Diel variations in bacterial heterotrophic activity and growth in the northwestern Mediterranean Sea

Abstract

Primary producers must respond to the diel changes in light availability. Therefore, detection of diel cycles in bacterial activity would imply tight coupling between the production of photosynthetic dissolved organic carbon (DOC) and its consumption by bacteria. Absence of diel cycles, on the contrary, would indicate that bacteria depend largely upon allochthonous organic carbon and that bacteria are not tightly dependent on photosynthetically produced autochthonous carbon. In 1993 and 1994 we sampled 3 sites in the NW Mediterranean Sea several times a day, and measured several microbial parameters as well as the vertical profiles of DOC along the diel cycle. The sites were selected so that one was on the continental shelf and, thus, was more Influenced by coastal runoff; a second one was over the shelf slope and a third, oceanlc one was located further offshore over a depth of 2000 m. LVe found clear die1 cycles in bacterial total and specific activity always in the oceanic stations and sometimes in the shelf slope stations. Diel changes were detected as changes in both DNA and protein synthesis rates. These diel cycles were accompanied by diel changes in the distribution of total DOC, and by diel changes in the proportion of bacteria containing visible nucleoids. Noon estimates of bactenal activity were more than twice the daily average in the oceanic site, but they were less different in the other 2 sites. DOC chanyed daily by 15 pM (5 to 15% of the total stock) For bacterial activity to explain the diel changes In DOC concentration, bacteria should have growth efficiencies lower than 10 O/o in general, and lower than 2 "h in the oceanic station.

The magnitude of spring bacterial production in the North Atlantic Ocean

“…Maximum accumulations near Bermuda, where productivity is lower, reach about 30 M C (Carlson et al 1994). Decay rates are poorly studied and not well constrained and appear closer to 1% than 10% d Ϫ1 (Anderson and Williams 1999;Carlson and Ducklow 1996;Zweifel et al 1996) but can be higher (Gasol et al 1998). Thus it seems that subsidies of previously stored DOC might be 50-100% of the PP (supporting BP : PP of 5-15%) for brief periods but are unlikely to be much greater and if they should become greater, then it would not be for very long.…”

Towards a better understanding of microbial carbon flux in the sea*

“…The existence of diel cycles of heterotrophic bacterial activity, especially at oligotrophic sites, has been long documented (Fuhrman et al 1985, Gasol et al 1998. Daily maxima around midday (Gasol et al 1998) were attributed to a tight coupling between the release of DOM by phytoplankton and its rapid uptake by bacteria, implying a major role of sunlight.…”

“…Daily maxima around midday (Gasol et al 1998) were attributed to a tight coupling between the release of DOM by phytoplankton and its rapid uptake by bacteria, implying a major role of sunlight. However, and despite claims concerning the necessity of evaluating the effects of light conditions on bacterial metabolism (Herndl et al 1993, Sommaruga et al 1997, Gasol et al 1998 Most studies dealing with the effects of solar radiation on bacterial growth have focused on UV radiation, but we will restrict this analysis of light vs. dark BP estimates to the PAR region of the irradiance spectrum (400 to 700 nm), the wavelength that passes through vials and flasks and is commonly used for determining microbial metabolic rates.…”

“…Daily maxima around midday (Gasol et al 1998) were attributed to a tight coupling between the release of DOM by phytoplankton and its rapid uptake by bacteria, implying a major role of sunlight. However, and despite claims concerning the necessity of evaluating the effects of light conditions on bacterial metabolism (Herndl et al 1993, Sommaruga et al 1997, Gasol et al 1998 Most studies dealing with the effects of solar radiation on bacterial growth have focused on UV radiation, but we will restrict this analysis of light vs. dark BP estimates to the PAR region of the irradiance spectrum (400 to 700 nm), the wavelength that passes through vials and flasks and is commonly used for determining microbial metabolic rates. There is now compelling evidence that light in the PAR region stimulates Leu incorporation rates relative to dark incubations, ranging from a 10% increase in the Gulf of Mexico (Aas et al 1996) and a mean 30% in a transect across the North Atlantic (Michelou et al 2007) to dramatic increases of >100% in the oligotrophic North Pacific (Church et al 2004).…”

Respiration in the mesopelagic and bathypelagic zones of the oceans

“…It may be calculated as the ratio between the bacterial production and the bacterial growth efficiency (BC = BP/BGE = (BP + BR)/BGE). The estimation of BC fluxes relies, however, on assumed factors that are highly uncertain: (i) the ratio of leucine incorporation to bacterial production, which may range from <1 to 20 Kg C mol −1 leucine (Kirchman et al 1985;Simon et al 1992;Gasol et al 1998;Sherry et al 1999), and (ii) the bacterial growth efficiency (BGE = BP/(BP + BR)), which ranges from 2% to 70% in surface waters of aquatic habitats (del Giorgio and Cole 2000), with no published estimates available for the mesopelagic oceanic waters. Hence the estimates of respiration rates derived from bacterial carbon flux in the dark ocean are highly sensitive to poorly constrained conversion factors and, provided these are not rigorously established, must be considered with some degree of caution.…”