Regeneration functions and microbial ecology of coral reefs

Abstract

Rapid rates of production and consumption on coral reefs have been indirectly measured by several investigators who suggested the existence of rapid regeneration rates in these ecosystems. During 15 months in 1967-68 I attempted to characterize mechanisms and rates of regenerative functioning in coral reefs at Kaneohe Bay, Oahu, Hawaii and Eniwetok Atoll. Marshall Islands. Emphasis was placed on the study of bacteria and other microorganisms based on their typically important regenerative roles in other ecosystems. Complex internal spaces in reef formations and non-living porous coral skeletons appeared to be major sites of regenerative processing. Dead coral heads were obtained from several reef stations and returned to the laboratory for study of internally contained regenerative sediments. Sediments were characterized by chemical and biological assays, and results were compared between local stations as well as between the two major study regions. Additional measurements were made on selected fragments of reef regenerative mass (dead coral). Eniwetok Atoll regenerative sediments contained less than 1% acid-soluble residues, while similarly collected sediments from Kaneohe Bay contained about 24% insoluble (terrigenous) residues. Station averages for total sediment organic matter by ashing ranged from 6 to 12%, and Kjeldhal nitrogen values ranged from 0.2 to 0.7%. Significant amounts of soluble phosphorus and amino nitrogen were released from the dead head spaces during sediment recovery. Regenerative sediments and adjacent skeletal substrates were heavily populated by bacteria, diatoms, protozoa and meiofauna. Bacterial plate counts gave average values of 10 8 - 10 9 colonies per gram sediment. Significant numbers of bacteria were chitin or agar digesting types. Visual counts of diatoms gave values as high as 10 6 cells per gram dry sediment, and were directly proportional to chlorophyll "a" content of the sediments. Bottle respirometry showed consumption of 0.06 to 0.50 mg 02 per g dry sediment per hour. Characteristic mean values obtained for each station appeared to be directly related to the wave and current energy at each station. Antibiotics significantly reduced sediment respiration. Respirometry of algal-encrusted dead coral fragments showed rapid rates of production (P) and consumption (R). Antibiotic treatment of these fragments interfered with their Rand P in a coupled manner. Respirometry of entire dead heads showed that sediment respiration accounted for about 10% of the total respiration in each head. Bacteria were actively removed from water circulated over living and dead coral heads in a laboratory reef simulation. Some infaunal animals apparently digested bacteria that were removed from the water. Observations on infauna of the regenerative system indicated an active role in sediment production and processing, and in maintenance of internal spaces. The infauna apparently acted in symbiosis with the microorganisms to promote rapid organic breakdown processes. Total organic matter, Kjeldahl nitrogen, terrigenous derivatives, and pheophytin showed highest mean levels in sediments from Kaneohe Bay nearshore heads, whereas bacterial counts, diatom counts, and sediment metabolism were highest in sediments from offshore heads. These comparative differences were indications of stressed regenerative function in nearshore reefs, possibly due to land-derived inputs. Parameters measured for outer Kaneohe Bay were strikingly similar to various measurements at Eniwetok, suggesting that regenerative function was similar in geographically separated reefs. Some simplified energy diagrams for simulation suggest how animals and microorganisms are coupled to perform effective mineral recycling and structural renewal of reefs.