Unicellular microbes are of special interest with regards to the overall aim of the centre: (i) they account for most of the biological activity in the ocean; (ii) unlike other organisms, they may be osmotrophs that utilize dissolved organic material (organic or inorganic); and (iii) they are particularly well suited for a trait-based approach because in many instances (at least bacteria and archaea), the species concept does not really apply and organisms are often much better characterized by their functional traits than by their phylogeny (Fenchel 2005). Trait-based descriptions of phytoplankton are well developed and here we initially focus on the heterotrophic activities of microbes.
The main purposes of this work package are to use a trait-based approach to understand the dynamics of the oceans' pool of dissolved organic material (which accounts for >90 % of the organic material in the ocean) and its significance in global carbon budgets, and to understand the spatial and seasonal distribution of mixotrophic organisms in the oceans. The work package includes definition of traits and trade-offs (T1), development of trait based models (T2) and comparison with observed trait distributions in the ocean (T3).
Project 1.1: Traits for bacterial carbon turnover in the marine environment: Chemical complexity meets bacterial diversity
Up to half of the carbon fixed by photosynthesis in the ocean is utilized as dissolved organic matter (DOM) by bacteria. While simple compounds are utilized directly the more complex structures persist for longer. Bacteria release enzymes to cleave complex material to simple monomers (Hoppe et al 1988). We will combine experiments and modelling to study how the chemical complexity and dilute nature of DOM interacts with bacterial functional traits, i.e., enzyme life-time and reactivity. Our hypothesis is that the bacterial turnover of DOM in marine environments can be predicted from two sets of key traits and associated trade-offs: i) characteristics of DOM that define its susceptibility to degradation and ii) bacterial enzyme production and its intrinsic energetic trade-offs; i.e., consequences of enzyme production for bacterial growth efficiency. Such a mechanistic description of the interplay between DOM and bacterial community function is unique, and essential for understanding and predicting nutrient cycling and productivity in pelagic waters. Moreover, it will help us explain the persistence of DOM in the world’s oceans and model its role in the global carbon cycle as an intermediate, delaying the return of carbon to the atmosphere.
Supervisors: Lasse Riemann, Colin A. Stedmon, Uffe Høgsbro Thygesen. External: Mick Follows.
Project 1.2: Mixotrophy trade-offs
Many phytoflagellates have the ability to ingest particulate organic matter, including other algae, thus combining photosynthesis and food uptake; they are mixotrophs (Stoecker 1998). The advantage of mixotrophy is the dual source of nutrition, and the trade-off is the investment in machineries for both photosynthetic and food uptake and degradation. Mixotrophy is a continuum from almost complete autotrophy to almost complete heterotrophy, even within a species (Jones 1994, Hansen 2011). Several attempts have been made to model the trade offs of mixotrophy and, hence, to predict the distribution of this trait in the ocean (e.g. Thingstad 1996, Ward et al. in press). However, these have mainly been theoretical exercises lacking experimental justification and physiological basis for the control of mixotrophy (Mitra & Flynn 2010). Thus, we will combine modelling and experimentation for a better description the mixotrophy trait in the sea. The aim of this project will be to 1) explore the competition for light and nutrients between complete autotrophs on one side and different kinds of mixotrophs (with varying degrees of dependency on food uptake) on the other side (T1), 2) the influence of grazers (protozooplankton versus metazooplankton) on the outcome of the competition between autotrophs and mixotrophs. Finally, 3) we will use our new insights to modify existing trait-based models to predict the seasonal distribution of the mixotrophy trait in the ocean (T2&3).
Supervisors: Per Juel Hansen, Torkel Gissel Nielsen, André William Visser. External: Kevin Flynn, Swansea University.
Project 1.3: Gain vs cost of viral resistance in marine plankton (late project)
Through infection and cell lysis, viruses cycle about 25 % of the carbon fixed by photosynthesis in the ocean. Viral resistance in microbes have clear trade-offs in terms of reduced nutrient uptake capability. We will examine the benefits and costs of viral resistance in bacterioplankton experimentally using molecular and physiological approaches, and develop generic models to quantify the relative fitness of different trade-off strategies.
Supervisors: Lasse Riemann, Torkel Gissel Nielsen, Uffe Høgsbro Thygesen, Mathias Middelboe.
Project 1.4: Response of marine biota to low pH/high CO2 (late project)
Ocean acidification is considered the most serious threat to life in the oceans due to its influence on primary producers (via CO2 availability) and calcifying organisms. This project aims to describe patterns in and mechanisms of sensitivity to low Ph among marine organisms (Melzner et al. 2009 and quantify the associated tradeoffs. We will specifically determine variation in growth rate and other physiological traits among non-calcifying and calcifying marine phytoplankton in response to acidification.
Supervisors: Per Juel Hansen, Benni Winding Hansen. External: Jörg Dutz.
References
Fenchel, T. 2005. Cosmopolitan microbes and their 'cryptic' species. Aquat. Microb. Ecol. 41: 49-54.
Hansen, P.J., 2011. The role of photosynthesis and food uptake for the growth of marine mixotrophic dinoflagellates. Journal of Eukaryotic Microbiology. In press.
Hoppe, H-G., Kim, S-J., and Gocke K., 1988. Microbial decomposition in aquatic environments: combined process of extracellular enzyme activity and substrate uptake. Appl Environ Microbiol 54: 784-790.
Jones, R.I., 1994. Mixotrophy in planktonic protists as a spectrum of nutritional strategies. Marine Microbial Food Webs, 8: 87-96.
Thingstad, T. F., Havskum, H., Garde, K., et al. 1996. On the strategy of ''eating your competitor'': A mathematical analysis of algal mixotrophy. Ecology 77: 2108-2118.
Ward, B.A., Barton, A.D., Dutkiewicz, S., and Follows, M.J., In press. Biophysical aspects of mixotrophic acquisition and competition. Am. Nat.