40Hietanen Meri No. 37:37-44, 1998availability <strong>of</strong> nutrients (nutrient recycling by attached bacteria, nitrogen fixati<strong>on</strong>) and turbulence(keeping the cells in the productive water layer <strong>or</strong> allowing them sink). Theref<strong>or</strong>e aggregati<strong>on</strong> can eitherenhance <strong>or</strong> diminish the primary producti<strong>on</strong> <strong>of</strong> phytoplankt<strong>on</strong> cells. As a general rule, the c<strong>on</strong>tributi<strong>on</strong> <strong>of</strong>primary producti<strong>on</strong> in <strong>aggregates</strong> to the total primary producti<strong>on</strong> in the euphotic layer is significant <strong>on</strong>lyin newly f<strong>or</strong>med, phytoplankt<strong>on</strong>-<strong>or</strong>iginated <strong>aggregates</strong> (Alldredge & Gotschalk 1990).5.2 Heterotrophic producti<strong>on</strong>Abundance and heterotrophic producti<strong>on</strong> <strong>of</strong> attached bacteria has already been studied intensively f<strong>or</strong> acouple <strong>of</strong> decades, but the results are still somewhat c<strong>on</strong>tradict<strong>or</strong>y. Depending <strong>on</strong> the method used, locati<strong>on</strong>and time <strong>of</strong> year, the abundance and producti<strong>on</strong> <strong>of</strong> bacteria in <strong>aggregates</strong> has been claimed to beeither significant <strong>or</strong> negligible compared to that <strong>of</strong> free-living bacteria (Table 2). This might also reflectdiscrepancies in the timing <strong>of</strong> sampling since microbial activity <strong>on</strong> marine snow is highly variable anddepends <strong>on</strong> aggregate <strong>or</strong>igin as well as aggregate age (Alldredge & Silver 1988, Herndl 1988, Alldredge& Gotschalk 1990).Table 2. The abundance and producti<strong>on</strong> <strong>of</strong> attached bacteria compared to that <strong>of</strong> total pelagic bacteriaat some locati<strong>on</strong>s.Locati<strong>on</strong>Attached bacteria,% <strong>of</strong> allMethodProd. <strong>of</strong> attached bacteria,% <strong>of</strong> total Reference-1ng C l -1 hBilbao Coast, Spain 1-22 Iriberri & al. 1987Elbe Estuary 75 Zimmermann 1997Freshwater p<strong>on</strong>d
<str<strong>on</strong>g>Literature</str<strong>on</strong>g> <str<strong>on</strong>g>review</str<strong>on</strong>g> <strong>on</strong> <strong>microbiology</strong> <strong>of</strong> <strong>aggregates</strong> <strong>or</strong>iginating from phytoplankt<strong>on</strong> blooms 411996). This finding might explain some differences observed between free-living and attached bacteriasince all bacterial groups found <strong>on</strong> <strong>aggregates</strong> in these studies are known to have special properties thatmake them well-adjusted to attached life (DeL<strong>on</strong>g & al. 1993, Weiss & al. 1996). These special featuresinclude a tendency to associate and glide <strong>on</strong> surfaces as well as an ability to produce differentexoenzymes and to degrade a variety <strong>of</strong> high molecular weight compounds (DeL<strong>on</strong>g & al. 1993).5.3 Nitrogen processesAggregates are known to be sites <strong>of</strong> intense nutrient cycling (e.g. Shanks & Trent 1979, Car<strong>on</strong> & al.1986, Fowler & Knauer 1986, Alldredge & Cohen 1987, Alldredge & Silver 1988, Gotschalk &Alldredge 1989, Alldredge & Gotschalk 1990, Riebesell 1991b, Kaltenböck & Herndl 1992). This <str<strong>on</strong>g>review</str<strong>on</strong>g><strong>on</strong>ly discusses nitrogen processes in cyanobacterial <strong>aggregates</strong>.The biological nitrogen cycle c<strong>on</strong>sists <strong>of</strong> reacti<strong>on</strong>s that are mainly carried out by bacteria. Plants canassimilate amm<strong>on</strong>ia and nitrate, but nitrogen fixati<strong>on</strong>, amm<strong>on</strong>ificati<strong>on</strong>, nitrificati<strong>on</strong> and denitrificati<strong>on</strong> arestrictly microbial processes (Stanier & al. 1971). Likewise, anaerobic amm<strong>on</strong>ium oxidati<strong>on</strong> to dinitrogengas has been shown to be a microbially mediated process (van de Graaf & al. 1995).Cyanobacteria capable <strong>of</strong> fixing dissolved molecular nitrogen (N 2) regularly aggregate during intensiveblooms. Since the sedimentati<strong>on</strong> and grazing <strong>of</strong> these <strong>aggregates</strong> is min<strong>or</strong> (Hoppe 1981, Heiskanen &K<strong>on</strong><strong>on</strong>en 1994), they have been c<strong>on</strong>sidered a net nitrogen input in the ecosystem. Nevertheless, the fate <strong>of</strong>nitrogen fixed by cyanobacteria still remains uncertain. The alternatives in the<strong>or</strong>y are that fixed nitrogenis released into the surrounding water as the cyanobacterial cells lyse at the end <strong>of</strong> the bloom (Heiskanen& K<strong>on</strong><strong>on</strong>en 1994), <strong>or</strong> that nitrogen is at least partly c<strong>on</strong>verted back into gaseous dinitrogen by anaerobicdenitrificati<strong>on</strong> processes.Both nitrogen fixati<strong>on</strong> and denitrificati<strong>on</strong> are oxygen-sensitive processes. Denitrifying bacteria arefacultative anaerobes that denitrify in anaerobic envir<strong>on</strong>ments, although some bacteria may simultaneouslynitrify and denitrify in aerobic envir<strong>on</strong>ments (Roberts<strong>on</strong> & Kuenen 1984). Theref<strong>or</strong>e, the f<strong>or</strong>mati<strong>on</strong><strong>of</strong> microscale anoxic patches in <strong>aggregates</strong> plays a key role in both nitrogen fixati<strong>on</strong> and denitrificati<strong>on</strong>rates in <strong>aggregates</strong>. Anoxic microz<strong>on</strong>es and denitrificati<strong>on</strong> have been dem<strong>on</strong>strated in activated sludgeflocs at waste water treatment plants (de Beer & al. 1997). Recently, Michotey & B<strong>on</strong>in (1997)dem<strong>on</strong>strated particle-associated denitrificati<strong>on</strong> in the Mediterranean Sea. Nitrogen fixati<strong>on</strong> has beenshown to be m<strong>or</strong>e active in cyanobacterial <strong>aggregates</strong> than in suspended filaments (Paerl 1985, Paerl &Prufert 1987). The existence <strong>of</strong> micr<strong>on</strong>iches in cyanobacterial <strong>aggregates</strong> has also been proved experimentallyin many studies (Paerl 1985, Alldredge & Cohen 1987, Paerl & Prufert 1987, Bianchi & al.1992, Shanks & Reeder 1993, Ploug & al. 1997), but the fact<strong>or</strong>s regulating their quantity are not totallyunderstood. Aggregates, like sediments, provide both a col<strong>on</strong>isable surface and a c<strong>on</strong>centrated source <strong>of</strong>readily metabolisable <strong>or</strong>ganic matter. Microbial degradati<strong>on</strong> <strong>of</strong> <strong>or</strong>ganic matter results in oxygen-po<strong>or</strong>microz<strong>on</strong>es around and inside particles (Paerl & Prufert 1987). Anoxic patches remain even in welloxygenatedsurroundings as the physical structure provided by the aggregate slows down the diffusi<strong>on</strong> inand out <strong>of</strong> the microz<strong>on</strong>e (Shanks & Reeder 1993). Paerl (1985) and Ploug & al. (1997) point to theimp<strong>or</strong>tance <strong>of</strong> low envir<strong>on</strong>mental turbulence and de Beer & al. (1997) to the complicated structure <strong>of</strong><strong>aggregates</strong> in microz<strong>on</strong>e f<strong>or</strong>mati<strong>on</strong>. These requirements are easily met in cyanobacterial <strong>aggregates</strong> thatf<strong>or</strong>m under almost n<strong>on</strong>-existent turbulence and seem to actively fix both carb<strong>on</strong> and nitrogen.6. SUMMARYPhytoplankt<strong>on</strong>-derived <strong>aggregates</strong> seem to be an imp<strong>or</strong>tant phenomen<strong>on</strong> in pelagic ecosystems, albeitephemeral in an annual time scale.Regular aggregati<strong>on</strong> <strong>of</strong> phytoplankt<strong>on</strong> blooms provides unique microenvir<strong>on</strong>ments with highly specializedpopulati<strong>on</strong>s and trophic interacti<strong>on</strong>s, thus increasing the biodiversity <strong>of</strong> oligotrophic pelagial waters.
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