Microbial associations with macrobiota in coastal ecosystems: patterns and implications for nitrogen cycling

N (N) is often a limiting nutrient in coastal marine systems, but human activities have doubled the availability of this nutrient over the past century, particularly via fertilizer production to sustain increases in agriculture (Galloway et al. 1995). In coastal ecosystems receiving anthropogenic N, excess N can fuel harmful algal blooms, and the decomposition of organic (dead algal) material via bacterial respiration depletes dissolved oxygen (DO), potentially leading to hypoxia (Howarth et al. 2011). However, coastal ecosystems (estuaries, marshes, reefs, and the nearshore pelagic ocean) are also wellknown sites of N removal via microbial activity. This removal attenuates the land–sea flux of N and can ameliorate the effects of N pollution in coastal regions. At the same time, newly recognized forms of microbial N metabolism, such as anammox, conversion of nitrite (NO− 2 ) and ammonium (NH+ 4 ) directly into gaseous nitrogen (N 2 ) (Table 1), and the identification of new microbial contributors (eg archaea; Könneke et al. 2005) have increased our appreciation of the complexity of coastal N cycling. In open waters, light and DO gradients in the water column govern microbial transformations of the marine N cycle, and “new” N is primarily supplied by upwelling and biological N fixation (Figure 1; Table 1). Coastal systems, however, also host a diversity of macrobiota (multicellular flora and fauna) that contribute to N cycling through N production, retention, and removal (eg Nelson et al. 2013). Macrobiota can also locally change DO concentrations via metabolic activities, thereby promoting a range of N metabolisms over a scale of only micrometers to millimeters (Figures 2 and 3; Table 1). Macrofauna and macroflora clearly host microbial communities that are capable of a diversity of N metabolisms (Figures 2 and 3), though our understanding is still limited. As the distribution and abundance of marine macrobiota change rapidly due to harvest pressure, invasions, habitat fragmentation, pollution, other local disturbances, and global climate change, the effects Microbial associations with macrobiota in coastal ecosystems: patterns and implications for nitrogen cycling