Invariable biomass‐specific primary production of taxonomically discrete picoeukaryote groups across the Atlantic Ocean

Abstract

Oceanic photosynthetic picoeukaryotes (< 3 µm) are responsible for > 40% of total primary production at low latitudes such as the North-Eastern tropical Atlantic. In the world ocean, warmed by climate changes, the expected gradual shift towards smaller primary producers could render the role of photosynthetic picoeukaryotes even more important than they are today. Little is still known, however, about how the taxonomic composition of this highly diverse group affects primary production at the basin scale. Here, we combined flow cytometric cell sorting, NaH¹⁴CO₃ radiotracer incubations and class-specific fluorescence in situ hybridization (FISH) probes to determine cell- and biomass-specific inorganic carbon fixation rates and taxonomic composition of two major photosynthetic picoeukaryote groups on a ∼7500-km-long latitudinal transect across the Atlantic Ocean (Atlantic Meridional Transect, AMT19). We show that even though larger cells have, on average, cell-specific CO₂ uptake rates ∼5 times higher than the smaller ones, the average biomass-specific uptake is statistically similar for both groups. On the other hand, even at a high taxonomic level, i.e. class, the contributions to both groups by Prymnesiophyceae, Chrysophyceae and Pelagophyceae are significantly different (P < 0.001 in all cases). We therefore conclude that these group's carbon fixation rates are independent of the taxonomic composition of photosynthetic picoeukaryotes across the Atlantic Ocean. Because the above applies across different oceanic regions the diversity changes seem to be a secondary factor determining primary production.

A global perspective on marine photosynthetic picoeukaryote community structure

“…The high Prymnesiophyceae signal detected across all ocean basins (Supplementary Table 3) supports the observation that 19 0 -hexanoyloxyfucoxanthin, a prymnesiophyte-specific pigment (though also present in a few other Heterokont algae, see Andersen, 2004), often dominates oceanic pigment analyses Liu et al, 2009). Recent fluorescent in situ hybridisation studies confirm the high abundance of these pico-prymnesiophytes in the Atlantic Ocean (Jardillier et al, 2010;Grob et al, 2011;Kirkham et al, 2011b), Indian Ocean and in open ocean regions of the Arctic Ocean . However, Prymnesiophyceae have been consistently underestimated in previous amplificationbased studies using primers targeting the nuclear 18S rRNA gene, potentially a result of PCR bias (Liu et al, 2009).…”

“…Although there is a general trend in the PPE/total picophytoplankton (that is, PPEs þ picocyanobacteria) ratio increasing systematically with increasing latitude and decreasing temperature (Bouman et al, 2012), a considerable amount of variability is observed across the range of latitudes and temperatures. It is important to remember that even at very low cell abundances, PPEs are now known to contribute significantly to marine primary production because of a multifactorial effect of greater biovolume, higher growth rates and high grazing mortality rates (Li, 1994;Worden et al, 2004;Jardillier et al, 2010;Grob et al, 2011). Marine PPE community structure AR Kirkham et al PPE a-diversity Clone sequence data of the 16S rDNA from all the transects studied here (except the BEAGLE and Indian Ocean transects) representing 31 clone libraries allowed us to calculate species richness values (a-diversity) using the Margalef index (Dmg) ( Table 2).…”

“…In particular, cloning and sequencing of both nuclear (for example, see Moon van der Staay et al, 2001;Not et al, 2008) and plastid (for example, see Fuller et al, 2006b;MacDonald et al, 2007;Lepère et al, 2009) small subunit rRNA genes have revealed a vast, previously unsuspected diversity within this assemblage. This focus is largely because of the increased recognition of PPEs as key contributors to phytoplankton biomass and primary production across various marine pelagic ecosystems (Li, 1994;Worden et al, 2004;Richardson and Jackson, 2007;Cuvelier et al, 2010;Jardillier et al, 2010;Grob et al, 2011). Moreover, PPEs have recently been shown to be key bacterivores controlling bacterioplankton abundance in both temperate (Zubkov and Tarran, 2008) and oligotrophic gyre (Hartmann et al, 2012) waters.…”

Threats to Ultraoligotrophic Marine Ecosystems

“…Even though photosynthetic picoplankton are dominated numerically by Prochlorococcus and Synechococcus, much of the carbon is fixed by photosynthetic picoeukaryotes such as the exceptionally diverse haptophytes (Grob et al, 2011). Picohaptophytes are thought to contribute 30-50% of the total photosynthetic standing stock across the world ocean with their competitive success attributed to their mixed mode of nutrition as some are able to photosynthesize as well as engulf bacteria (Liu et al, 2009).…”

Picoeukaryote distribution in relation to nitrate uptake in the oceanic nitracline

“…Yet, despite general descriptions of a deep layer of new production having been available for over 20 years, identification of the responsible organisms and explanations for the fate of the consumed NO 3 − remain equivocal. This is in part driven by our limited understanding of how factors other than slowly changing irradiance intensities impact the lower reaches of the euphotic zone (Letelier et al 2004, Dore et al 2008, Dave & Lozier 2010, but also by limited understanding of picoplankton community structure at depth (Fuller et al 2006, Worden & Not 2008, Grob et al 2011, Kirkham et al 2013.…”