Patterns of photosynthetic carbon metabolism in light-limited phytoplankton

Patterns of phytoplankton carbon (C) metabolism were examined in å combined laboratory and field study to assess the influence of light conditions on ¹⁴C assimilation into photosynthetic end-products. Laboratory studies with three species representing distinct size classes and taxonomic groups tested the influence of low light on patterns of C flow. Prorocentrum mariae-lebouriae (dinoflagellate) and Ditylum brightwellii (diatom) showed decreased movement of photoassimilated ¹⁴C into protein following a shift to low light ¹⁴C assimilation into lipids and photosynthetic pigments increased in low light and was paralleled by increased chl a per cell. The proportion of ¹⁴C fixed into protein returned to the pre-shift level upon return to initial light conditions. Monochrysis lutheri (chrysophyte) did not show this pattern of reduced % ¹⁴C protein. Incubations of 12 and 24 h demonstrated significant rearrangements in labeling patterns at night, wherein ¹⁴C flow into protein in darkness was favored. % ¹⁴C protein at night was lower for M. lutheri than for the other species, suggesting some interspecific differences in the low light response. Measurements of ¹⁴C assimilation in phytoplankton assemblages from Chesapeake Bay demonstrated movement of a higher proportion of photo-assimilated C into protein in samples collected in the surface mixed layer than in those below the pycnocline. In comparison, phytoplankton collected below the pycnocline fixed a higher proportion of ¹⁴C into lipids, photosynthetic pigments, and low molecular weight metabolites, as was observed in low light laboratory cultures. A comparison of 12- and 24-h incubations for measuring patterns of C flow into photosynthetic end-products confirmed the inadequacy of short-term measurements, as significant changes in ¹⁴C allocation occurred in the dark phase of the photocycle. Together, these results suggest that ¹⁴C assimilation into photosynthetic end-products can be a useful measure of adaptive state in changing light conditions, but point out some difficulties in applying this approach in situ.     

Observations on organic carbon utilization by photosynthetic marine microalgae

Organic carbon utilization was studied in 13 species of axenic unicellular marine algae. The compounds tested were 15 sugars, 19 acids, and 16 alcohols. Heterotrophy occurred in only three chlorophyte strains with acetate and ethanol; one strain with glucose. Mixotrophy was observed with the following compounds: glyceraldehyde, 1 species; acetate, 4 species; glycolic acid, 5 species; pyruvic acid, 9 species; caproic acid, 3 species; lactic acid, 1 species; ethanol, 3 species; glycerol, 9 species. Some ecological implications are discussed.     

Release of photoassimilated carbon as dissolved organic matter by marine phytoplankton

The release of photoassimilated carbon as dissolved organic matter was studied in situ in oligotrophic and eutrophic marine waters and in axenic laboratory cultures. Percentage extracellular release (PER), integrated for the trophogenic zone, ranged from 6 to 12% in eutrophic waters and from 17 to 27% in oligotrophic seas. Most of the algal cultures released low amounts of dissolved organic matter (5%) during exponential growth. The highest levels of PER were observed in surface and deep samples, possibly as a result of elevated photorespiration and senescent cells. The generally lower values for extracellular release reported in this work as compared to other studies may be partly due to improved experimental techniques which minimized previously encountered artifacts.     

Nitrate uptake in marine phytoplankton: Energy sources and the interaction with carbon fixation

Field studies of whole natural phytoplankton communities from Knight Inlet, B. C., Canada and laboratory cultures of the diatom Skeletonema costatum indicate inorganic carbon fixation may be temporarily suppressed following 10 to 15% enrichment with NO ₃ ^- or NH ₄ ⁺ . (This effect is suggested to be due to competition between inorganic carbon and nitrogen for adenosine triphosphate (ATP), and is reduced when chlorophyll a is increased intracellularly after 6 to 8 h.) Results imply that the source of ATP for nitrate uptake is primarily from Photosystem I (cyclic photophosphorylation) in the presence of light. It would appear that a transient nutrient-adaptive response occurs upon addition of extracellular nitrogen.     

Measurement of photosynthetic production of a marine phytoplankton population using a stable 13C isotope

The use of stable isotope of carbon, ¹³C, for the determination of the photosynthetic rate of a marine phytoplankton population was examined. Particular concern was paid to the effects of non-phytoplanktonic organic carbon and the enrichment of inorganic carbon on the estimation of the photosynthetic rate. Photosynthetic rates determined by the ¹³C method showed a remarkable agreement with those determined by the ¹⁴C method. Insitu determinations of photosynthetic rate were made in three different water types: open ocean, coastal and neritic waters, which included oligo- and mesotrophic waters, by using the ¹³C method established.     

Hydrocarbons of marine phytoplankton

The hydrocarbon contents of 23 species of algae (22 marine planktonic), belonging to 9 algal classes, were analyzed. The highly unsaturated 3,6,9,12,15,18-heneicosalhexaene predominates in the Bacillariophyceae, Dinophyceae, Cryptophyceae, Haptophyceae and Euglenophyceae. Rhizosolenia setigera contains n-heneicosane, presumably derived from the hexaolefin by hydrogenation. Two isomeric heptadecenes have been isolated: the double bond is located in 5-position in the bluegreen alga Synechococcus bacillaris and in 7-position in 2 green algae. Our complete analyses are discussed in the context of earlier data; some generalizations appear no longer valid. Hydrocarbon analysis of marine algae should provide a tool for the investigation of the dynamics of the marine food chain. Knowledge now available provides the background needed for distinguishing between hydrocarbons of recent biogenic origin and hydrocarbon pollutants from fossil fuels.Contribution No. 2576 of the Woods Hole Oceanographic Institution, USA.     

Effects of oxygen and inorganic carbon concentrations on the photosynthetic quotients of marine algae

The photosynthetic quotients of the marine prymnesiophyte Pavlova lutheri and the marine dinoflagellate Glenodinum sp. were measured at different concentrations of dissolved oxygen and inorganic carbon. Dissolved oxygen concentration appeared to be the most important factor controlling the photosynthetic quotient. Photosynthetic quotients generally were between 1.0 and 1.8 at oxygen concentrations less than saturation, were approximately 1.0 at oxygen saturation, and generally were from 0.1 to 1.0 at oxygen concentrations greater than saturation. The photosynthetic quotients greater than 1.0 were not caused by lipid synthesis. They may have been partially caused by the presence of KNO₃ rather than an ammonium salt in the growth media. The lowered photosynthetic quotients at higher oxygen concentrations were probably caused by algal photorespiration.Contribution No. 202 from the Department of Biology, The Pennsylvania State University; 202 Buckhout Laboratory, University Park, Pennsylvania 16802, USA     

Diel periodicity of photosynthesis in marine phytoplankton

Short-term changes in photosynthesis were documented for 17 of 24 marine phytoplankton species, representing a range of taxonomic groups. Periodicity in phytoplankton photosynthesis on light-dark cycles (diel periodicity) was widespread but not universal for the species studied. The centric diatoms Lauderia borealis, Ditylum brightwellii, Stephanopyxis turris, Coscinodiscus rex, Chaetoceros gracile, and Biddulphia mobiliensis had strong diel periodicity in photosynthetic capacity (P _max). Amplitudes of the daily variations ranged from 2.9 to >50, with maxima in the morning or near midday, and with minima during the dark period, and these variations were not dependent on changes in cell pigmentation. There was some evidence for sustained photosynthetic periodicity in constant conditions in several diatoms, and an endogenous rhythm may have been present. The photosynthesis-irradiance (P-I) relationship was time-dependent for representative marine diatoms, with both the initial slope () and the asymptote (P _max) of P-I curves exhibiting significant synchronous diel oscillations. Moreover, detailed studies of the amplitude and timing of photosynthetic periodicity for the diatoms L. borealis and D. brightwellii demonstrated large temporal variations in photosynthesis with morning maxima. These P-I oscillations are discussed with reference to models of primary production which use the relationship between photosynthesis and light as a component of predictive equations for phytoplankton growth in the sea.     

Increased carbon dioxide availability alters phytoplankton stoichiometry and affects carbon cycling and growth of a marine planktonic herbivore

Rising levels of CO₂ in the atmosphere have led to increased CO₂ concentrations in the oceans. This enhanced carbon availability to the marine primary producers has the potential to change their nutrient stoichiometry, and higher carbon-to-nutrient ratios are expected. As a result, the quality of the primary producers as food for herbivores may change. Here, we present experimental work showing the effect of feeding Rhodomonas salina grown under different pCO₂ (200, 400 and 800 μatm) on the copepod Acartia tonsa. The rate of development of copepodites decreased with increasing CO₂ availability to the algae. The surplus carbon in the algae was excreted by the copepods, with younger stages (copepodites) excreting most of their surplus carbon through respiration and adult copepods excreting surplus carbon mostly as DOC. We consider the possible consequences of different excretory pathways for the ecosystem. A continued increase in the CO₂ availability for primary production, together with changes in the nutrient loading of coastal ecosystems, may cause changes in the trophic links between primary producers and herbivores.     

Diel changes in phytoplankton photosynthetic efficiency in Brackish waters

From 18 to 23 September 1974, investigations on the diel changes in phytoplankton were carried out in the Baltic Sea. Every 4 h, water samples were collected from 2 and 15 m, and PO₄, chlorophyll a, temperature, salinity, pH, phytoplankton composition and phytoplankton light photosynthesis relationship were determined. Continuous measurements of surface irradiance and some estimations of zooplankton were also made. P ^B (photosynthesis per unit chlorophyll a at low light levels of 2·10^-2 cal cm^-2 min^-1) revealed only random variation during the sampling period, i.e., 1.0 to 1.6 mg C (mg chlorophyll a)^-1 h^-1. P _m ^B (Light-saturated photosynthesis per unit of chlorophyll a) displayed pronounced diel fluctuations with the highest value of about 6 mg C (mg chlorophyll a)^-1 h^-1 around noon, and the lowest value of about 2.5 mg C (mg chlorophyll a)^-1 h^-1 during the night, during which latter period the value of P _m ^B was more or less constant. Reasons for the diel fluctuations are discussed, and an equation which describes these fluctuations is proposed. Using this equation, the daily phytoplankton production estimated in incubators by a previously described method can be corrected for the time of day at which samples are collected.     

Light stimulation of phosphate uptake in marine phytoplankton

A phosphate uptake system responding positively to light would offer the algae a competitive advantage over bacteria, particularly when phosphorus is limiting. Effects of light on phosphate uptake in phytoplankton were investigated in view of conflicting literature reports, to test the hypothesis that both the P status and the light history of the population controlled the response. Maximum stimulation, to about 2.7x dark uptake, was observed at a Strait of Georgia, British Columbia, Canada station where phytoplankton was phosphorus sufficient but light limited. Stimulation was similar, about 1.8 x the dark rate, in the Northern Sargasso Sea, where phytoplankton was probably limited by phosphate but not by light. Minimal stimulation was observed in the Sheepscot Estuary, Maine, USA, where the population appeared to be neither phosphorus nor light limited.     

Cycling of selenite and selenate in marine phytoplankton

The marine phytoplankton Dunaliella tertiolecta, Cachonina niei, Thalassiosira nordenskioldii, Phaeodactylum tricornutum, and Chaetoceros sp. were incubated with a range of molar concentrations of sodium-selenite (Na₂-SeIVO₃) and sodium-selenate (Na₂-SeVIO₄) to examine further their role in metabolic cycling of selenium in ocean waters. At low selenium concentrations, approaching those found naturally in seawater (10^-10 to 10^-9 M), all species distinguished between selenite and selenate, and actively concentrated selenite from the incubating medium while only marginally accumulating selenate. At much higher concentrations (10^-8 to 10^-6 M), selenate was also taken up. At the highest concentration tested, i.e., 10^-5 M with C. niei, after an immediate rapid uptake in the first 24 h, the intracellular selenite and selenate levels dropped to about 35 to 50% of the initial peak values. These observations suggest an uptake mechanism in these algae which, at normal ambient concentrations of selenium (10^-9 M), preferentially selects selenite and excludes selenate. At much higher concentrations (10^-8 M), the mechanism becomes overloaded and both selenium species enter the cells. Intracellularly, selenite became associated primarily with protein and amino acid fractions, in approximately equal proportions, while only ca. 4% of total intracellular selenium was found in the lipid fraction. Trace amounts of selenate that entered the cells, mainly during the first minutes of exposure, also entered the protein and amino acid components, but over time were increasingly associated with the protein fraction only. At the end of a 10-d incubation of algal cells in selenite-spiked medium, less than 25% of total Se in the medium could in fact be identified analytically as selenite. This suggests the presence of a non-selenite metabolite, possibly released back into the medium from the algae.     

Questions on the mechanism of temperature adaptation in marine phytoplankton

The rate of light-saturated photosynthesis in 3 marine algae [Phaeodactylum tricornutum Bohlin, Nitzschia closterium (Ehrenberg) Smith and Dunaliella tertiolecta Butcher] varies during growth in batch culture. The photosynthetic rare declines most rapidly during growth at the higher temperatures. Because of these changes in photosynthesis rate, the previously reported enhanced photosynthetic abilities caused by growth at lower temperatures (generally interpreted as evidence for higher enzyme levels) can only be observed when measurements are made late in the exponential phase or after the onset of the stationary phase of growth. When allowance is made for the earlier peak of photosynthetic ability in cultures growing at higher temperatures, there is no evidence for adaptation to lower temperatures being caused by increased levels of the enzymes required for carbon-dioxide fixation. When the changes due to growth in batch culture are taken into account, certain effects of temperature can be recognized. the dry weight: chlorophyll ratio of all 3 algae increases with decreasing growth temperatures. For P. tricornutum and N. closterium, growth at lower temperatures reduces the cellular content of chlorophyll a, but has little effect on the chlorophyll content of D. tertiolecta. The dry weight: cell-number ratio of D. tertiolecta and P. tricornutum increases with lower growth temperatures, but growth temperature has little effect on the cell mass of N. closterium. Growth of the 3 algae at lower temperatures does not increase their ability to photosynthesize at these lower temperatures. Rather, it reduces their ability to assimilate carbon dioxide at the higher temperatures.     

DCMU-enhanced fluorescence as an index of photosynthetic activity in phytoplankton

Tests have been carried out, both on phytoplankton cultures and in the field, on the relation between the ratio of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)-enhanced to normal fluorescence (F _D /F _N ) and the specific photosynthetic rate (P/B). In all cases observed, significant linear correlations were found between these two values. Differences in this relation were observed according to species and physiological conditions. Nutrient stress that occurs in batch cultures decreases both F _D /F _N and P/B, while lowered light intensity has a different effect on both, increasing F _D /F _N and decreasing P/B. This is interpreted as indicating that F _D /F _N is an index of photosynthetic efficiency at different light levels and an indicator of the specific photosynthetic rate (P/B) when physiological conditions vary at a given light intensity. The practical use of DCMU-enhanced fluorometry to estimate instantaneous P/B values in the field is discussed, stressing the frequent calibrations needed, as in all in situ fluorometry studies.     

Buoyancy of natural populations of marine phytoplankton

Buoyancy of natural populations of marine phytoplankton was studied in a fjord in western Norway during the diatom bloom and in autumn. The study was carried out under approximate in situ conditions by means of an apparatus described in the paper. During the spring bloom, positive buoyancy was observed only once. Sinking rates of individual fractions ranged from 0 to more than 9 m day^-1, and the mean sinking rates of the total chlorophyll content from 0 to at least 2. 2 m day^-1. The highest rates occurred in the post-bloom period, while sinking appeared negligible from the onset of the bloom up to its culmination. In autumn, the population was dominated by small, flagellated cells. Positive buoyancy, or upward migration, was then observed in two out of three experiments.     

Iron-mediated effects on nitrate reductase in marine phytoplankton

The potential activity of nitrate reductase was determined in uni-algal cultures in the laboratory and in natural marine phytoplankton assemblages. In the laboratory bioassays, distinct differences in nitrate reductase activity were observed in iron replete versus depleted cultures for Emiliania huxleyi, Isochrysis galbana and Tetraselmis sp. Cells from iron-depleted cultures had 15 to 50 percent lower enzyme activity than those from iron-replete cultures. Upon addition of iron, nitrate reductase activity was enhanced in depleted cells up to levels comparable to those of the replete cells. Bioassays in the northern North Sea conducted in 1993, under low iron conditions, demonstrated similar results. Upon addition of 2.5 nM iron, a distinct enhancement, to a maximum of three times, of nitrate reductase activity was observed within 32 h after addition. Therefore, iron can stimulate nitrate reductase activity. In spite of the clean techniques used, some nitrate reductase activity was always observed. Iron deficiency was shown to impair nitrate reductase activity, but it is unlikely that nitrate reduction would cease completely.     

Coagulation efficiency and aggregate formation in marine phytoplankton

Flocculation of phytoplankters into large, rapidly sinking aggregates has been implicated as a mechanism of vertical transport of phytoplankton to the sea floor which could have global significance. The formation rate of phytoplankton aggregates depends on the rate at which single cells collide, which is mainly physically controlled, and on the probability of adhesion upon collision (=coagulation efficiency, stickiness), which depends on physico-chemical and biological properties of the cells. We describe here an experimental method to quantify the stickiness of phytoplankton cells and demonstrate that three species of diatoms grown in the laboratory (Phaeodactylum tricornutum, Thalassiosira pseudonana, Skeletonema costatum) are indeed significantly sticky and form aggregates upon collision. The dependency of stickiness on nutrient limitation and growth was studied in the two latter species by investigating variation in stickiness as batch cultures aged. In nutrient repleteT. pseudonana cells stickiness is very low (< 5 × 10^?3), but increases by more than two orders of magnitude as cell growth ceases and the cells become nutrient limited. Stickiness ofS. costatum cells is much less variable, and even nutrient replete cells are significantly sticky. Stickiness is highest (> 10^?1) forS. costatum cells in the transition between the exponential and the stationary growth phase. The implications for phytoplankton aggregate formation and subsequent sedimentation in the sea of these two different types of stickiness patterns are discussed.     

Short-term variability of photosynthesis in natural marine phytoplankton populations

Harmonic regression analysis has been used to determine the short-term variability in the photosynthetic rate (mgC/mg chlorophyll a/h) of phytoplankton in three inlets of Japan. In natural water without large zooplankton present, the photosynthetic rate [log P=log (100xmgC/mg chlorophyll a/h)] can be expressed as (B+A cos T). Factor B represents the average photosynthetic rate, of which the maximum is usually designated as P _max, and Factor A corresponds to the slope of the regression line. The phase of the periodicity, represented by T, is adjusted to give the highest correlation: usually T is expressed as [360/24 x (local time + 4)] in degrees. The correlation between Factors A and B is very high (r=0.95, P<0.001), indicating that Factor A may depend upon Factor B (potential activity of chlorophyll a). Both Factors A and B decrease with decreasing irradiance, but the slope of each regression between Factor A and irradiance varies with season. Continuous darkness reduces the phase of the periodicity to one cycle a day when phytoplankton has multiple cycles of photosynthetic rate per day. Adequate nutrient supply from zooplankton regeneration may cause an increase in Factor B; however, excess density of zooplankton decreases Factor A.     

Circadian variations in photosynthetic assimilation and estimation of daily phytoplankton production

In August 1984, hourly measurements of photosynthetic characteristics were carried out during 96 h, at 5 and 10 m, on a natural population of phytoplankton in the St Lawrence Estuary. Synchronous circadian variations of similar amplitude (max./min.: 2 to 3) were observed at the two depths in both the photosynthetic capacity (P _m ^B ) and the photosynthetic efficiency (^B). Maximum values occurred at around noontime and minima during the night. Estimates of daily specific productivity were computed with and without the observed circadian variability. Large differences (15 to 70%) were evidenced between estimates.Contribution to the programs of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec) and of the Maurice Lamontagne Institute (Department of Fisheries and Oceans)     

Persistent diel rhythms in the chlorophyll fluorescence of marine phytoplankton species

The diel periodicities of in-vivo chlorophyll fluorescence and DCMU-enhanced chlorophyll fluorescence of 47 marine phytoplankton species were examined for 2 d in a 14 h L:10 h D light: dark cycle and then in continous light for another 2 to 3 d. Almost all phytoplankton species exhibit a more rapid increase in in-vivo fluorescence and DCMU-enhanced fluorescence during the light phase than during the dark phase. About one-half of the species examined exhibited persistent diel rhythms in continous light, indicating the operation of a biological clock. No phylogenetic or habitat related trends as to which species exhibited persistent rhythms were apparent. Of the phyla of eukaryotic phytoplankton adequately examined, none lacked biological clocks. Contrary to past hypotheses, some phytoplankton species maintain a persistent diel rhythm in a constant environment while reproducing at a rate greater than one division per day.