Effects of inorganic nutrients and organic matter on microbial euendolithic community composition and microbioerosion rates

ABSTRACT: Closed cages were used in Glovers Atoll, Belize to test the effects of organic matter (OM) and inorganic nutrient (N + P) additions on microbial euendolithic communities (bacteria, fungi, and algae) and their rates of bioerosion of Strombus gigas shells during a 49 d fertilization experiment. We hypothesized that the addition of OM would release heterotrophic microborers from C-limitation and at the same time reduce light levels for euendolithic microalgae, thus changing the microborers’ community composition and their bioerosion rates. The addition of OM increased the abundance of heterotrophs (particularly fungi), but only when OM was added alone, not when it was combined with inorganic nutrients. In contrast, both cyanobacteria and green algae were stimulated by the addition of inorganic nutrients, but were not affected by OM; these taxa were 4 times more abundant in treatments with inorganic nutrients than in control and OM treatments. Green algae dominated the control, N + P and N + P + OM treatments (77 to 87% green algae, 6 to 8% cyanobacteria, 4 to 16% heterotrophs), whereas in the OM treatment, heterotrophs represented nearly 50% of the total area colonized (52% green algae, 4% cyanobacteria, 44% heterotrophs). Bioerosion rates in treatments with added inorganic nutrients (396 g CaCO₃ m^–2 yr^–1 in the N + P treatment and 370 g CaCO₃ m^–2 yr^–1 in the N + P + OM treatment) were 9-fold greater than bioerosion rates in the control and OM treatments (43 and 48 g CaCO₃ m^–2 yr^–1, respectively), and were not affected by OM addition. We conclude that inorganic nutrients are a major factor controlling the microbioerosion rates and the abundance of euendolithis algae over fungi in carbonate substrata. Microborers act in synergy with the grazers that feed on them and with macroborers that increase the internal surfaces available for microborers to colonize. As a result, increased nutrients can initiate a feedback loop where bioerosion processes reinforce one another, leading to accelerated erosion of the reef framework.