second, nitrite is oxidized to nitrate by Nitrobacter spp. and similar bacteria. Nitrobacter spp. benefit from their association with Nitrosomonas spp. because they use nitrite to obtain energy for growth. Commensalistic associations also occur when one microbial group modifies the environment to make it better suited for another organism. The synthesis of acidic waste products during fermentation stimulates the proliferation of more acid-tolerant microorganisms, which may be only a minor part of the microbial community at neutral pH. A good example is the succession of microorganisms during milk spoilage. Biofilm formation provides another example. The colonization of a newly exposed surface by one type of microorganism (an initial colonizer) makes it possible for other microorganisms to attach to the microbially modified surface. Octocorals and Brittle Stars In deep-sea, benthic environments there is an associative relationship between octocorals and brittle stars. Due to the currents flowing upward along seamount ridges, atop these ridges there are colonies of suspension feeding corals and sponges, and brittle stars that grip tight to them and get up off the sea floor. A specific documented commensal relationship is between the ophiuran Ophiocreas oedipus Lyman and the octocoral primnoid Metallogorgia melanotrichos. Historically, commensalism has been recognized as the usual type of association between brittle stars and octocorals. In this association, the ophiurans benefit directly by being elevated through facilitating their feeding by suspension, while the octocorals do not seem to benefit or be harmed by this relationship. Recent studies in the Gulf of Mexico have suggested that there are actually some benefits to the octocorals, such as receiving a cleaning action by the brittle star as it slowly moves around the coral. In some cases, a close relationship occurs between cohabiting species, with the interaction beginning from their juvenile stages. Arguments Whether the relationship between humans and some types of gut flora is commensal or mutualistic is still unanswered. Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party, and that relationships identified as commensal are likely mutualistic or parasitic in a subtle way that has not been detected. For example, epiphytes are "nutritional pirates" that may intercept substantial amounts of nutrients that would otherwise go to the host plant. Large numbers of epiphytes can also cause tree limbs to break or shade the host plant and reduce its rate of photosynthesis. Similarly, phoretic mites may hinder their