Growth Rates Of Synechococcus Research Articles

Investigation of the Growth and Mortality of Bacteria and Synechococcus spp. in Unvegetated and Seagrass Habitats

There is no doubt that seagrass beds constitute one of the most productive ecosystems in shallow coastal waters. Despite this, picoplankton in seagrass ecosystems has received relatively little attention. The purpose of this study was to compare picoplankton growth and mortality rates between seagrass and unvegetated habitats using chamber incubations. We tested two main hypotheses: (i) incubation with seagrass would result in higher bacterial growth rates due to increased DOM release from seagrass photosynthesis, and (ii) Synechococcus spp. would be lower in the presence of seagrass due to competition for inorganic nutrients. Bacterial growth rates were higher in seagrass chambers (2.44 d–1) than in non-seagrass chambers (2.31 d−1), respectively, suggesting that organic carbon coming from the seagrass community may support bacterial production. Furthermore, the growth rate of Synechococcus spp. was significantly lower in the seagrass treatment than in the non-seagrass treatment, likely reflecting nutrient competition with the seagrass. Small-scale chambers proved to be a useful tool for studying the factors controlling spatial and temporal patterns of picoplankton across different habitats. Furthermore, future studies should examine picoplankton growth over a wider range of spatial scales in seagrass beds and adjacent unvegetated sediment.

Annual Cycle of the Synechococcus spp. and Picoeukaryotic Growth and Loss Rates in a Subtropical Coastal Ecosystem

Seasonal variations in the picophytoplankton community structure (Synechococcus spp. and picoeukaryotes) were studied by flow cytometry in the coastal ecosystem of the subtropical western Pacific from October 2019 to September 2020. Synechococcus spp. was dominant in abundance during the study period, with its density ranging from 0.05 to 5.6 × 104 cells mL−1; its maximum occurred in July 2020. Picoeukaryotes were less abundant, with their density ranging from 0.2 to 13.6 × 103 cells mL−1. Their highest abundance was recorded in January 2020. The growth rates of Synechococcus spp. and picoeukaryotes ranged from −0.39 to 1.42 d−1 and 0.38 to 2.46 d−1, respectively, throughout the study period. Overall, the growth rate of the picoeukaryotes was significantly higher than that of Synechococcus spp. It is interesting to note that the grazing mortality of Synechococcus spp. and picoeukaryotes during the warmer period (April to September) was relatively low. Based on this study, we suggest that mixotrophic nanoflagellates lowered their feeding activity that obtained nutrients from prey and instead used additional nutrients during the incubation experiments. Our study demonstrated that a shift in the picophytoplankton community composition and grazing activity of predacious nanoflagellates in cold and warm periods can impact on the seasonal dynamics of the microbial food web.

Response of Growth and Grazing Rate of Nanoflagellates on Synechococcus spp. to Experimental Nutrient Enrichment

As important bacterivores in planktonic food webs, mixotrophic nanoflagellates cancause mortality in marine Synechococcus spp. Our previous study found that the pigmented nanoflagellate (PNF) has a significant grazing impact on Synechococcus spp. In the current study, we applied the dilution approach to test the growth and grazing rates of nanoflagellates on Synechococcus spp. We then compared the differences between experimental nutrient additions and in situ conditions in the coastal waters of the East China Sea during the summer season from July to September. The growth rates of Synechococcus spp. in the ambient environment were between 0.54 and 0.62 day−1, which were slightly higher than the 0.56 and 0.66 day−1 with nutrient enrichment in summer. In contrast, our nutrient enrichment experiments produced a marked decline approximately from 21% to 58% in the nanoflagellate grazing rate on Synechococcus spp. The reason was that the mixotrophic PNFs directly used the added nutrients and reduced their supply of nutrients from prey during the incubation experiments.

Growth of Mutant Synechococcus SP. PCC 7002: Effects of Multi-Parameters and Prediction of Growth Rate

Understanding of the correlative effects of combined variables on the growth rate of the cyanobacteria is fundamental to the exploitation of cyanobacteria as a biological mechanism to produce biofuels. Cyanobacteria (blue-green algae) are phototrophic microorganisms that offers attractive benefits, among which is a direct conversion of CO2 to a range of valuable products such as carbon-based biofuels. One model of cyanobacteria species is the cyanobacterium Synechococcus sp. PCC 7002. This paper describes the model developed to investigate the combined impacts of the variables on the growth of the Synechococcus sp. PCC 7002. The variables understudy include the temperature of the media, light intensity, the concentration of NaNO3, and the concentration of the NPK. The data is obtained from a lab scale study in which the Synechococcus sp. PCC 7002 underwent mutagenesis procedures. It is hypotheses that certain combination of the variables plays a key role in determining the growth rate of Synechococcus sp. 7002. The growth rate is determined through the measurement of four response variables, carbohydrate concentration, percentage of CO2 uptake, cell dry weight (CDW), and optical density (OD). A multivariate PCA model was developed which unearths the underlying relationship between the variables. Promising results were yield from the proposed model. Distinctive correlations between the variables were clearly described by the PCA model.

Reduced daytime net growth rate of Synechococcus spp. in the East China sea in summer estimated using a dilution approach

Viruses are thought to play a critical role in aquatic nutrient cycling, yet few field experiments have quantified their significance. We conducted fifteen modified dilution experiments in the East China Sea in summer to determine the growth rate and effect of mortality on growth rates of Synechococcus spp. They were measured in a 0.2 μm dilution series in which the abundance of Synechococcus spp. and nanoflagellates were diluted but the viruses were retained. They were also measured in a 30 kDa filtered natural samples in which the abundance of Synechococcus spp., nanoflagellates and viruses were diluted. We found positive relationships between dilution factor and Synechococcus spp. net growth rates. Instantaneous growth rate of Synechococcus spp in 100% whole water generally exceeded rates in other dilution fraction treatments, a result of net growth rates of Synechococcus spp. in undiluted incubations. Throughout the entire study area, growth rate of Synechococcus app. was highly variable and ranged from 0.01 h−1 to 0.17 h−1, with a maximum value appearing in waters with a salinity of around 31 psμ. Most importantly, net growth rates of Synechococcus spp. were suppressed in the 30 kDa dilution series compared to the 0.2 μm dilution series dilution experiments, suggesting that dissolved organic matter available to heterotrophic bacteria and viral lysis contributed to nutrient cycling and made a difference in the growth rates found in the two dilution series.

Comparative genomics reveals the molecular determinants of rapid growth of the cyanobacterium Synechococcus elongatus UTEX 2973

Cyanobacteria are emerging as attractive organisms for sustainable bioproduction. We previously described Synechococcus elongatus UTEX 2973 as the fastest growing cyanobacterium known. Synechococcus 2973 exhibits high light tolerance and an increased photosynthetic rate and produces biomass at three times the rate of its close relative, the model strain Synechococcus elongatus 7942. The two strains differ at 55 genetic loci, and some of these loci must contain the genetic determinants of rapid photoautotrophic growth and improved photosynthetic rate. Using CRISPR/Cpf1, we performed a comprehensive mutational analysis of Synechococcus 2973 and identified three specific genes, atpA, ppnK, and rpaA, with SNPs that confer rapid growth. The fast-growth-associated allele of each gene was then used to replace the wild-type alleles in Synechococcus 7942. Upon incorporation, each allele successively increased the growth rate of Synechococcus 7942; remarkably, inclusion of all three alleles drastically reduced the doubling time from 6.8 to 2.3 hours. Further analysis revealed that our engineering effort doubled the photosynthetic productivity of Synechococcus 7942. We also determined that the fast-growth-associated allele of atpA yielded an ATP synthase with higher specific activity, while that of ppnK encoded a NAD+ kinase with significantly improved kinetics. The rpaA SNPs cause broad changes in the transcriptional profile, as this gene is the master output regulator of the circadian clock. This pioneering study has revealed the molecular basis for rapid growth, demonstrating that limited genetic changes can dramatically improve the growth rate of a microbe by as much as threefold.

Identifying the Metabolic Differences of a Fast-Growth Phenotype in Synechococcus UTEX 2973

The photosynthetic capabilities of cyanobacteria make them interesting candidates for industrial bioproduction. One obstacle to large-scale implementation of cyanobacteria is their limited growth rates as compared to industrial mainstays. Synechococcus UTEX 2973, a strain closely related to Synechococcus PCC 7942, was recently identified as having the fastest measured growth rate among cyanobacteria. To facilitate the development of 2973 as a model organism we developed in this study the genome-scale metabolic model iSyu683. Experimental data were used to define CO2 uptake rates as well as the biomass compositions for each strain. The inclusion of constraints based on experimental measurements of CO2 uptake resulted in a ratio of the growth rates of Synechococcus 2973 to Synechococcus 7942 of 2.03, which nearly recapitulates the in vivo growth rate ratio of 2.13. This identified the difference in carbon uptake rate as the main factor contributing to the divergent growth rates. Additionally four SNPs were identified as possible contributors to modified kinetic parameters of metabolic enzymes and candidates for further study. Comparisons against more established cyanobacterial strains identified a number of differences between the strains along with a correlation between the number of cytochrome c oxidase operons and heterotrophic or diazotrophic capabilities.

Phytoplankton growth and microzooplankton grazing dynamics across vertical environmental gradients determined by transplant in situ dilution experiments.

The Costa Rica Dome (CRD) represents a classic case of the bloom-forming capacity of small phytoplankton. Unlike other upwelling systems, autotrophic biomass in the CRD is dominated by picocyanobacteria and small eukaryotes that outcompete larger diatoms and reach extremely high biomass levels. We investigated responses of the subsurface phytoplankton community of the CRD to changes associated with vertical displacement of water masses, coupling in situ transplanted dilution experiments with flow cytometry and epifluorescence microscopy to assess group-specific dynamics. Growth rates of Synechococcus (SYN) and photosynthetic picoeukaryotes (PEUK) were positively correlated with light (Rpearson_SYN = 0.602 and Rpearson_PEUK = 0.588, P < 0.001). Growth rates of Prochlorococcus (PRO), likely affected by photoinhibition, were not light correlated (Rpearson_PRO = 0.101, P = 0.601). Overall, grazing and growth rates were closely coupled in all picophytoplankton groups (Rspearman_PRO = 0.572, Rspearman_SYN = 0.588, Rspearman_PEUK = 0.624), and net growth rates remained close to zero. Conversely, the abundance and biomass of larger phytoplankton, mainly diatoms, increased more than 10-fold in shallower transplant incubations indicating that, in addition to trace-metal chemistry, light also plays a significant role in controlling microphytoplankton populations in the CRD.

The effect of grazing and viral lysis on the diel variations of Synechococcus spp. abundance in the East China Sea

Diel variations in the nanoflagellate grazing and viral-mediated mortality of Synechococcus spp. were simultaneously estimated using a dilution and size-fractionation approach in the inner (I-1 and I-2) and outer regions (O-1, O2 and O-3) of the Changjiang River plume in the East China Sea during summer 2014. Synechococcus spp. abundance generally tended to increase during the dark period, followed by a plateau until midnight at all sampling stations. Overall, gross growth rate of Synechococcus spp. ranged from 0.069 h−1 to 0.122 h−1 during the growth phase. Microzooplankton, nanoflagellate grazing, and viral lysis had no effect on the Synechococcus spp. abundance during this phase. Moreover, nanoflagellate grazing was the largest cause of Synechococcus spp. mortality during the loss phase at nighttime. Compared to the predators, viruses exerted only a minor impact on mortality at St. I-1, where we detected some of the effect that this community had on Synechococcus spp.. Little is known about the impact of nanoflagellates and viruses on the short-term dynamics of Synechococcus spp. in the East China Sea. Therefore, this study's characterization of the relative importance of nanoflagellates and viruses may help provide a better understanding of trophic structures and the energy flow within the microbial loop.

Did sulfate availability facilitate the evolutionary expansion of chlorophyll a+c phytoplankton in the oceans?

During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent primary producers in the oceans, succeeding an earlier biota in which green algae and cyanobacteria had been proportionally more abundant. This transition occurred during an interval marked by increased sulfate concentration in seawater. To test whether increasing sulfate availability facilitated the evolutionary transition in marine phytoplankton, the cyanobacterium Synechococcus sp., the green alga Tetraselmis suecica and three algae containing chlorophyll a+c (the diatom Thalassiosira weissflogii, the dinoflagellate Protoceratium reticulatum and the coccolithophorid Emiliania huxleyi) were grown in media containing 1, 5, 10, 20, or 30 mm SO(4) (2-) . The cyanobacterium and the green alga showed no growth response to varying [SO(4) (2-) ]. By contrast, the three chlorophyll a+c algae showed improved growth with higher [SO(4) (2-) ], but only up to 10 mm. The chlorophyll a+c algae, but not the green alga or cyanobacterium, also showed lower C:S with higher [SO(4) (2-) ]. When the same experiment was repeated in the presence of a ciliate predator (Euplotes sp.), T. suecica and T. weissflogii increased their specific growth rate in most treatments, whereas the growth rate of Synechococcus sp. was not affected or decreased in the presence of grazers. In a third experiment, T. suecica, T. weissflogii, P. reticulatum and Synechococcus sp. were grown in conditions approximating modern, earlier Paleozoic and Proterozoic seawater. In these treatments, sulfate availability, nitrogen source, metal availability and Pco(2) varied. Monospecific cultures exhibited their highest growth rates in the Proterozoic treatment. In mixed culture, T. weissflogii outgrew other species in modern seawater and T.suecica outgrew the others in Paleozoic water. Synechococcus sp. grew best in Proterozoic seawater, but did not outgrow eukaryotic species in any treatment. Collectively, our results suggest that secular increase in seawater [SO(4) (2-) ] may have facilitated the evolutionary expansion of chlorophyll a+c phytoplankton, but probably not to the exclusion of other biological and environmental factors.

Seasonal variations in trophic dynamics of nanoflagellates and picoplankton in coastal waters of the western subtropical Pacific Ocean

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 51:263-274 (2008) - DOI: https://doi.org/10.3354/ame01196 Seasonal variations in trophic dynamics of nanoflagellates and picoplankton in coastal waters of the western subtropical Pacific Ocean An-Yi Tsai1, Kuo-Ping Chiang1,3,4,*, Jeng Chang2,3, Gwo-Ching Gong3 1Institute of Environmental Biology and Fishery Science, 2Institute of Marine Biology, 3Institute of Marine Environmental Chemistry and Ecology, and 4Center for Marine Bioscience and Biotechnology, National Taiwan Ocean University, Keelung 202-24, Taiwan, ROC *Corresponding author. Email: kpchiang@mail.ntou.edu.tw ABSTRACT: This investigation was undertaken between August 2002 and July 2003 at a coastal station on the southern edge of the East China Sea. We found a 2-phase (warm season, >25°C [June to October] and cold season, <25°C [November to May]) seasonal cycle with a 10-fold variation in the growth of bacteria (heterotrophic bacteria only) and picophytoplankton, primarily coccoid cyanobacteria (Synechococcus spp.), and nanoflagellate grazing rates upon them. Growth rate in bacteria and Synechococcus spp. appeared to be affected by changes in temperature, and the nanoflagellate grazing rate was controlled by concentrations of bacteria and Synechococcus spp. The seasonal cycles of abundance in bacteria and Synechococcus spp. were a reflection of their changing net growth rates (i.e. picoplankton growth rates – nanoflagellate grazing rates), which were highest at the beginning of the warm season. During the warm season, nanoflagellates consumed an equal amount of bacteria and Synechococcus spp.; therefore, growth in both groups was affected equally by grazing in the warm season. However, during the cold season, bacteria contributed more to nanoflagellate carbon consumed than did Synechococcus spp. because the growth rate of Synechococcus spp. was low. We conclude that during the warm season a significant part of bacteria and Synechococcus spp. carbon is channeled through the microbial loop, possibly making it an important link between primary production and higher trophic levels. KEY WORDS: Synechococcus spp. · Picoplankton · Nanoflagellate · Microbial loop Full text in pdf format PreviousNextCite this article as: An-Yi T, Chiang KP, Chang J, Gong GC (2008) Seasonal variations in trophic dynamics of nanoflagellates and picoplankton in coastal waters of the western subtropical Pacific Ocean. Aquat Microb Ecol 51:263-274. https://doi.org/10.3354/ame01196 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 51, No. 3. Online publication date: June 16, 2008 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2008 Inter-Research.

Abundance, biomass and growth rates of Synechococcus sp. in a tropical coastal ecosystem (Philippines, South China Sea)

The abundance, biomass and growth rates of Synechococcus sp. were estimated in a tropical coastal ecosystem (Philippines, South China Sea). The patterns of change of these parameters were further examined in relation to human-derived disturbance such as siltation, and by short-term episodic disturbances such as the typhoons, which are frequent in the region. The average abundance and biomass of Synechococcus sp. in the coastal ecosystem ranged from 0.13 to 21×10 6 cells l −1, and from 0.01 to l.6 mg C m −3, respectively, with higher biomass occurring near river sources rich in inorganic nutrients. There was, however, a significant decline of specific growth rates and maximum frequency of cells in division with increasing siltation, which suggests a deterioration of the environmental conditions to support picocyanobacterial populations. The low biomass of Synechococcus sp. in more pristine sites, in spite of relatively high growth rates there suggests that loss factors (i.e. grazing) are important in controlling the biomass in the area. The temporal pattern of picocyanobacterial abundance in the tropical ecosystem studied was tightly coupled with their temporal patterns of growth indicating that changes in abundance may result from changes in growth rate. There was not, however, a clear annual pattern of Synechococcus sp. abundance in the study site but there was some evidence for effects of storms on Synechococcus sp. abundance.

Physiological characteristics of picophytoplankton, isolated from Lake Kinneret: responses to light and temperature

Two different phylogenetic groups of picophytoplankton, namely picocyanobacteria and picoeukaryotes, are represented in Lake Kinneret. Three species were isolated from the lake and identified as the picoeukaryote Mychonastes homosphaera and two picocyanobacteria, Synechococcus sp. A and B. Picocyanobacterial and M. homosphaera cultures grew well at light intensities up to 330 and 700 μmol photons m -2 s -1 , respectively, but poorly below 10 μmol photons m -2 s -1 . Picoganobacterial and M. homosphaera cultures photoacclimated to low light by increasing their chlorophyll per cell through increase in photosynthetic unit (PSU) size and PSU numbers, respectively. Growth rates of Synechococcus A and B were higher at temperatures characteristic of summer-autumn in the epilimnion, when maximum abundances of picocyanobacteria occur Growth rates of M. homosphaera were higher at 14°C, corresponding to lake water temperatures during their occurrence in winter-spring. Temperature is a dominant factor influencing the seasonal dynamics of both picocyanobacteria and picoeukaryotes in Lake Kinneret, while the vertical distribution is controlled by acclimation to different light conditions. Differences in temperature tolerance and photoacclimation suggest that Synechococcus A belongs to picocyanobacteria found in summer below surface waters, while Synechococcus B represents picocyanobacteria found throughout the year at all depths. Photoacclimation to high light as shown in M. homosphaera cultures, may account for the relatively high abundance of picoeukaryotes in surface waters in Lake Kinneret.

Response of Mediterranean Synechococcus growth and loss rates to experimental nutrient inputs

The response of Synechococcus sp. growth, primary production and loss rates was examined in a large-scale mesocosm nutrient enrichment experiment in a coastal NW Mediterranean bay community during the summer of 1997. The mesocosm units (33 m 3 ) received N, P and Si at a sto- chiometric ratio of 20N:7Si:1P, at the normal nutrient loading rate for the site (5 mmol N m -2 d -1 ), and at 0.5-, 2-, 4-, 8-, and 16-fold the normal nutrient loading input into the bay. Growth and primary pro- duction of Synechococcus during the early phase of the experiment increased 2- to 4-fold in the mesocosms receiving ♢ 4-fold the normal nutrient loading rate compared to those receiving less, pro- viding evidence of nutrient-limited growth of the population in the Mediterranean bay studied dur- ing that summer. Synechococcus growth was saturated at approximately 0.25 µM DIN, and was unsustained as nutrient inputs continued, showing growth inhibition at relatively high DIN levels (> 8 µM). The response of loss rates of Synechococcus to the experimental nutrient inputs was similar to the growth responses, although the dynamics of Synechococcus population size seemed to be an interplay between growth and loss rates. The population size of Synechococcus increased early by almost 3-fold, indicative of limitation of Synechococcus biomass in the Bay of Blanes during summer. The increase is suggested to result from the lag between Synechococcus growth and loss rate responses during the early phase of the experiment when growth rates exceeded loss rates, resulting in an increase in net production with increased loading. The increase of Synechococcus population size towards the end of the experiment was a result of decreased grazing pressure on Synechococcus, despite the low growth rates of Synechococcus at the high nutrient inputs. Grazing of Synechococcus is suggested to be the main loss process (>50% of calculated loss rates) except towards the end of the experiment when grazing was only 13% of the calculated losses.

Prochlorococcus and Synechococcus growth rates and contributions to production in the Arabian Sea during the 1995 Southwest and Northeast Monsoons

We investigated the growth rates of Prochlorococcus and Synechococcus spp. and their relative contributions to carbon production at five stations in the Arabian Sea during the late Southwest and early Northeast Monsoon seasons in 1995. Estimates of Prochlorococcus growth rates were based on diel cell cycle analysis. Accurate determination of the duration of the cell cycle terminal event, e.g., t S+G2 was not possible for Synechococcus because of its highly variable cell cycle patterns (e.g., imperfect phasing, multiple DNA-replication peaks, and dark-arrested division). Consequently, growth and mortality rates of Synechococcus were estimated from diel variations in population abundance. The assumptions of this approach were validated by observations that Synechococcus cell division occurred only during the daytime as well as good agreement between growth rate estimates for Prochlorococcus compared to cell cycle analyses. Prochlorococcus growth rates were typically less than 1 doubling per day, although higher rates occurred in the surface waters at an offshore site (Stn. N7) during the SW Monsoon and at a coastal station (S2) during the NE Monsoon. For Synechococcus, maximum growth rates >2 d -1 were observed at mesotrophic (nitrate concentration 0.1–3 μm) onshore stations during both seasons. Synechococcus spp. grew much faster than Prochlorococcus in the upper water column at almost every station during both seasons, but the depth range of its maximum growth rate was shallower and its growth and abundance decreased sharply in deeper waters. In addition, growth rates of Synechococcus increased with nutrient availability whereas Prochlorococcus growth rates did not vary dramatically with nutrients. Although there was no significant difference in Synechococcus growth rates between the late SW and early NE Monsoon seasons, the estimated carbon production and relative contribution to primary production were greater during the early NE Monsoon due to the larger biomass of Synechococcus and lower total primary production. Maximum Prochlorococcus production was found only in the most oligotrophic regions, and Prochlorococcus was not a major contributor of primary production for the most part of the Arabian Sea during the SW and NE Monsoons. Overall, Prochlorococcus and Synechococcus were inversely related in terms of their relative contributions to primary production.

GROWTH AND PHOTOSYNTHESIS OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) UNDER IRON STRESS

Prokaryotic picoplankton such as Synechococcus are relatively abundant in putatively Fe‐limited high‐nutrient, low‐chlorophyll (HNLC) regions of the oceans. The physiology of Synechococcus under Fe stress has been studied less than eukaryotic algae. Recent evidence suggests that although biomass and growth rates of Synechococcus are not typically Fe limited in situ, cells may still exhibit symptoms of Fe stress. We grew Synechococcus A2169 and WH7803 in laboratory batch cultures in the artificial medium Aquil and enriched natural seawater, at a series of Fe concentrations and Fe:macronutrient ratios, and with either nitrate or ammonium as the sole nitrogen source. Cell yields, and in some experiments exponential specific growth rate (μ), were more readily Fe limited in the Atlantic isolate WH7803 than in the equatorial Pacific isolate A2169. In both strains, final cell yields spanned about an order of magnitude and decreased continuously with Fe concentration from 900 to 3.6 nM (150 μM N, 10 μM P), whereas μ decreased much less and only at Fe concentrations below 90 nM. Synechococcus yield was controlled by both absolute Fe concentration and Fe:macronutrient ratio, but μ was determined primarily by absolute Fe concentration. Contrary to theoretical predictions, neither yield nor μ was higher in Fe‐limited cells grown in ammonium compared to nitrate. Under severe Fe stress, cellular chlorophyll (Chl) content and light‐saturated gross photosynthetic capacity (Pcellm) decreased proportionately, and dark respiration (Rcelld) increased, such that net Pcellm was extremely low but gross PChlm was unchanged. This is the first report of an absolute increase in Rcelld under Fe stress in phytoplankton.

The significance of viruses to mortality in aquatic microbial communities

A variety of approaches including enumeration of visibly infected microbes, removal of viral particles, decay of viral infectivity, and measurements of viral production rates have been used to infer the impact of viruses on microbial mortality. The results are surprisingly consistent and suggest that, on average, about 20% of marine heterotrophic bacteria are infected by viruses and 10-20% of the bacterial community is lysed daily by viruses. The effect of viruses on phytoplankton is less certain, but ca. 3% of Synechococcus biomass may be lysed daily. The fraction of primary productivity this represents depends upon the relative biomass and growth rate of Synechococcus. Virus enrichment experiments suggest that the productivity of eukaryotic phytoplankton would be ca. 2% higher in the absence of viruses. Overall, probably about 2-3% of primary productivity is lost to viral lysis. There is considerable variation about these estimates; however, they represent a starting point for incorporating viral-mediated processes into aquatic ecosystem models.

Diel patterns of cell division and growth rates of Synechococcus spp. in Long Island Sound

Diel patterns of cell division and growth rates of Synechococcus spp. in Long Island Sound

Effect of irradiances up to 2000 μE m −2 s −1 on marine Synechococcus WH7803—II. Photosynthetic responses and mechanisms

We investigated the photosynthetic behavior of Synechococcus WH7803 when grown over the irradiance range of 30–2000 μE m −2 s −1 in nutrient-replete, continuous light, pre-adapted batch cultures. For each of 8 growth irradiances investigated, we found a unique photosynthesis vs irradiance ( P vs I) relationship. Cell- and carbon-specific photosynthetic light harvesting efficiencies (α (cell) and α (C)) decreased 10-fold from the lowest to highest growth irradiances. Chlorophyll and phycocyanin-specific efficiencies also decreased but to a lesser extent. Phycoerythrin-specific efficiencies increased ca. 2-fold with increasing irradiances. Photosynthetic capacity ( P max) approximated the in situ rate of photosynthesis ( P i ) only at growth irradiances which saturated growth rate; at light limiting irradiances, P max increased to 2.9 times P i . Photoinhibition of photosynthesis was only observed in light-limited cells. The mechanism of photoadaptation which resulted in the observed growth rate vs irradiance (μ vs I) response involved regulation of α (cell) by changes in phycobilisome size over saturating irradiances and phycobilisome numbers over limiting irradiances. Our results are consistent with previously reported field observations of α (Chl), α (cell) and P max increasing with depth, and of no photoinhibition of near-surface collected assemblages, suggesting that high irradiances in near-surface waters can support maximal growth rates of Synechococcus. These results raise important questions regarding the interpretation of parameters of P vs I measurement from field studies. Our results do not conform to the previously reported hypothesis that a significant percentage of phycoerythrin serves as a non-photosynthetic nitrogen storage compound in these cells.

Photosynthetic characteristics of phycoerythrin-containing marine Synechococcus spp.

Marine Synechococcus spp. are sufficiently abundant to make a significant contribution to primary productivity in the ocean. They are characterized by containing high cellular levels of phycoerythrin which is highly fluorescent in vivo. We sought (Jan.–Apr., 1984) to determine the adaptive photosynthetic features of two clonal types of Synechococcus spp., and to provide a reliable physiological basis for interpreting remote sensing data in terms of the biomass and productivity of this group in natural assemblages. It was found that the two major clonal types optimize growth and photosynthesis at low photon flux densities by increasing the numbers of photosynthetic units per cell and by decreasing photosynthetic unit size. The cells of clone WH 7803 exhibited dramatic photoinhibition of photosynthesis and reduction in growth rate at high photon flux densities, accompanied by a large and significant increase in phycoerythrin fluorescence. Maximal photosynthesis of cells grown under 10–50 μE m-2 s-1 was reduced by 20 to 30% when the cells were exposed to photon flux densities greater than 150 μE m-2 s-1. However, steady-state levels of photosynthesis maintained for brief periods under these conditions were higher than those of cells grown continuously at high photon flux densities. No photoinhibition occurred in clone WH 8018 and rates of photosynthesis were greater than in WH 7803. Yields of in-vivo phycoerythrin fluorescence under all growth photon flux densities were lower in clone WH 8018 compared to clone WH 7803. Since significant inverse correlations were obtained between phycoerythrin fluorescence and Pmax and μ for both clones grown in laboratory culture, it may be possible to provide a reliable means of assessing the physiological state, photosynthetic capacity and growth rate of Synechococcus spp. in natural assemblages by remote sensing of phycoerythrin fluorescence. Poor correlations between phycoerythrin fluorescene and pigment content indicate that phycoerythrin fluorescence may not accurately estimate Synechococcus spp. biomass based on pigment content alone.