Phytoplankton-bacteria interactions: an apparant paradox? Analysis of a model system with both competition and commensalism

Abstract

Mineral nutrient limitation seems to stimulate phytoplankton excretion of extracellular organic carbon (EOC). Bacterial growth on EOC requires additional uptake of mineral nutrients. A paradoxical situation is thereby created: algae stressed by lack of mineral nutrients respond in a manner whereby they stimulate their competitors for the lacking nutrients. To investigate this paradoxical ecological relation, a simple mathematical model was analysed and compared to a biological model system. The biological model system consisted of the diatom Skeletonema costaturn and a strain of a marine bacterium cultured together in chemostats with phosphate as the limiting mineral nutrient. Increasing mineral nutrient limitation by decreasing dilution rate resulted in chemostat equilibria with more bacteria and less algae, confirming the proposed paradox. In a certain sense, algae outcompeted themselves at low dilution rates. Predictions of the simple mathematical model were found to be in agreement both with observed distribution of phosphorus between the 2 populations and w~t h amount of algal carbon biomass in the mixed culture. Implications of results for natural aquatic ecosystems are briefly discussed.

Episodic meteorological and nutrient-load events as drivers of coastal planktonic ecosystem dynamics: a time-series analysis

“…Whenever these episodic perturbations imply an input of allochthonous material, situations in which algae and bacteria compete for new nutrients may appear (Bratbak & Thingstad 1985). The outcome of this competition will determine whole-ecosystem metabolism: if primary production dominates, the expected organic matter exported by sedimentation is potentially higher than in a situation dominated by the microbial food web.…”

“…Besides, during the last century, the increment in human activity and population has altered the composition and increased the amounts of nutrient load derived from freshwater inputs, particularly in the Mediterranean (Vollenweider et al 1996). Whenever these episodic perturbations imply an input of allochthonous material, situations in which algae and bacteria compete for new nutrients may appear (Bratbak & Thingstad 1985). The outcome of this competition will determine whole-ecosystem metabolism: if primary production dominates, the expected organic matter exported by sedimentation is potentially higher than in a situation dominated by the microbial food web.…”

Bridging the Gap between Knowing and Modeling Viruses in Marine Systems—An Upcoming Frontier

“…The argument for the inclusion of heterotrophic bacteria, and with it the whole microbial loop, relied on their use of DOM; models with heterotrophic bacteria were able to simulate microbial loop processes, achieving greater detail and dynamics. Other reasons for explicit treatment of the microbial loop in models was their link between the microbial and metazoan food webs, and the possibility to address bacteria-phytoplankton competition for inorganic nutrients (Bratbak and Thingstad, 1985;Fasham, 1993). Since viruses mediate much of these processes, and can change dramatically the fluxes within the microbial loop (Figure 3), their inclusion is obvious.…”

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

“…Heterotrophic bacteria have been shown to both enhance and inhibit the growth of marine and freshwater algae (Grossart et al, 2006;Grossart and Simon, 2007;Mayali et al, 2008) and cyanobacteria (Bratbak and Thingstad, 1985;Manage et al, 2000;Morris et al, 2008) in liquid culture and on solid media. Through these and similar studies we have come to recognize specific mechanisms of interaction, which can occur in the marine environment, such as facilitation of iron uptake (Amin et al, 2009;D'Onofrio et al, 2010), transfer of essential vitamins (Croft et al, 2005), inter-and intra-specific communication (Bassler and Losick, 2006;Vardi et al, 2006) and allelopathy (Mayali et al, 2008;Hibbing et al, 2009).…”