Moran Xose Anxelu G.

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Xose Anxelu G.

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  • Article
    Single-cell physiological structure and growth rates of heterotrophic bacteria in a temperate estuary (Waquoit Bay, Massachusetts)
    (Association for the Sciences of Limnology and Oceanography, 2011-01) Moran, Xose Anxelu G. ; Ducklow, Hugh W. ; Erickson, Matthew
    Flow cytometric determinations of membrane integrity, nucleic acid content, and respiratory activity were combined with dilution cultures in Waquoit Bay Estuary (Massachusetts) to estimate specific growth rates of total, live, high (HNA), and low (LNA) nucleic acid content and actively respiring (CTC+) cells. Bacterial abundance ranged from 106 to 107 cells mL-1, with live cells generally contributing > 85% to total numbers, 42-82% HNA cells, and 3-36% CTC+ cells. Specific growth rates (µ) from all physiological groups were positively correlated, but they showed different temperature dependences, with activation energies ranging from 0.28 (live) to 0.97 eV (LNA). The µ values of live cells (0.14-2.40 d-1) were similar to those of total bacteria (0.06-1.53 d-1). LNA bacteria were not dormant but showed positive growth in most experiments, although HNA cells greatly outgrew LNA cells (µ ranges of 0.28-2.26 d-1 vs. 0-0.69 d-1), and CTC+ cells showed the highest values (0.12-2.65 d-1). Positive correlations of HNA bacteria µ with total and phytoplankton-derived dissolved organic carbon support the previously hypothesized strong bottom-up control of HNA cells. Bacterial production estimated from leucine incorporation and empirical conversion factors agreed well with estimates based on growth rates. HNA cells were always responsible for the largest share of bacterial production in the estuary. The contribution of CTC+ cells significantly increased with temperature in the 7-27°C range, reaching values of 40% at temperatures higher than 20°C.
  • Article
    Towards a better understanding of microbial carbon flux in the sea
    (Inter-Research, 2008-09-18) Gasol, Josep M. ; Pinhassi, Jarone ; Alonso-Saez, Laura ; Ducklow, Hugh W. ; Herndl, Gerhard J. ; Koblizek, Michal ; Labrenz, Matthias ; Luo, Ya-Wei ; Moran, Xose Anxelu G. ; Reinthaler, Thomas ; Simon, Meinhard
    We now have a relatively good idea of how bulk microbial processes shape the cycling of organic matter and nutrients in the sea. The advent of the molecular biology era in microbial ecology has resulted in advanced knowledge about the diversity of marine microorganisms, suggesting that we might have reached a high level of understanding of carbon fluxes in the oceans. However, it is becoming increasingly clear that there are large gaps in the understanding of the role of bacteria in regulating carbon fluxes. These gaps may result from methodological as well as conceptual limitations. For example, should bacterial production be measured in the light? Can bacterial production conversion factors be predicted, and how are they affected by loss of tracers through respiration? Is it true that respiration is relatively constant compared to production? How can accurate measures of bacterial growth efficiency be obtained? In this paper, we discuss whether such questions could (or should) be addressed. Ongoing genome analyses are rapidly widening our understanding of possible metabolic pathways and cellular adaptations used by marine bacteria in their quest for resources and struggle for survival (e.g. utilization of light, acquisition of nutrients, predator avoidance, etc.). Further, analyses of the identity of bacteria using molecular markers (e.g. subgroups of Bacteria and Archaea) combined with activity tracers might bring knowledge to a higher level. Since bacterial growth (and thereby consumption of DOC and inorganic nutrients) is likely regulated differently in different bacteria, it will be critical to learn about the life strategies of the key bacterial species to achieve a comprehensive understanding of bacterial regulation of C fluxes. Finally, some processes known to occur in the microbial food web are hardly ever characterized and are not represented in current food web models. We discuss these issues and offer specific comments and advice for future research agendas.
  • Preprint
    Increasing importance of small phytoplankton in a warmer ocean
    ( 2009-03-19) Moran, Xose Anxelu G. ; Lopez-Urrutia, Angel ; Calvo-Diaz, Alejandra ; Li, William K. W.
    The macroecological relationships between marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric size-scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the -0.6 to 22ºC range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift towards smaller primary producers in a warmer ocean. Since the fate of photosynthesized organic carbon largely depends on phytoplankton size we anticipate future alterations in the functioning of oceanic ecosystems.
  • Article
    Total and phytoplankton mediated bottom-up control of bacterioplankton change with temperature in NE Atlantic shelf waters
    (Inter-Research, 2010-01-20) Moran, Xose Anxelu G. ; Calvo-Diaz, Alejandra ; Ducklow, Hugh W.
    The regulation of heterotrophic bacterial growth by resource supply (bottom-up control) was temperature-dependent in our analysis of data obtained during 2006 in the euphotic layer of the southern Bay of Biscay (NE Atlantic) continental shelf. The dataset was split into 2 subgroups using 16°C as the boundary between warm and cool waters based on differences in associated physico-chemical conditions, e.g. inorganic nutrient limitation at higher temperatures. The linear regressions between bacterial biomass (BB) and leucine incorporation rates (LIR) were significantly positive in both temperature regimes, thus indicating similar total bottom-up control, albeit with a slightly higher slope in warm waters (0.33 vs. 0.22). However, the relationship of LIR with phytoplankton biomass (chl a), which is an indicator of bottom-up control that is mediated by phytoplankton, was only significant in waters below 16°C. The analysis of bimonthly variations in the BB-LIR and LIR-chl a correlations indicated that the strength of total bottom-up control significantly increased while the role of phytoplankton in supplying DOM to bacteria diminished with mean temperatures over the 12 to 19°C range, suggesting a seasonal switch in the major source of substrates used by bacteria. We show that the abundance of cells with relatively high nucleic acid content (HNA), which are hypothesized to be the most active ones, was positively associated with bacterial production and specific growth rates in cool but not in warm conditions. These results suggest that HNA bacteria are good predictors of bulk activity and production in temperate ecosystems only when the community relies principally on phytoplankton substrates for growth and metabolism.