Palmer LTER: Hydrogen peroxide in the Palmer LTER region: IV. Photochemical interactions with dissolved organic matter

-the coupled production and utilization of dissolved X organic matter (DOM) through the activities of phyto plankton production, predator)' consumption, and microheterotrophic growth define the "microbial loop," which constitutes an important pathway of carbon and energy' flow in all marine ecosystems (Azam et al. 1983). Comprehensive results from both the AMERIEZ (Antarctic Marine Ecosystem Research at the Ice-Edge Zone) and RACER (research on antarctic coastal ecosystem rates) programs indicate, how ever, a dramatic uncoupling of these anticipated algal-bac terial metabolic processes (Cota et al. 1990; Karl et al. 1991) to the extent that microbial loop processes appear to be neg ligible in the southern oceans coastal ecosystem. Several hypotheses, including organic substrate limitation, lowtemperature inhibition, and chemical antagonism have been presented to explain these fundamental differences from other marine ecosystems (Karl 1993, pp. 1-63). To date, these hypotheses have not been explicitly tested by direct field experiments. There is a new 20th-century challenge to southern oceans microbial assemblages in the form of an increased flux of ultraviolet (UV) light caused by an erosion of the atmospheric ozone layer (Frederick and Snell 1988). Smith et al. (1992) and Helbling et al. (1992) recently documented growth inhibitory, UV-dependent effects on antarctic marine phytoplankton populations. We hypothesized that UV radiation might also play a role in microheterotrophic microbial processes by direct interactions with DOM. Two independent pathways are proposed: (1) partial photolytic degradation or alteration of refractory DOM resulting in a supply of readily available organic substrates for bacterial metabolism and (2) complete photolytic degradation of DOM. The former pathway would enhance heterotrophic bacterial processes; the latter would effectively constitute a form of competition for the availability of organic nutrients and, perhaps, provide an independent nonbiological pathway for DOM decomposition. Our initial testing of the DOM-photolysis hypothesis was during cruise 92-09 of the R/V Polar Duke (November 1992).