Form II of D1 is important during transition from standard to high light intensity in Synechococcus sp. strain PCC 7942

In Synechococcus sp. strain PCC 7942 the D1 protein of Photosystem II is encoded by a multigene family; psbAI encodes Form I of D1 whereas both psbAII and psbAIII encode Form II. The psbA genes are differentially regulated in response to changes in light intensity, such that psbAI expression and Form I predominate at standard light intensity, whereas psbAII and psbAIII are induced at high light intensity, causing insertion of Form II into the thylakoids. The present study addressed whether high-light induced Form II is important for Synechococcus cells during adaptation to high light intensity. Wild-type Synechococcus, and mutants which produce only Form I (R2S2C3) or only Form II (R2K1), were co-cultured at standard light (130 E · m^–2 · s^–1) and then shifted to high light (750 E·m^–2·s^–1). Measurement of the proportion of each cell type at various time intervals revealed that the growth of R2S2C3, which has psbAII and psbAIII inactive, and thus lacks Form II, is transiently impaired upon shift to high light. Both mutants R2S2C3 and R2K1 maintained normal levels of psbA messages and D1 protein under standard and high light through an unknown mechanism that compensates for the inactive psbA genes. Thus, the impairment of R2S2C3 at high light is not due to a deficiency of D1 protein, but results from lack of Form II. We discounted the influence of possible secondary mutations by re-creating the psbA-inactivated mutants and testing the newly isolated strains. We conclude that Form II of D1 is intrinsically important for Synechococcus cells during a critical transition period after exposure to high light intensities.     

Lakewide odours in Ontario and New Hampshire caused by Chrysochromulina breviturrita Nich. (Prymnesiophyceae)

At densities of 500–9 000 cells/ml, the planktonic alga Chrysochromulina breviturrita Nich. produced obnoxious lake-wide odours in five lakes between 1978 and 1980. In one case, CuSO₄ was used to kill the organism and control the odour, but in the other four lakes, the species persisted for 4–6 weeks before populations declined and odours disappeared. This is the first recorded instance of odour production by any member of the Prymnesiophyceae.     

Heterotrophic growth of Thiobacillus acidophilus in batch and chemostat cultures

Heterotrophic growth of the facultatively chemolithoautotrophic acidophile Thiobacillus acidophilus was studied in batch cultures and in carbon-limited chemostat cultures. The spectrum of carbon sources supporting heterotrophic growth in batch cultures was limited to a number of sugars and some other simple organic compounds. In addition to ammonium salts and urea, a number of amino acids could be used as nitrogen sources. Pyruvate served as a sole source of carbon and energy in chemostat cultures, but not in batch cultures. Apparently the low residual concentrations in the steady-state chemostat cultures prevented substrate inhibition that already was observed at 150 M pyruvate. Molar growth yields of T. acidophilus in heterotrophic chemostat cultures were low. The Y _max and maintenance coefficient of T. acidophilus grown under glucose limitation were 69 g biomass · mol^–1 and 0.10 mmol · g^–1 · h^–1, respectively. Neither the Y _max nor the maintenance coefficient of glucose-limited chemostat cultures changed when the culture pH was increased from 3.0 to 4.3. This indicates that in T. acidophilus the maintenance of a large pH gradient is not a major energy-requiring process. Significant activities of ribulose-1,5-bisphosphate carboxylase were retained during heterotrophic growth on a variety of carbon sources, even under conditions of substrate excess. Also thiosulphate- and tetrathionate-oxidising activities were expressed under heterotrophic growth conditions.     

Nitrogen fixation by Nodularia spumigena Mertens (Cyanobacteriaceae). 2: Laboratory studies

The effects of changes in diurnal light patterns, salinity, and phosphorus on nitrogen fixation (as measured by acetylene reduction) by Nodularia spumigena Mertens were examined. As well, the effects of added inorganic nitrogen on growth, nitrogen fixation and heterocyt frequencies, and changes in nitrogen fixation and heterocyst frequencies during the growth cycle of Nodularia in cultures were determined.The diurnal pattern of nitrogenase activity in Nodularia was primarily light-induced, though dark activity did occur. Nitrogenase activity following a period of darkness exceeded the normal light rate (> 90 compared to 50 nmol · C₂H₂ reduced · ml^–1 · h^–1). Nitrogen fixation was reduced by high and very low salinities (5 to 10 was the optimum range), and added phosphorus stimulated nitrogenase in P-starved cells. Added nitrogen (ammonium or nitrate) had no effect on the growth of Nodularia, but in short term studies, ammonium completely inhibited nitrogenase activity. Heterocyst frequencies were greatest in the log phase of growth (to 40 per mm). During stationary phase, nitrogenase activity was negligable.     

Influence of vitamin B12 and light on the formation of chlorosomes in green- and brown-colored Chlorobium species

The specific Bchl a and c content of the vitamin B₁₂-dependent Chlorobium limicola strain 1230 decreased strongly under vitamin B₁₂ limitation. In comparison to a regularly grown culture (20 g vitamin B₁₂/l) the specific Bchl c content of a B₁₂-limited culture was reduced to 20% and the specific Bchl a content to 42%. By ultrathin sections it could be clearly demonstrated that B₁₂-deficient cells contained no chlorosomes. After the addition of vitamin B₁₂ to a deficient culture, chlorosomes were formed and the Bchl a and c content increased again to the level of regularly grown cells. The brown-colored Chlorobium phaeobacteroides strain 2430 (type strain) and the extremely low-light-adapted strain MN1 were compared with respect to the influence of light on the formation of chlorosomes and the Bchl e and carotenoid content. By ultrathin sections it could be demonstrated that strain MN1 produced two-fold larger chlorosomes. Chlorosome dimensions of strain MN1 decreased with increasing light intensities. The number of chlorosomes per cell in both strains did not change with different light intensities. Strain MN1 formed twice as much Bchl e as the type strain when grown at 30 or below 1 mol · m^-2 · s^-1. Under comparable light conditions strain MN1 formed 14–57% more carotenoids than the type strain. Low light intensities aaused the carotenoid content to increase by 25% in strain 2430 in comparison to high light intensity.     

Effect of light internsity on vesicle formation in Chlorobium

1. Chlorobium limicola forma sp. thiosulfatophilum was cultivated at 22 and 22000 lux. 2. The content of bchl d on a protein basis in the low light intensity cultures was about twice that of the high light intensity cultures. 3. After growth at 22 lux the red bchl d peak was at c. 743 nm, while at the higher intensity this peak was at c. 732 nm. 4. Electron microscopy of thin sections of Chlorobium revealed that vesicle size was greater at the low light intensity than at the high. 5. This was confirmed by sucrose density gradient centrifugation of differentially ¹⁴C-labelled vesicles from cultures grown at the two intensities. 6. The optimum temperature for growth was about 35°C. Incubation at the optimum temperature was particularly beneficial at high light intensity.Abbreviation bchl bacteriochlorophyll     

Gas vesicle formation and buoyancy regulation in Pelodictyon phaeoclathratiforme (Green sulfur bacteria)

Gas vesicle formation and buoyancy regulation in Pelodictyon phaeoclathratiforme strain BU1 (Green sulfur bacteria) was investigated under various laboratory conditions. Cells formed gas vesicles exclusively at light intensities below 5 mol · m^-2 · s^-1 in the stationary phase. No effect of incubation temperature or nutrient limitation was observed. Gas space of gas vesicles occupied always less than 1.2% of the total cell volume. A maximum cell turgor pressure of 330 kPa was determined which is comparable to values determined for cyanobacterial species. Since a pressure of at least 485 kPa was required to collapse the weakest gas vesicles in Pelodictyon phaeoclathratiforme, short-term regulation of cell density by the turgor pressure mechanism can be excluded.Instead, regulation of the cell density is accomplished by the cease of gas vacuole production and accumulation of carbohydrate at high light intensity. The carbohydrate content of exponentially growing cells increased with light intensity, reaching a maximum of 35% of dry cell mass above 10 mol · m^-2 · s^-1. Density of the cells increased concomitantly. At maximum density, protein and carbohydrate together accounted for 62% of the total cell ballast. Cells harvested in the stationary phase had a significantly lower carbohydrate content (8–12% of the dry cell mass) and cell density (1010–1014 kg · m^-3 with gas vesicles collapsed) which in this case was independent of light intensity. Due to the presence of gas vesicles in these cultures, the density of cells reached a minimum value of 998.5 kg · m^-3 at 0.5 mol · m^-2 · s^-1.The cell volume during the stationary phase was three times higher than during exponential growth, leading to considerable changes in the buoyancy of Pelodictyon phaeoclathratiforme. Microscopic observations indicate that extracellular slime layers may contribute to these variations of cell volume.     

Effect of light intensity on macromolecular synthesis in cyanobacteria

The light-dependent incorporation of NaH¹⁴CO₃ into low molecular weight compounds, polysaccharide, or protein was determined in cultures of the cyanobacteriumMerismopedia tenuissima incubated at a series of light intensities. There was an inverse relationship between incorporation into polysaccharide and protein. At light intensities of 90 E/m²/sec or above, relative incorporation of radioisotope into polysaccharide was greatest and relative incorporation into protein was lowest. Optimal relative protein accumulation occurred in samples incubated at 20 E/m²/sec. A broader optimum of light intensity for maximal protein accumulation was found if ammonia rather than nitrate was the nitrogen source. Physiological adaptation of cultures to growth at a particular light intensity did not alter the pattern of macromolecular incorporation when those cultures were tested over the series of light intensities. The response of cultures ofOscillatoria rubescens to light intensity was similar to that ofM. tenuissima, although incorporation into low molecular weight compounds was significantly greater.The effect of light intensity on macromolecular synthesis in a natural population ofOscillatoria rubescens was also determined. A pattern similar to that observed in batch cultures ofO. rubescens was occasionally found, but in other experiments there was no increase in relative protein incorporation when light intensity was decreased.     

Mass development of the flagellate Gonyostomum semen (Raphidophyta) in Swedish forest lakes - an effect of acidification?

During the last 20–30 years the flagellate Gonyostomum semen has become more abundant in lakes, especially small humic lakes, in Scandinavia. Mass development of the alga has been reported from areas affected by anthropogenic acid deposition, and reports from bathers of health problems are becoming frequent. Although there is an apparent connection between the appearance of Gonyostomum and acidification, it seems not to be the low pH per se that is the cause, but rather interlinked factors. The present tendency towards a wider distribution of the alga in non-humic lakes, emphasizes the importance of a better understanding of small humic lakes.     

Characteristics of mixotrophic growth of Synechocystis sp. in an enclosed photobioreactor

Synechocystis sp. PCC 6803 was grown in a 2.5 l enclosed photobioreactor on medium with or without glucose. The incident light intensities ranged from 1.5 klux to 7 klux. The highest average specific growth rates of mixotrophic culture and photoautotrophic culture were, respectively, 1.3 h^–1 at a light intensity of 7 klux on 3.2 g l^–1 glucose and 0.3 h^–1 at both light intensities of 5 klux and 7 klux. The highest cell density 2.5 g l ^–1 was obtained at both of light intensities 5 klux and 7 klux on 3.2 g glucose l^–1. Glucose consumption decreased with decreasing light intensity. The energy yields of mixotrophic cultures were 4 to 6 times higher than that of photoautotrophic cultures. Light favored mixotrophic growth of Synechocystis sp. PCC 6803, especially at higher light intensities (5–7 klux).     

Interaction of nitrogen source and light intensity on the growth and photosynthesis of the brown tide alga Aureococcus anophagefferens

High ratios of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) have been suggested to favor the growth of the brown tide alga Aureococcus anophagefferens. DON could provide a particular advantage in low light levels, as occur when blooms induce self-shading. We examined the effects of varying DON:DIN ratios on the photosynthetic abilities of cultured Aureococcus at two light intensities, 93 and 17 μmol photons m⁻² s⁻¹. Glutamic acid and urea were used as DON sources, and the remainder of the nitrogen was added as nitrate.In experiments examining Aureococcus growth with varying ratios of DON_Glu:DIN_Nitrate at two light intensities in batch culture, higher growth rates and biomass were observed in treatments containing DIN than in those with DON only, which contrasts with the results of previous studies. In semi-continuous growth experiments with varying DON_Urea:DIN_Nitrate ratios, low light cultures with urea had higher growth rates than those without urea. Also, the effective target area for light absorption per cell and photosystem II efficiency were greater for the low light cultures of each nutrient treatment, particularly when DON:DIN mixtures (33 and 67% N_Urea) were used. The same pattern was seen in the maximum photosynthetic rates per cell in the light-saturated (P_m^cell) and in the initial slope (α^cell) of the PE (photosynthesis versus irradiance) curve, and in PON, POC and chlorophyll a cell⁻¹. This indicates that the ability of Aureococcus to acclimate to low light conditions may be enhanced by the presence of both organic and inorganic nitrogen sources. These results suggest that Aureococcus physiology and photosynthesis are different during growth on a mixture of urea-N and nitrate than when either nitrogen source is present alone. Results of this study suggest that Aureococcus may not respond to all DON substrates in the same way, and that mixtures of DON and DIN may provide for higher photosynthetic rates, especially at low light. Our results did not, however, support earlier suggestions that growth on DON alone provides the brown tide alga with a large advantage at low light levels.     

Novel ion channels in the protists, Mougeotia and Saprolegnia, using sub-gigaseals

Protoplasts of the filamentous alga, Mougeotia, and the filamentous fungal oomycete, Saprolegnia ferax, exhibit two K⁺ ion channels (2–6 pA) using the patch-clamp technique when the seals are less than 1 GΩ (about 100 MΩ). The membrane potential of the protoplasts was near 0 mV as measured intracellularly with double-barreled micropipettes; thus, inward K⁺ flux is due solely to concentration differences. Although conductances are in the range expected for K⁺ channels, the activity at 0 mV is not seen in other organisms under gigaseal conditions. This paper draws attention to the usefulness of this subsidiary patch-clamp technique and the novel characteristics of ion channels in Mougeotia and Saprolegnia.     

Effect of salinity and pH on growth and agar yield of Gracilaria tenuistipitata var. liui in laboratory and outdoor cultivation

Acclimation responses of the red alga Gracilaria tenuistipitata var. liui collected on the northwest coast of Philippines were determined in laboratory setups and outdoor cultivation tanks in Haifa, Israel. Growth under laboratory conditions was influenced by all three variables studied, namely, temperature (20 or 30 ^°C), salinity (20, 30 or39‰) and seawater pH (6.5, 7.0, 8.0 or ≥ 9.0). In 250 mL flasks lacking pH control growth was influenced by temperature only at 20 ‰, whereas at 39 ‰, growth rates were similar at 20 or 30 ^°C. In 500 mL cylinders in which pH was controlled, growth rates were significantly different at a pH of 6.5 and 7.0 for all salinities, with maximal rates occurring in 39 ‰. At pH 8.0, and above, growth rates between salinities were similar and reduced to approximately 50% at a pH of 9.0 compared to rates at a pH of 6.5. Photosynthesis responses generally resembled growth responses both, in 250 mL and 500 mL cultures. In 40-L outdoor tanks, weekly growth and agar yields were apparently enhanced by increasing light intensities (up to full sunlight) and nutrient concentrations (up to 0.2 mM PO₃ ^2- and 2.0 mM NH₄ ⁺), and rates averaged four times higher than rates determined in the smaller flask cultures. This study shows broad salinity tolerance of G. tenuistipitata var. liui and its ability to sustain growth rates that are among the highest measured for Gracilaria spp. in outdoor cultures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most (75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 E m^–2 sec^–1), or in a light intensity of 150 E m^–2 sec^–1 for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod. Offprint requests to: David A. Caron     

Culture studies onCaulerpa (caulerpales,chlorophyceae) III. Reproduction,development and morphological variation of laboratory-culturedC. racemosa var.peltata

Reproduction, development and morphological variation of the marine green algaCaulerpa racemosa var.peltata from the southern part of Japan were studied in culture in the laboratory. Anisogamous biflagellate male and female gametes were produced monoeciously and copulated with each other. Settled zygotes became spherical and increased in volume. After five weeks, they formed two germ tubes which extended in opposite directions. Both germ tubes became elongated and branched, resulting in the formation of creeping, filamentous, protonema-like plants. These plants formed primary shoots which differentiated into creeping rhizomes and upright axes. Each upright axis successively formed ramuli and developed into an assimilator. The morphology of assimilators, i.e., shape and arrangement of ramuli, varied with culture coditions. The effects of temperature and light intensity on the formation of assimilators were investigated with 25 combinations of 5 temperatures (20.0–30.0C) and 5 light intensities (0.5–8.0 klux). The morphological plasticity of this alga is identical to that ofCaulerpa racemosa var.laetevirens, which was previously described by the present authors. Thus, apparently, the plasticity of this taxon is correlated with environmental factors. It appears, moreover, thatC. racemosa var.peltata andC. racemosa var.laetevirens are ecophenes (ecads) of a single species.     

Anti-inflammatory oleanane triterpenes from Tripterygium wilfordii cell suspension cultures by fungal elicitation

Summary Treatment of cell suspension cultures of Tripterygium wilfordii with an autoclaved Botrytis sp. homogenate rapidly increased the synthesis of a family of oleanane and friedelane triterpenes, including the antiinflammatory oleanane triterpene 3,22-dihydroxyolean-12-en-29-oic acid. This compound exceeded 30 mg · l^–1 in 13 day elicitations with 12 l bioreactors, in contrast to control levels of less than 5 mg · l^–1. Cell cultures treated with the fungal elicitor provided higher triterpene yields in less time than cultures in a diterpene production medium or whole plants. Elicited production has been developed for commercial application in light of the successful treatment of rheumatoid arthritis with Tripterygium extracts.     

Phytochrome-mediated uptake of calcium in Mougeotia cells

Ca²⁺ is proposed to function as a messenger in such phytochrome-mediated responses as localized cell growth, intracellular movements, and control of plasma membrane properties. To test this hypothesis, the uptake of Ca²⁺ in irradiated and non-irradiated regions of individual threads of the green alga Mougeotia was studied with the aid of ⁴⁵Ca²⁺ and low temperature autoradiography: 10–20 cells within 40–60 cell-long threads were irradiated for up to 1 min, transferred to darkness for 3 to 10 min, submersed in a radioactive medium for 1 min, washed in an unlabelled medium for 30 min, and then autoradiographed at-80° C for several days.The autoradiographs show that those cells which had been pre-irradiated with red light did take up 2–10 times more Ca²⁺ than the adjacent non-irradiated cells of the same thread. Cells pre-irradiated with farred light or red light followed by far-red light showed no enhanced uptake of Ca²⁺. These results might be interpreted to indicate, firstly, that phytochrome-Pfr is involved in the enhanced uptake of Ca²⁺ and secondly, that the accumulation of radioactive Ca²⁺ in red light irradiated cells is an expression of an increased intracellular concentration of Ca²⁺. This interpretation is based on the data that (i) the dark interval between irradiation and labelling precluded the involvement of photosynthesis, (ii) the effect of red light was reversible with far-red light, and (iii) the accumulation of Ca²⁺ persisted during the long wash-out period. We speculate, that the red light-enhanced accumulation of Ca²⁺ in Mougeotia cells is caused by a Pfr-mediated increase of the Ca-permeability of the plasma membrane, and perhaps by a Pfr-impeding of an active Ca²⁺-extrusion.Abbreviations APW artificial pond water - EGTA ethylene glycol-bis-(-amino ethyle ether) N,N-tetraacetic acid - R red irradiation - D darkness - FR far-red irradiation - Pfr physiologicallyactive form of phytochrome - Pr physiologically inactive form of phytochrome This paper is part of a Ph. D. Thesis submitted to the University of Erlangen-Nürnberg by E.M. Dreyer     

Utilization of l-alanine by Rhodopseudomonas acidophila

Rhodopseudomonas acidophila strain 7050 can satisfy all its nitrogen and carbon requirements from l-alanine. Addition of 100 M methionine sulfoximine to alanine grown cultures had no effect on growth rate indicating that deamination of alanine via alanine dehydrogenase and re-assimilation of the released NH ₄ ⁺ by glutamine synthetase/glutamate synthase was an insignificant route of nitrogen transfer in this bacterium. Determination of aminotransferase activities in cell-free extracts failed to demonstrate the presence of direct routes from alanine to either aspartate or glutamate. The only active aminotransferase involving l-alanine was the alanine-glyoxylate enzyme (114–167 nmol·min^–1·mg^–1 protein) which produced glycine as end-product. The amino group of glycine was further transaminated to yield aspartate via a glycineoxaloacetate aminotransferase (117–136 nmol·min^–1 ·mg^–1 protein). No activity was observed when 2-oxoglutarate was substituted for oxaloacetate. The formation of glutamate from aspartate was catalysed by aspartate-2-oxoglutarate aminotransferase (85–107 nmol·min^–1·mg^–1 protein). Determinations of free intracellular amino acid pools in alanine and alanine+100 M methionine sulfoximine grown cells showed the predominance of glutamate, glycine and aspartate, providing further evidence that in alanine grown cultures R. acidophila satisfies its nitrogen requirements for balanced growth by transamination.Abbreviations ADH alanine dehydrogenase - GDH glutamate dehydrogenase - GS glutamine synthetase - GOGAT glutamate synthase - MSO methionine sulfoximine - GOT glutamate-oxaloacetate aminotransferase - GPT glutamate-pyruvate amino-transferase - AGAT alanine-glyoxylate aminotransferase - GOAT glycine-oxaloacetate aminotransferase - GOTAT glycine-2-oxoglutarate aminotransferase - AOAT alanine-oxaloacetate aminotransferase     

The hydrochemical regime of the habitat of the red algaAhnfeltia tobuchiensis population in the Bay of Izmena (Kunashir Island)

In 1989–1990 and 1997, in the Bay of Izmena (Kunashir Island), the spatial distribution ofAhnfeltia tobuchiensis (Kanno et Matsubara) Mak, an unattached agar-containing red alga, was studied, and the availability of biogenic elements for the process ofA. tobuchiensis primary production was assessed. The concentrations of dissolved O₂, ammonium [NH ₄ ⁺ ], and orthophosphate [PO ₄ ³⁻ ], were highest in the central and northeastern areas of the bay, where the bulk of the population is situated. During the day, the mean oxygen concentration above the alga bed increased from 8.2±0.09 to 8.7±0.08 mg/l, showing a high rate of photosynthesis in the population. As a result of diurnal ammonium and orthophosphate assimilation by the alga, their concentration in bottom waters dropped from 5.1 to 3.5 and from 0.45 to 0.35 μM/l, respectively. Proceeding from the mean values of the nitrogen to phosphorus atomic ratio in the bay waters (11.5±3.3 in the morning and 10.6±3.3 in the evening) and in algal thalluses (33 in the case ofA. tobuchiensis), we conclude that the concentration of major biogenic elements is sufficient for the processes of primary production in the Bay of Izmena.     

Adaptation by hot spring phototrophs to reduced light intensities

Photosynthesis was measured by the ¹⁴C method on natural as well as low light adapted populations of Chloroflexus (a photosynthetic bacterium) and Synechococcus (a blue-green alga) from hot springs in Yellowstone National Park (Wyoming U.S.A.), to test the ability of these phototrophs to photosynthesize at a variety of light intensities. The herbicide 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) was used to distinguish uptake of the blue-green alga from that of the photosynthetic bacterium, while measurements of chlorophyll a and bacterio-chlorophyll c served to quantitate the standing crops of these organisms.Natural populations of Synechococcus were found to be slightly inhibited by full sunlight intensities (summer values can surpass 90000 Lux), whereas the Chloroflexus populations were not. Populations of both phototrophs subjected to reduced light intensities through the use of neutral density filters were found to adapt to low light, and then become severely inhibited by high light intensities. Adaptation to various light regimes may be an important ecological phenomenon to the survival of these hot spring phototrophs.