Adaptation to Nutrient Availability in Marine Microorganisms by Gene Gain and Loss

An important regulator of both primary productivity and respiration is nutrient availability [e.g., Sanudo-Wilhelmy et al., 2001; Bonnet et al., 2008; Mills et al., 2008; Wambeke et al., 2008]. The concentration and ratio of nitrogen, phosphorus, and iron in the surface ocean varies predictably among different oceanic regions. For example, phosphate is often present at very low concentration in the North Atlantic compared to the North Pacific Subtropical Gyre, whereas the case is reversed for nitrate [Wu et al., 2000]. Similarly, iron is generally low in the Eastern Equatorial Pacific and Southern Ocean compared to most other areas of the ocean [Coale et al., 1996]. Despite this variation in nutrient availability, both photosynthetic and heterotrophic bacteria like Prochlorococcus, Synechococcus , and the SAR11 group are often detected in high abundances across ocean regions [Partensky et al., 1999; Morris et al., 2002; Rusch et al., 2007]. Thus, a fundamental question is whether and how specific bacterial lineages adapt to spatial variation in nutrient availability? It is becoming increasing clear that adaptation to local nutrient conditions is often mediated by gene gain and loss of entire cassettes containing the functionality responsible for nutrient acquisition. For example, a comparative analysis of Prochlorococcus genomes showed large variation in the presence of phosphate uptake genes [Martiny et al., 2006]. Furthermore, Prochlorococcus cells proliferating in low P environments like the Sargasso