The electron transport system of respiring organisms reduces 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) to INT-formazan. Respiring bacteria deposit accumulated INT-formazan intracellularly as dark red spots. Corresponding to electron transport system activity, these deposits attain a size and a degree of optical density which allows them to be examined by light microscopy. If polycarbonate filters and epifluorescence microscopy are applied to analyze an INT-treated water sample, it is possible to differentiate between respiring and apparently nonrespiring bacteria. This differentiation, which permits determinations of the total number of bacteria and the proportion thereof involved in respiration, is realized directly within one and the same microscopic image. Initial applications of the present method for hydrobiological purposes showed that the proportion of respiring aquatic bacteria ranged between 6 to 12% (samples taken from coastal areas of the Baltic Sea) and 5 to 36% (samples taken from freshwater lakes and ponds). Cells of 1.6 to 2.4 ym (freshwater) and 0.4 ttm (Baltic Sea) account for the highest proportion of respiring bacteria.

Snow algae occupy a unique habitat in high altitude and polar environments. These algae are often subject to extremes in nutrient availability, acidity, solar irradiance, desiccation, and ambient temperature. This report documents the accumulation of secondary carotenoids by snow algae in response to the availability of nitrogenous nutrients. Unusually large accumulations of astaxanthin esters in extra‐chloroplastic lipid globules produce the characteristics red pigmentation typical of some snow algae (e.g. Chlamydomonas nivalis (Bauer) Wille). Consequently these compounds greatly reduce the amount of light available for absorption by the light‐harvesting pigment‐protein complexes, thus potentially limiting photoinhibition and photodamage caused by intense solar radiation. The esterification of astaxnthin with fatty acids represents a possible mechanism by which this chromophore can be concentrated within cytoplasmic globules to maximize its photoprotective efficiency.

Ciliated Protozoa (Euplotes sp, and Uronema sp.) and natural microzooplankton assemblages (10 to 200 pm) -collected from Los Angeles Harbor, a coastal lagoon, and San Pedro Channelwere fed bacteria previously labeled with 14C(U)-glucose. Dissolved organic carbon-l4 (D014C) released during grazing was collected and subsequently fractionated by AMICON ultrafiltration into 5 nominal molecular weight (NMW) classes: > 105, 104 to 105, 103 to 104, 5 X 102 to 103, and < 5 X 102 daltons. Release of labeled organic compounds with low (< 5 X 102 daltons) and intermediate NMW (103 to 104 daltons) was most rapid in experimental and control samples and was enhanced in the presence of grazers. Unlabeled bacterioplankton assemblages were incubated 24 h with either fractionated or unfractionated DO14C which was released during grazing experiments. Bacterioplankton utilized 196 % d-I + 100 more of the DO14C released in presence of cultured ciliates or microzooplankton than D014C released in their absence. Low (< 5 X 102 daltons) and intermediate NMW (103 to 104 daltons) labeled fractions were incorporated and respired more rapidly by bacteria than the other 3 NMW labeled fractions. Fractionation of D014C after bacterial incubation revealed a decrease in all NMW classes and a shift toward a dominance of low NMW labeled fractions in the D0I4C released in the presence of grazers, but not in the controls. Bactivorous ciliates and microzooplankton contribute quantitatively and qualitatively to the dissolved organic matter pool in nearshore planktonic systems. Dissolved organic matter released in the presence of bactivores is a readily available carbon source for bacterioplankton and may thereby influence bacterioplankton metabolism and growth activity.

New techniques used during a recent study in the oligotrophic North Pacific Ocean demonstrated that coccoid cyanobacteria were the most abundant photoautotrophs. Despite their small size, they accounted for 64 % of the total photosynthesis, with specific growth rates of 1.6 d-'. Grazing experiments indicated that a diverse assemblage of micrograzers is able to consume and effectively metabolize chroococcoid cyanobacteria. In the open ocean, a significant trophic interaction occurs between cyanobacteria and micrograzers, components not included in the classical paradigm of the oceanic food web.In oligotrophic ocean systems, picoplankton (Sieburth et al. 1978) account for approximately 60 % of the inorganic carbon fixation (Li et al. 1983 and apparently have high specific growth rates , Douglas 1984, Landry et al. 1984. Chroococcoid cyanobacteria have been observed in the marine environment (Johnson & Sieburth 1979, Waterbury et al. 1979), but only recently have they been implicated as a substantial component of the autotrophic picoplankton. Information about cyanobacteria in situ growth rates, photosynthetic characteristics and contribution to higher trophic levels is still limited. Studies conducted with cultures of Synechococcus sp. demonstrated net specific growth rates of u p to 1.5 d-', saturating at low irradiance (Morris & Glover 1981). Preliminary field data indicate that picoplankton photosynthesis saturates at low irradiance, with maximal contributions to primary production at the base of the euphotic zone (Morris & Glover 1981, Li et al. 1983). However, the relative contribution of cyanobacteria to total photosynthesis has not been quantified. Few studies have investigated the cya- demonstrated their suitability as food for micrograzers, while ingestion, but not digestion, by calanoid copepods has been observed (Johnson et al. 1982).In the North Pacific gyre (35"N, 128"W), we studied the distribution and photosynthesis of cyanobacteria and their consumption by micrograzers. This region is typical of the oligotrophic subtropical open ocean in spring, with a mixed layer of 60 to 80 m, and a deep chlorophyll maximum between 90 and 105 m as shown by in situ profiles of density, fluorescence, and beam attenuation (Fig. 1). Integrated euphotic zone chlorophyll a was approximately 25 mg m-2. Fig. 2 illustrates the vertical distribution of cyanobacteria and acetone extracted chlorophyll a concentrations for discrete samples at the study site. Daily specific growth and grazing rates were calculated from the general exponential relation:where N, = concentration at time t; No = concentration at time zero; r = specific rate coefficient; t = time. References to rates in this paper are specific natural log rates as defined above. If the cyanobacteria population is in steady state, the specific rate constants for growth, U , grazing, g, and residual losses, 1, satisfy the equation: r = U -g -1 = 0. For our experiments, the loss term, 1, includes respiration and exudation by cyanobacteria, respiration and e...

Measurements of photosynthetic p~g m e n t s , nutnents, spectral Irradiance and vanous physical parameters were performed in the western Sargasso Sea (35"N, 70°W) to ~n v e s t~g a t e the factors affecting phytoplankton blomass dlstribut~ons Algal plgment concentrations and c o m p o s~t~o n s measured during spring 1985 showed cons~derable time-dependent v a n a t~o n s which were cons~stent with those documented by dlrect mlcroscoplc observation D u r~n g early Apnl, 2-fold Increases In chlorophyll a and fucoxanthln were measured on a relatively short tlme scale of days The presence of a d~atom-dominated community malnly specles of the genera Rhlzosolen~a and Chaetoceros, suggested that we were witnessing a stage of the spring bloom Upon return to thls location 2 wk later, the diatom bloom was replaced by a considerably more diverse phytoplankton assemblage consisting of prymnes~ophytes, cyanobacter~a, d~noflagellates, green algae (~ncludlng praslnophytes) and d~a t o m s The vert~cal structures d~splayed by ~n d i v~d u a l accessory plgments dunng Apnl were markedly simllar and suggest that the major phytoplankton taxa were not un~formly distributed In the upper 200 m The phytoplankton were d~s t r~b u t e d as broadly overlapp~ng layers, 1~1th cyanobacterla and diatoms most abundant In the m~x e d layer, prymnesiophytes at lntermed~ate depths, and green algae (Including prasinophytes) deeper In the water column Results p r o v~d e descnptive evidence for a rapid succession of chromatically-adapted phytoplankton during springtime in the Sargasso Sea