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Although the principal source of marine organic matter is phytoplankton, experimental data on carbon and nitrogen mass balance during their growth cycle are lacking. Phytoplankton from diverse taxonomic groups (Synechococcus bacillaris, Phaeocystis sp., Emiliania huxleyi, Skeletonema costatum) were grown in synthetic seawater media, and changes in particulate and dissolved carbon, nitrogen, and carbohydrates were followed for 14 d. There was a close molar balance between dissolved inorganic carbon (DIC) uptake and total organic carbon (TOC) production in all phytoplankton except Emiliania, which synthesizes carbonate-containing coccoliths. Rates of dissolved organic carbon (DOC) production during phytoplankton growth ranged from 5 to 13 FM DOC d-l (O.Ol-0.06 pmol DOC pM cell C-l d-l) and constituted a substantial (10-3256) fraction of TOC production. The carbohydrate content of both the particulate and dissolved pools increased over the growth cycle and constituted 18-45% and 26-80% of TOC, respectively. The dissolved carbohydrate pool was predominantly composed of polysaccharides (70-94%). Despite some species-specific variability, phytoplankton cellular (particulate) and extracellular (dissolved) organic matter C: N ratios did not deviate far from Redfield values. However, phytoplankton synthesized compositionally distinct pools of high molecular weight dissolved organic matter (> 1,000 Da, average C : N ratio -21) and low molecular weight dissolved organic matter (< 1,000 Da, average C : N ratio -6.0).Most of the organic matter in the sea originates from phytoplankton production. Phytoplankton influence seawater composition by the uptake of inorganic carbon and nutrients for synthesis of cellular organic materials (Eppley and Peterson 1979) and exudation and loss of dissolved organic matter (Mague et al. 1980). The occurrence of phytoplankton blooms at sea have been observed to cause physicochemical changes in the seawater milieu through redistribution of inorganic macronutrients (McAllister et al. 1961) and synthesis and release of organic compounds (Jenkinson and Biddanda 1995). According to Redfield et al. (1963, p. 26), "the influence of organisms on the composition of seawater is profound, with elements being withdrawn from seawater by the growth of phytoplankton in the proportions required to synthesize protoplasm of specific composition and being returned to it as excretions and decomposition products of an equally specific nature."The major portion of DOM released by phytoplankton in the sea consists of small molecules (low-molecular-weight dissolved organic matter [LMW DOM], < 1,000 Da;Jensen 1983;Lancelot 1984). Past studies demonstrate that 20-30% of oceanic DOM is high molecular weight (HMW, Carlson et al. 1985;Benner et al. 1992) and that this pool is carbohydrate rich (25-50%) relative to bulk DOM (Benner et al. AcknowledgmentsThis work was supported by NSF grant OCE 94-13843. We are thankful to Marisa Garza for helping with preliminary culture studies, Andy Biersmith for assistance wit...

Heterotrophic bacteria are a key component driving biogeochemical processes in aquatic ecosystems. In 1998, we examined the role of heterotrophic bacteria by quantifying plankton biomass and bacterial and planktonic respiration across a trophic gradient in several small Minnesota lakes as well as Lake Superior. The contribution of bacteria (<1‐ µm fraction) to total planktonic respiration ranged from ~10 to 90%, with the highest contribution occurring in the most oligotrophic waters. The bacterial size fraction constituted a substantial reservoir of planktonic carbon, nitrogen, and phosphorus (14‐58%, 10‐49%, and 14‐48%, respectively), being higher in oligotrophic than in eutrophic waters. However, we saw no clear evidence for the selective enrichment of either nitrogen or phosphorus in the bacteria size fraction relative to total plankton. Carbon : nitrogen and carbon : phosphorus ratios in both the total particulate matter and <1‐ µm fractions were similar and above Redfield values in oligotrophic waters, but approached them in eutrophic waters. Carbon‐based bacterial growth efficiencies (BGE) were variable (4‐40%) but were lowest in oligotrophic systems and increased in eutrophic systems. BGE varied negatively with carbon : nitrogen : phosphorus ratios, suggesting increased maintenance costs in low‐nutrient waters. In oligotrophic waters most of the organic matter is dissolved, supporting a predominantly microbial food web, whereas in eutrophic waters there is an increased abundance of particulate organic matter supporting a food web consisting of larger autotrophs and phagotrophic heterotrophs.

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The effects of photochemical transformations on the bioavailability of marine dissolved organic matter (DOM) were investigated in surface and deep water from the Gulf of Mexico. Seawater samples were collected from eight depths (15–1,000 m), passed through 0.2‐µm pore‐size filters, and exposed to sunlight in quartz bottles in a flowing seawater deck incubator for 5‐9 h. Following sunlight exposure, samples were inoculated (1:10) with unfiltered seawater from 15‐m depth, and bacterial growth rates were estimated from rates of 3H‐leucine incorporation in dark incubations. Exposure of surface‐water DOM to sunlight resulted in a 75% reduction in bacterial production, whereas exposure of deep‐water DOM resulted in a 40% enhancement in bacterial production. Photomineralization of bioreactive DOM likely contributed to the reduction of bacterial growth in surface water, but the photoproduction of biorefractory DOM also appeared to contribute to reduced bacterial growth. Enhanced bacterial growth in irradiated deep water was consistent with previous studies demonstrating the photoproduction of bioavailable substrates from deep‐water DOM. Phototransformations of DOM appeared to be multifaceted and to play a critical role in the cycling of DOM in the ocean.