Plastic responses that mediate the experienced resource environment

Although the levels of key resources such as food and oxygen are generally viewed as a given property of any ecological setting, organisms in fact make behavioral, developmental, and physiological adjustments that often allow them to experience a given resource environment as more plentiful. This fundamental aspect of experiential niche construction can be carried out by means of a variety of responses in very different types of organisms. In this case, as with temperature, the organism's environment is determined not purely externally but through close interaction with the phenotype.

Microbial niche construction for food and oxygen availability

It has become increasingly well recognized that microbial habitats comprise tremendous spatial and temporal complexity. Microorganisms show a range of behavioral responses to this heterogeneity, just as they contribute to its creation. Microscale habitat choices allow microbes to experience these shifting, heterogeneous environments as consistently rich in nutrients.

Microorganisms are largely motile, using amoeboid movement to glide across surfaces, or flagella to swim in liquid media (Fenchel 2002 and references therein). These motile behaviors are informed by diverse chemosensory signals, allowing bacteria to accumulate at high-nutrient patches within their habitats (J. Seymour et al. 2010). This type of habitat choice or "relocational" niche construction is most well studied in marine microbes, including phytoplankton, heterotrophic bacteria, and the microzooplankton that consume them. Although ocean water is generally low in nutrients, tiny patches or "microzones" of dissolved organic matter (e.g., phytoplankton photosynthates, lysed algal cells, and zooplanktonic wastes) occur at a spatial scale of micrometers to centimeters (Stocker et al. 2008). These high-nutrient microzones occur only briefly before being dissipated by diffusion and water movement (Fenchel 2002). Marine microorganisms rely on "infochemicals" (such as dimethylsulfonio- propionate released by grazed plankton cells) to locate nutrient patches as well as prey organisms (J. Seymour et al. 2010).

Experiments with the marine bacterium Pseu- doalteromonas haloplanktis showed that rapid chemo- tactic swimming responses allowed the bacteria to exploit transient resource patches: in response to fine-scale nutrient pulses such as those generated by lysed algal cells, bacteria clustered within tens of seconds in foraging "hot spots" containing three times more bacterial cells than other microsites. As a result of these rapid foraging behaviors, the fastest- moving 20% of the population experienced a tenfold higher nutrient environment than nonmotile cells (Stocker et al. 2008). Motile bacteria were also able to quickly colonize nutrient plumes of sinking organic particles with environmentally realistic dimensions and dynamics, resulting in fourfold enhanced nutrient availability compared with that obtained by nonmotile cells (Stocker et al. 2008). In nature, such rapid, targeted movements guided by fine-scale "chemical landscapes" (J. Seymour et al. 2010) allow microorganisms to experience their generally nutrient-poor surroundings as nutrient rich. Similarly, chemosensory movements also allow these tiny organisms to seek out microsites that have very specific, preferred oxygen levels (Fenchel 2002).

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