Picoplankton Diel Variability and Estimated Growth Rates in Epipelagic and Mesopelagic Waters of the Central Red Sea

Najwa Al-Otaibi,Francisca C García,Xosé Anxelu G Morán

doi:10.3389/fmars.2021.752910

Abstract

The diel variability of the abundance and cell size of picoplanktonic groups in the central Red Sea was monitored every 2 h in situ on 4 occasions (once per season) from 2015 to 2016. We distinguished Prochlorococcus, low (LF-Syn) and high (HF-Syn) fluorescence Synechococcus, small (Speuk) and large (Lpeuk) picoeukaryotes and two groups of heterotrophic prokaryotes of low (LNA) and high (HNA) nucleic acid content. The diel variability in abundance was less marked than in cell size and more apparent in autotrophs than heterotrophs. Specific growth rates were estimated by an empirical relationship from measurements obtained in bottle incubations of surface and deep samples collected in the winter compared with in situ variations in cell size over 24 h. Autotrophic picoplankton groups generally grew faster (0.23–0.77 d–1) than heterotrophic prokaryotes (0.12–0.50 d–1). Surface to 100 m depth-weighted specific growth rates displayed a clear seasonal pattern for Prochlorococcus, with maxima in winter (0.77 ± 0.07 d–1) and minima in fall (0.52 ± 0.07 d–1). The two groups of Synechococcus peaked in spring, with slightly higher growth rates of LF-Syn (0.57 ± 0.04 d–1) than HF-Syn (0.43 ± 0.04 d–1). Speuk and Lpeuk showed different seasonal patterns, with lower values of the former (0.27 ± 0.02 and 0.37 ± 0.04 d–1, respectively). HNA consistently outgrew LNA heterotrophic prokaryotes, with a higher growth in the epipelagic (0–200 m, 0.36 ± 0.03 d–1) than in the mesopelagic (200–700 m, 0.26 ± 0.03 d–1), while no differences were found for LNA cells (0.19 ± 0.03 d–1 and 0.17 ± 0.02 d–1, respectively). With all data pooled, the mean diel abundances of autotrophic picoplankton in the upper epipelagic and of HNA cells in the epipelagic and mesopelagic layers were significantly correlated with the specific growth rates estimated from cell size variations. Our high-resolution sampling dataset suggests that changes in growth rates underlie the noticeable seasonality of picoplankton recently described in these tropical waters.

Highlights

Picoplankton play an essential role in the productivity and biogeochemical cycles of the oceans and frequently dominate in oligotrophic regions (Olson et al, 1990a; Zubkov et al, 2000; DuRand et al, 2001)
Seasonal differences were more conspicuous for temperature, which ranged from 26.3 ± 0.1◦C (SD) in spring to 31.5 ± 0.2◦C in summer, while low variability was found between the day and night values (CVs = 0.3– 0.7%, Figure 1A)
By being more accessible than the open Atlantic, Indian or Pacific oceans, the oligotrophic and quasi-permanently stratified Red Sea is an appropriate model for studying the spatio-temporal changes in picoplankton communities, the dominant planktonic size class in low latitude waters (Campbell et al, 1997; DuRand et al, 2001; Agawin and Agustí, 2005)

Summary

Introduction

Picoplankton (i.e., organisms with cell sizes ranging from 0.2 to 2 μm) play an essential role in the productivity and biogeochemical cycles of the oceans and frequently dominate in oligotrophic regions (Olson et al, 1990a; Zubkov et al, 2000; DuRand et al, 2001). Most of them have revealed clear diel changes in Prochlorococcus and Synechococcus abundance, with typically maxima in the late afternoon or early night accompanied by an inverse pattern in cell size (Binder and DuRand, 2002; Lefort and Gasol, 2014), despite considerable variability exists among depths and sampling locations These two variables are undoubtedly connected through synchronized cell division (Durand and Olson, 1996; Vaulot and Marie, 1999), and would be useful for calculating growth rates in the absence of loss processes, in situ changes in cell abundance alone do not allow for an accurate estimation

Methods

Results

Conclusion