Synechococcus as an indicator of trophic status in the Cochin backwaters,west coast of India

Eutrophication is a major problem in coastal water bodies. Information about the trophic status of water bodies will enable proper management of coastal ecosystems. In this regard, biological organisms which are sensitive to environmental changes can serve as indicators of ecosystem trophic status. In this study, seasonal and spatial variations of picophytoplankton (PP; <3 μm size) community structure was assessed in the Cochin backwaters (CB) with respect to the prevailing environmental conditions during three seasons, post-monsoon (PM-I; October 2011 and PM-II; November 2012), pre-monsoon (PrM; May 2012) and monsoon (MON; August 2012). CB, along the west coast of India, receives continuous load of nutrients throughout the year through anthropogenic wastes. Trophic status index (TRIX) scores showed that CB is highly eutrophic with a high phytoplankton biomass. Synechococcus was the dominant PP observed in the study area. Seasonal and spatial salinity variations influenced the PP distribution, especially Synechococcus where PE-rich Synechococcus (SYN-PE) were dominant in higher saline (>30) and PC-rich Synechococcus (SYN-PC) in lower saline (<30) waters. SYN-PC showed a significant positive relation with chlorophyll a suggesting that this group contributes substantially to the total phytoplankton biomass. TRIX scores and SYN-PC: SYN-PE abundance ratio were negatively correlated with salinity suggesting an influence of the tidal amplitude. SYN-PC correlated positively and SYN-PE negatively with TRIX scores suggesting that these groups occupy contrasting ecological niches. These findings imply that PP distribution pattern can serve as an indicator of the trophic status of coastal water bodies.     

Phytoplankton and chemico-physical parameters of the Scardovari Lagoon (Po Delta,North Adriatic Sea) during 1991 and 1992

Abstract

During research directed towards the employment of the biological resources of the North Adriatic lagoons, from January 1991 to December 1992, in both tidal phases, a survey was carried out on the phytoplankton and the chemico-physical parameters of the Scardovari lagoon. Data analyses allowed two different areas to be distinguished: one inner, which was generally characterized by high phytoplanktonic densities (St. 3–4), the other outer, which was more influced by marine load (St. 1–2). Photosynthetic picoplankton was dominant in terms of cell number in most samples. Phytoplanktonic fraction >2 μm was represented mainly by diatoms, which showed the higher species number. During the sample observation, some dinoflagellates, considered toxic or potentially toxic, were found. Their presence was important because of their effect on the bivalve mollusc cultures.     

Dynamic responses of picophytoplankton to physicochemical variation in the eastern Indian Ocean

Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.     

Urea degradation by picophytoplankton in the euphotic zone of Lake Biwa

The ability of photoautotrophic picoplankton Synechococcus to degrade urea was examined in the euphotic zone of Lake Biwa. Samples were divided into pico (0.2–2.0 μm) and larger (>2.0 μm) size fractions by filtration. The rates of urea degradation (the sum of the rates of incorporation of carbon into phytoplankton cells and of liberation of CO₂ into water) measured by radiocarbon urea were 8 and 17 μmol urea m⁻³ day⁻¹ in June and July, respectively, for the picophytoplankton in the surface water, and 196 and 96 μmol urea m⁻³ day⁻¹, respectively for the larger phytoplankton. The rates decreased with depth, somewhat similar to the vertical profiles of the photosynthetic rate. The urea degradation rates were obviously high under light conditions. In daylight, urea was degraded into two phases, carbon incorporation and CO₂ liberation, whereas in the dark it was degraded only into the CO₂ liberation phase. The contribution of picophytoplankton to total phytoplankton in urea degradation was high in the subsurface to lower euphotic layer. Urea degradation activity was higher in the picophytoplankton fraction than in the larger phytoplankton fraction. Shorter residence times of urea were obtained in the upper euphotic zone. The contribution of picophytoplankton to urea cycling was 4% to 35%. The present results suggest that the picophytoplankton Synechococcus is able to degrade urea and effectively makes use of regenerated urea as a nitrogen source in the euphotic layer, and that picophytoplankton play an important role in the biogeochemical nitrogen cycle in Lake Biwa. Received: June 25, 1998 / Accepted: February 10, 1999     

Composition and significance of picophytoplankton in Antarctic waters

Filter fractionated picophytoplankton from Antarctic coastal waters (summer 2001) represented only 7–33% of total phytoplankton, even though total stocks were low (average Chl a = 0.32 μg l⁻¹, range = 0.13–1.03 μg l⁻¹). Though all cells passed a 2 μm filter, electron microscopy revealed most cells were over 2 μm, principally Parmales, Phaeocystis sp., and small diatoms. CHEMTAX analysis of HPLC pigment data suggested type 8 haptophytes (e.g. Phaeocystis sp. plus Parmales and pelagophytes) contributed 7–58% of picoplanktonic chlorophyll a, type 6 haptophytes (e.g. coccolithophorids) 18–59%, diatoms 0–18% (mostly type 2 diatoms, e.g. Pseudonitzschia sp., 0–15%), prasinophytes 0–17%, with cell fragments of cryptophytes 0–40%, and dinoflagellates 0–11%. Only stocks of type 8 haptophytes and prasinophytes differed significantly due to successional changes. Zeaxanthin concentrations exceeded estimates from previous cyanobacterial counts and may derive from non-photosynthetic bacteria.     

Abundance and pigment type composition of picocyanobacteria in Barguzin Bay, Lake Baikal

In Lake Baikal, picocyanobacteria are the most important primary producers during the summer. Freshwater picocyanobacteria are discriminated into either the phycoerythrin (PE)-rich or the phycocyanin (PC)-rich types according to their pigment composition. The distributions of these two types of picocyanobacteria were investigated in Barguzin Bay. The PC-rich type accounted for >98% of the total picocyanobacteria at the station near the shore of the bay where river water flows directly in. In the offshore area of the lake, all of the picocyanobacteria cells were of the PE-rich type. In addition, the occurrence of the PC-rich type was restricted to the station, where the attenuation coefficient exceeded 0.25 m⁻¹. Near the shore, where the turbidity was high (>1 NTU), the cell densities of both the PE- and PC-rich types increased away from the river mouth. This indicates that the PC-rich type cells grow near the shore of the bay where turbidity is high. Since the PC-rich type could not grow well when cells were incubated in offshore lake water, restricted distribution of the PC-rich type could also be explained by their growth capability. The present study clearly demonstrated the shift in the pigment type composition of picocyanobacteria from the coastal to the pelagic zone of Lake Baikal. The co-existence of the two pigment types probably enables the abundance of the picocyanobacterial community to be stable over a broader range of environmental conditions than would be possible for a single pigment type.     

Seasonal distribution of species of the toxic dinoflagellate genus Dinophysis in Maizuru Bay (Japan), with comments on their autofluorescence and attachment of picophytoplankton

The seasonal distribution of the dinoflagellate genus, Dinophysis, in Maizuru Bay, Japan, was investigated from May 1997 to December 1999. Seven species of Dinophysis were detected, including the toxic species of Dinophysis acuminata and D. fortii. The most dominant species wasD. acuminata, detected year-around and more abundantly during periods when water temperatures were between 15 and 18 °C. No relationship was found between cell abundance of Dinophysis spp. and concentrations of dissolved inorganic nutrients. Phycoerythrin containing nano- and picophytoplankton (cryptophytes and cyanobacteria), suspected to be prey of mixotrophic Dinophysis, were enumerated simultaneously. A clear relationship was not found among the cell abundances of Dinophysis spp. and nano- and picophytoplankton. Autofluorescence of Dinophysis spp. (mainly D. acuminata and D. fortii) under blue-light excitation was usually of a yellow-orange color. Occasionally, Dinophysis spp. had red autofluorescencing and yellow-orange autofluorescencing particles. The proportion of cells possessing red autofluorescence tended to be higher in the warm season. Numerous coccoid cells of picophytoplankton (ca. 1–2 μm in diameter) were found attached to the cell surface of D. acuminata, D. fortii, etc. and food vacuole-like structures also observed. These observations suggest there is a close relationship between mixotrophic Dinophysis spp. and certain picophytoplankton. Based on our observations, the possibility that the picophytoplankton found to be attached onto Dinophysis cell surfaces are a food source for Dinophysis, and a source of DSP toxins, is discussed.     

Vertical planktonic structure in the central basin of Lake Baikal in summer 1999, with special reference to the microbial food web

Planktonic microbial interactions in the central basin of Lake Baikal were examined on a summer day in 1999. The subsurface maxima of bacterial abundance and chlorophyll concentration were recorded at the same depth, whereas the vertical distribution of heterotrophic nanoflagellates was the inverse of those of bacteria and picophytoplankton. Release of extracellular organic car-bon (EOC) from phytoplankton was estimated by the NaH¹⁴CO₃ method as 2.4µgCl^–1day^–1. Bacterial production (4.3µgCl^–1day^–1), estimated in a bottle incubation experiment using size-fractionated water samples, exceeded the EOC released. Thus, other supplying sources of organic matter are needed for the bacterial production. Grazing (2.6µgCl^–1day^–1) was also estimated in the experiment and accounted for 60% of the bacterial production. This is the first report on the microbial food web in the central basin of Lake Baikal.     

Rapid Diversification of Marine Picophytoplankton with Dissimilar Light-Harvesting Structures Inferred from Sequences of Prochlorococcus and Synechococcus (Cyanobacteria)

Cultured isolates of the unicellular planktonic cyanobacteria Prochlorococcus and marine Synechococcus belong to a single marine picophytoplankton clade. Within this clade, two deeply branching lineages of Prochlorococcus, two lineages of marine A Synechococcus and one lineage of marine B Synechococcus exhibit closely spaced divergence points with low bootstrap support. This pattern is consistent with a near-simultaneous diversification of marine lineages with divinyl chlorophyll b and phycobilisomes as photosynthetic antennae. Inferences from 16S ribosomal RNA sequences including data for 18 marine picophytoplankton clade members were congruent with results of psbB and petB and D sequence analyses focusing on five strains of Prochlorococcus and one strain of marine A Synechococcus. Third codon position and intergenic region nucleotide frequencies vary widely among members of the marine picophytoplankton group, suggesting that substitution biases differ among the lineages. Nonetheless, standard phylogenetic methods and newer algorithms insensitive to such biases did not recover different branching patterns within the group, and failed to cluster Prochlorococcus with chloroplasts or other chlorophyll b-containing prokaryotes. Prochlorococcus isolated from surface waters of stratified, oligotrophic ocean provinces predominate in a lineage exhibiting low G + C nucleotide frequencies at highly variable positions. Received: 18 January 1997 / Accepted: 18 May 1997     

A comparison of phytoplankton size-fractions in Mondsee,an alpine lake in Austria: distribution,pigment composition and primary production rates

Production rates, abundance, chlorophyll a (Chl a) concentrations and pigment composition were measured for three size classes (<2 μm, 2–11 μm and >11 μm) of phytoplankton from May to December 2000 in deep, mesotrophic, alpine lake Mondsee in Austria. The study focuses on differences among phytoplankton size fractions characterised by their surface area to volume ratio ([mm² l⁻¹: mm³l⁻¹]), pigment distribution patterns and photosynthetic rates. Particular attention was paid to autotrophic picophytoplankton (APP, fraction <2 μm) since this size fraction differed significantly from the two larger size fractions. Among the three fractions, APP showed the highest surface area to volume ratios and a high persistence in the pattern of lipophilic pigments between temporarily and spatially successive samples (about 80% similarity of pigment composition between samples over seasons and depths). The epilimnetic abundance of APP varied seasonally with an annual maximum of 180 × 10³ cells ml⁻¹ in June (at 4–9 m). The minimum (October at 12 m) was more than an order of magnitude lower (4.9 × 10³ ml⁻¹). APP peaked during autumn and contributed between 24% and 42% to the total area-integrated Chl a (10–23 mg m⁻²) and between 16% and 58% to total area-integrated production (5–64 mg m⁻²  h⁻¹) throughout seasons.