Use of stable isotope‐labelled cells to identify active grazers of picocyanobacteria in ocean surface waters

Abstract

Prochlorococcus and Synechococcus are the two most abundant marine cyanobacteria. They represent a significant fraction of the total primary production of the world oceans and comprise a major fraction of the prey biomass available to phagotrophic protists. Despite relatively rapid growth rates, picocyanobacterial cell densities in open-ocean surface waters remain fairly constant, implying steady mortality due to viral infection and consumption by predators. There have been several studies on grazing by specific protists on Prochlorococcus and Synechococcus in culture, and of cell loss rates due to overall grazing in the field. However, the specific sources of mortality of these primary producers in the wild remain unknown. Here, we use a modification of the RNA stable isotope probing technique (RNA-SIP), which involves adding labelled cells to natural seawater, to identify active predators that are specifically consuming Prochlorococcus and Synechococcus in the surface waters of the Pacific Ocean. Four major groups were identified as having their 18S rRNA highly labelled: Prymnesiophyceae (Haptophyta), Dictyochophyceae (Stramenopiles), Bolidomonas (Stramenopiles) and Dinoflagellata (Alveolata). For the first three of these, the closest relative of the sequences identified was a photosynthetic organism, indicating the presence of mixotrophs among picocyanobacterial predators. We conclude that the use of RNA-SIP is a useful method to identity specific predators for picocyanobacteria in situ, and that the method could possibly be used to identify other bacterial predators important in the microbial food-web.

Phytoplankton Response to Saharan Dust Depositions in the Eastern Mediterranean Sea: A Mesocosm Study

“…It was found that ciliates which are their main predators presented an increase at the end of the experiment (Pitta et al, in review). Furthermore, recent studies have shown that many small flagellates may be mixotrophic, grazing on cyanobacteria and heterotrophic bacteria (Frias-Lopez et al, 2009;Unrein et al, 2014). The decline of populations of mixotrophic nanoflagellates would extenuate their grazing pressure on cyanobacteria, resulting in the accumulation of the latter as was observed in our study.…”

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

“…Many studies highlight the herbivore role of dinoflagellates (Stoecker, 1999;and references therein), and although we did not estimate algal predation in our studies, pigmented dinoflagellates were recurrently observed with ingested haptophytes or cryptophytes (Figure 4). Dictyochophytes are also bacterial and picocyanobacterial grazers (Havskum and Riemann, 1996;Frias-Lopez et al, 2009), although the few cells of Pseudopedinella sp. observed in our samples did not allow a reliable measurement of grazing rates.…”

“…In addition, pelagophytes and dictyochophytes (both stramenopiles), which were present in Blanes Bay samples (see pyrosequencing data above) but not quantified here, were also potential candidates to explain part of the total bacterivory exerted by mixotrophs. Both groups are known to actively graze on picocyanobacteria (Frias-Lopez et al, 2009), and probably also on heterotrophic bacteria.…”

“…In addition, Jardillier et al (2010) estimated that haptophytes are responsible for a very significant percentage of the total CO 2 fixation in the tropical and subtropical Northeast Atlantic Ocean. Frias-Lopez et al (2009) identified mixotrophic haptophytes as one of the most important picocyanobacterial predators in the sea, although the methodology they used did not allow to obtain grazing rates estimations. The high diversity, abundance and wide distribution of haptophytes in the oceans, together with their capability to ingest organic particles, suggest they can have a key role as grazers in the microbial food web.…”

Response of Prochlorococcus ecotypes to co-culture with diverse marine bacteria

“…While Prochlorococcus have been extensively studied vis-a`-vis the role of environmental factors, such as light, temperature and nutrient availability in shaping their ecology (Moore et al, 1998(Moore et al, , 2002Bouman et al, 2006;Johnson et al, 2006;Coleman and Chisholm, 2007), and 'top down' processes, such as predation and viral lysis have also been studied to some degree (Lindell et al, 2005Sullivan et al, 2005;Frias-Lopez et al, 2009), systematic studies of their interaction with heterotrophic bacteria are limited to that of Morris and Zinser (Morris et al, 2008) described above, who focused on the growth-enhancing role of bacteria in low-density cultures of Prochlorococcus. Inspired by this work, and by systemic analyses of Long and Azam (2001), we undertook a broad-based and quantitative analysis of co-cultures of two axenic Prochlorococcus ecotypes (Saito et al, 2002;Moore et al, 2005) with hundreds of diverse heterotrophic bacteria, examining the response of the Prochlorococcus cells to the presence of bacteria over the entire growth curve of the cultures.…”