the sand dollar Melitta quinquiesperforata

Sperm and eggs of sand dollars, Melitta quinquiesperforata (Leske), were subjected to two petroleum oils, and effects determined. The oils chosen were Kuwait crude and No. 2 fuel oil, supplied by the American Petroleum Institute. Water-soluble extracts (WSF) from oil-sea water mixes were prepared and the major aromatic components in the WSF of the fuel oil were identified. WSF of No. 2 fuel oil depressed respiration, mobility of sperm, interfered with fertilization and cleavage, and retarded larval development. The effects were detectable at dilutions of 4% and less (about 0.6 ppm of WSF). Kuwait crude was much less toxic. There was no effect on water permeability of the egg membrane. Results are compared with similar studies on other marine organisms.     

Temporal dynamics of arsenic uptake and distribution: food and water risks in the Bengal basin

Abstract

Contaminated food chain is a serious contender for arsenic (As) uptake around the globe. In Nadia, West Bengal, we trace possible means of transfer of As from multiple sources reaching different trophic levels, and associated seasonal variability leading to chronic As uptake. This work considers possible sources-pathways of As transfer through food chain in rural community. Arsenic concentration in groundwater, soil, rice, and vegetable-samples collected detected in different harvest seasons of 2014 and 2016. Arsenic level in shallow groundwater samples ranged from 0.1 to 354?µg/L, with 75% of the sites above the prescribed limit by WHO (10?µg/L) during the boro harvest season. High soil As content (～20.6?mg/kg), resulted in accumulation of As in food crops. A positive correlation in As conc. with increase over period in all sites indicating gradual As accumulation in topsoil. Unpolished rice samples showed high As content (～1.75?mg/kg), polishing reduced 80% of As. Among vegetables, the plant family Poaceae with high irrigation requirements and Solanaceae retaining high moisture, have the highest levels of As. Contaminated animal fodder (Poaceae) and turf water for cattle are shown to contaminate milk (0.06 to 0.24?µg/L) and behoves strategies, practices to minimize As exposure.     

Seasonal variation in uptake and depuration of anthracene by the brackish water clam Rangia cuneata

Rangia cuneata were collected every six weeks for one year from February 1984 to January 1985 at the mouth of the Dog River, Alabama, USA and dosed with 200 ppb [9–¹⁴C] anthracene at 10, 20, and 30°C. Uptake rates of [9–¹⁴C] anthracene measured over a 15-h period were not significantly different (P>0.05), but depuration rates measured over a two-week period were significantly higher with increasing temperature. The seasonal changes in rates were related to the annual changes in biochemical composition of the clams associated with the reproductive cycle. Lipid levels were nearly constant over the year, averaging 88.0±2.0 mg/g dry wt. The level of carbohydrates peaked in July, a month before the maximal gonad index, and decreased steadily through September during spawning. The level of proteins increased from 76.6 mg/g dry wt prior to spawning to 138.3 mg/g dry wt after spawning. Uptake rates were highest in late summer during the peak of gametogenesis and decreased during spawning in the fall. Depuration rates remained low from March through August and rapidly increased to the highest levels during the fall spawning period. The results suggest that the stage of the reproductive cycle, with its associated biochemical changes, and temperature play an important role in the pattern of accumulation and release of anthracene in R. cuneata.     

Storage and depletion of lipid components in the pyloric caeca and ovaries of the seastarAsterias rubens during its annual reproductive cycle

Detailed information is presented on the changes in lipid-class profiles (polar lipids, total neutral lipids, free sterols, steryl esters, triacylglycerols, neutral ether lipids and free fatty acids), which are manifest during the annual reproductive cycle of the seastarAsterias rubens for both the storage organs (pyloric caeca) and utilizing organs (ovaries). The ovaries appear to accumulate all individual lipid components proportional to progression of vitellogenesis. Pyloric caeca, in contrast, show variable lipid-class levels, probably due to more complex processes in these organs, operating simultaneously: digestion of food, accumulation of storage lipids, mobilization of stored lipids and their release. The results are discussed in terms of current knowledge on the role of lipids in echinoderm reproduction.     

Nitrate uptake and assimilation in Thalassiosira weissflogii and Phaeodactylum tricornutum: interactions with photosynthesis and with the uptake of other ions

Interactions of the nitrate, phosphate, and ammonium uptake systems and the interactions of these systems with photosynthesis were investigated for Thalassiosira weissflogii and Phaeodactylum tricornutum preconditioned in continuous culture. The cultures were supplied with NO _- ³ and PO ₌ ⁴ in an N:P atomic ratio of 15:1, and residual concentrations of both nutrients in the growth chamber were very low. The rate of NO _- ³ uptake was reduced by the addition of NH ₊ ⁴ or PO ₌ ⁴ . The rate of PO ₌ ⁴ uptake by T. weissflogii was reduced by the addition of NH ₊ ⁴ . The rate of carbon fixation was reduced by NO _- ³ additions and slightly reduced by the addition of PO ₌ ⁴ . There were two components of NO _- ³ uptake, one light-dependent and one light-independent. Uptake inhibition by added PO ₌ ⁴ acted on the light-independent component. The change in the C fixation rate due to added NO _- ³ was equal to the rate of NO _- ³ uptake by the light-dependent component on a molar basis. Nitrate assimilation (reduction) rates were calculated from the time course of extracellular and intracellular NO _- ³ concentrations. The light-dark change in the assimilation rate was similar to the light-dark change in the uptake rate, suggesting close coupling between the light-dependent components of uptake and assimilation. The assimilation rate dropped upon exhaustion of extracellular NO _- ³ , implying that an uptake-coupled component of assimilation is unavailable for the assimilation of intracellular NO _- ³ . The reduction in the C fixation rate due to NO _- ³ was temporally associated with uptake rather than assimilation, but may reflect interaction with either the light-dependent uptake step or the closely coupled assimilation. Phosphate additions reduced the rate of NO _- ³ uptake, while the rate of assimilation was unaffected.