Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance.

This first comprehensive analysis of the global biogeography of marine protistan plankton with acquired phototrophy shows these mixotrophic organisms to be ubiquitous and abundant; however, their biogeography differs markedly between different functional groups. These mixotrophs, lacking a constitutive capacity for photosynthesis (i.e. non-constitutive mixotrophs, NCMs), acquire their phototrophic potential through either integration of prey-plastids or through endosymbiotic associations with photosynthetic microbes. Analysis of field data reveals that 40-60% of plankton traditionally labelled as (non-phototrophic) microzooplankton are actually NCMs, employing acquired phototrophy in addition to phagotrophy. Specialist NCMs acquire chloroplasts or endosymbionts from specific prey, while generalist NCMs obtain chloroplasts from a variety of prey. These contrasting functional types of NCMs exhibit distinct seasonal and spatial global distribution patterns. Mixotrophs reliant on 'stolen' chloroplasts, controlled by prey diversity and abundance, dominate in high-biomass areas. Mixotrophs harbouring intact symbionts are present in all waters and dominate particularly in oligotrophic open ocean systems. The contrasting temporal and spatial patterns of distribution of different mixotroph functional types across the oceanic provinces, as revealed in this study, challenges traditional interpretations of marine food web structures. Mixotrophs with acquired phototrophy (NCMs) warrant greater recognition in marine research.

Cooccurrence of elevated urea levels and dinoflagellate blooms in temperate estuarine aquaculture ponds.

In hybrid striped bass aquaculture ponds, dinoflagellate blooms were found on 10 of 14 occasions to co-occur with concentrations of urea in excess of 1.5 microM nitrogen. When urea levels were <1.5 microM nitrogen, on seven occasions, no evidence of dinoflagellate blooms was observed in these ponds.

Release of Dissolved Organic Nitrogen by Marine Diazotrophic Cyanobacteria, Trichodesmium spp.

Trichodesmium sp. is a filamentous, colonial cyanobacterium which contributes substantially to the input of nitrogen in tropical and subtropical oceanic waters through nitrogen fixation (N(2) fixation). We applied a N tracer technique to assess the rate of release of dissolved organic nitrogen (DON) from this cyanobacterium and compared those rates with rates of N(2) fixation determined for the same assemblages at the same times of day. Rates of release of DON showed considerable variation within replicate experiments and were variable depending on time of day and duration of time course experiments. On average, rates of DON release were ca. 50% the rates of N(2) fixation. We also fractionated the DON released by using ultrafiltration and found that 60 to 80% of the total organic release was of the size class <10,000 Da. The release of these organic compounds by Trichodesmium spp. is likely a significant source of new nitrogen for the associated bacteria or the non-nitrogen-fixing filaments of the Trichodesmium colonies.

Nitrogen uptake, dissolved organic nitrogen release, and new production.

In oceanic, coastal, and estuarine environments, an average of 25 to 41 percent of the dissolved inorganic nitrogen (NH(4) (+) and NO(3) (-)) taken up by phytoplankton is released as dissolved organic nitrogen (DON). Release rates for DON in oceanic systems range from 4 to 26 nanogram-atoms of nitrogen per liter per hour. Failure to account for the production of DON during nitrogen-15 uptake experiments results in an underestimate of gross nitrogen uptake rates and thus an underestimate of new and regenerated production. In these studies, traditional nitrogen-15 techniques were found to underestimate new and regenerated production by up to 74 and 50 percent, respectively. Total DON turnover times, estimated from DON release resulting from both NH(4) (+) and NO(3) (-) uptake, were 10 +/- 1, 18 +/- 14, and 4 days for oceanic, coastal, and estuarine sites, respectively.