Chroococcoid cyanobacteria: a significant component in the food web dynamics of the open ocean

Abstract

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...

Have we overlooked the importance of small phytoplankton in productive waters?

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Vertical and horizontal structure of the picophytoplankton community in a stratified coastal system off New Zealand

“…Picophytoplankton (photosynthetic picoplankton) have been shown to make a significant contribution to planktonic biomass (e.g., Li et al 1983;Takahashi et al 1985;Gradinger et al 1992) and productivity Iturriaga & Mitchell 1986) in oligotrophic regions of the ocean. Earlier observations suggested that these smallcelled phototrophs were primarily a feature of offshore oligotrophic environments (Stockner & Antia 1986) but more recent work indicates that they can also be a major component of coastal and estuarine waters, including nutrient rich environments (e.g., Hall & Vincent 1990).…”

Spatial variability of cyanobacterial community composition in Sanya Bay as determined by DGGE fingerprinting and multivariate analysis

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