Research

Single Phytoplankton Populations and Nutrient Biogeochemical Cycles in the Sargasso Sea.
Prochlorococcus is ubiquitous in the oligotrophic subtropical and tropical oceans, where it is often the most abundant autotroph, and can account for a significant fraction of primary production. Since the first description of Prochlorococcus, a considerable research effort has been directed at this organism. This work has included assessments of the distribution, abundance, in-situ growth rates and primary production of Prochlorococcus; experimental investigations of the influence of temperature, light, nutrients and trace metals on the growth and physiology of Prochlorococcus in culture and in the field; and molecular studies focusing on the genome and genetic diversity of Prochlorococcus. There have been few direct studies on natural populations, however, and our understanding of the biogeochemical role of Prochlorococcus is not well understood or constrained.

Culture studies have shown that low-light and high-light ecotypes of Prochlorococcus all grow well on NH4+ enriched media while only low-light ecotypes grow on NO2-. No cultured ecotypes have displayed growth in NO3- enriched media. For one low-light ecotypic strain, SS120, this inability to grow on NO3- has been confirmed by the absence of a recognizable narB gene. Despite these growth observations in culture, maxima in Prochlorococcus cell densities are frequently observed throughout the tropical and subtropical ocean coincident with the nitracline over a four order of magnitude range in NO3- concentrations (Fig. 1A,B). Moreover, Prochlorococcus displays a net seasonal population growth at the nitracline in the Sargasso Sea (Fig. 1C). This poses an interesting paradox: Prochlorococcus is associated with the presence of the most abundant nitrogen source NO3-, but is unable to assimilate it for growth; at least based upon culture studies.

We optimized a method combining flow cytometric sorting and stable isotope tracer protocols (FCM-SIP) for use in the oligotrophic Sargasso Sea. The benefit of FCM-SIP is that it permits the incubation of complete pelagic communities, the determination of taxon-specific nutrient assimilation rates using a specific isotopic tracer and thereby directly quantifies the biogeochemical ecology of the target population. Moreover, the accuracy of measured assimilation rates is significantly enhanced by the exclusion of bacterial populations and detrital nitrogen. We observed that deep chlorophyll maximum Prochlorococcus assimilated NO3- with no time lag (Figure 2A) suggesting that there is no ‘trophic processing’ of NO3- thereby making that labeled N available for assimilation. Direct assimilation rates of nitrate (Fig 2B) comprised 5-10% of total measured N uptake.

Despite the likelihood of high grazing losses, the Prochlorococcus DCM population displays a roughly five-fold seasonal increase during a time when nutrient inputs are at their seasonal lowest (Fig. 1C). What supports this net population growth? Our data suggests a daily growth rate attributable to NO3- alone ranging from 0.01 to 0.018 d-1; a value remarkably similar to the net population growth rate, ~0.011 d-1 (Fig 1C). The fact that these estimates agree so well is very encouraging, especially given that our sampling was conducted at the end of the growing season.

For more information on this research, please read our paper.

In a collaboration with Dr. Margie Mullholland and her graduate student, George Boniello, we are studying competitive interactions between phytoplankton and heterotrophic bacteria for the assimilation of inorganic and organic carbon and nitrogen substrates. We are conducting these studies in several sub-estuaries of the Chesapeake Bay and on the US east coast continental shelf. The research in the sub-estuaries is focusing on the competitive interactions between the harmful algal species Aureococcus anophagefferens and bacterial populations. Aureococcus is known for its ability to assimilate organic nutrient substrates when it form near monospecific blooms. Studies into the nutritional ecology of this organism before it blooms have been limited by the inability to determine 'who is taking up what substrate'. With this new technique, we can now easily separate Aureococcus from other organisms (Figure 3) and study the ecology of this important HAB species before it forms dense blooms. The first data from this work is being presented at the 2007 ASLO Aquatic Sciences Meeting. Below is a link to the abstract for this poster presentation.

Collaborators in this research

       Dr. Ger van den Engh and the entire team at Cytopeia Inc.

       Dr. Bethany Jenkins at University of Rhode Island

       Dr. Debbie Bronk and Mr. Paul Bradley at Virginia Institute of Marine Science

       Dr. Margie Mulholland at Old Dominion University

       Dr. Craig Carlson at University of California at Santa Barbara

Peer Reviewed Publications Resulting from this Research:

       Casey, J.R., Lomas, M.W., Mandecki, J. and Walker, D.E. (In Press Geophysical Research Letters) Prochlorococcus                         contributes to new production in the Sargasso Sea deep chlorophyll maximum. (download paper as a PDF).

Abstracts at International Meetings:
       Lomas, M.W., Sedwick, P.N., Casey, J.R. DOES IRON AVAILABILITY CONTROL NEW PRODUCTION BY                                     PROCHLOROCOCCUS IN SUBSURFACE WATERS OF THE SARGASSO SEA? (download Word document)

       Boniello, G.E., Lomas, M.W., Bernhardt, P.W., Mulholland, M.R. NITROGEN UPTAKE AUREOCOCCUS                                             ANOPHAGEFFERENS VERSUS CO-OCCURING BACTERIA DURING A BLOOM: A FLOW CYTOMETRY                                 APPROACH (download Word document)

'Popular' Articles Resulting from this Research:
       Microscopic Life: Taking a closer look at the ocean. BBSR 2004 Annual Report.

Research Awards Supporting this Research:
    This instrument was purchased through a National Science Foundation Major Research Instrumentation Award (OCE-0420821). Additionally many of the projects conducted by my lab group are supporting this instrumentation and benefiting from its availability at BIOS.

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